Princess Anne Plaza and The Lakes Neighborhoods Stormwater Management System Flood Mitigation Plan

Transcription

1 CITY OF VIRGINIA BEACH Princess Anne Plaza and The Lakes Neighborhoods Stormwater Management System Flood Mitigation Plan FINAL APRIL 5, 2018 osemont Road Extent London Bridge Creek Windsor Woods Canal Rosemont Rd Bow Creek Northgate Ditch London Bridge Creek

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3 Table of Contents Abbreviations and Acronyms v Executive Summary ES-1 Section 1 Introduction 1-1 Section 2 Model Development Basin Description Model Overview Hydraulic Model Setup Hydrologic Model Setup Soils Land Use Imperviousness 2-8 Section 3 Model Calibration Storm Events for Calibration Tidal Boundary Conditions Calibration Calibration Results Validation 3-14 Section 4 Existing Conditions Model Results Target Level of Service (LOS Results for 10-year Design Storm Results for 100-year Design Storm Results for All Storms 4-4 Section 5 Alternative Solution Sets Approach Alternatives Analysis Alternative Alternative 2: Addition of Moderate Storage Alternative 3: Expanded Storage at Bow Creek Golf Course Alternative 4: Potters Road Pump Station Recommended Alternatives year Storm year Storm Extreme Rainfall Event Simulation Other Storms Summary Alternatives Improvements Mitigating Adverse Offsite Impacts Phasing of System Improvements Recommended Further Evaluations 5-31 i

4 Table of Contents Section 6 Regulatory Feasibility and Probable Cost Feasibility Opinion of Probable Construction Cost Sea Level Rise 6-10 Section 7 Summary and Conclusions Recommended Solutions 7-1 List of Figures Figure ES-1 Alternative 3: Proposed Stormwater Improvements 5 Figure ES-2 Alternative 4: Proposed Stormwater Improvements 7 Figure 1-1 Study Area Overview 1-4 Figure 2-1 Topography 2-3 Figure 2-2 2D Model Extent 2-4 Figure 2-3 Model Schematic 2-5 Figure 2-4 Existing Conditions Surface Soil Percentages 2-7 Figure 2-5 Existing Conditions Land Use Percentages 2-8 Figure 2-6 Existing Conditions Imperviousness Percentages 2-9 Figure 2-7 Soil Types 2-10 Figure 2-8 Existing Land Use 2-11 Figure 3-1 City Flood Complaint Database 3-3 Figure 3-2 Model Calibration and Validation Locations 3-4 Figure 3-3 October 8-9, 2016 Rainfall Public Works London Bridge 3-6 Figure 3-4 March 31, 2017 Rainfall Public Works London Bridge 3-6 Figure 3-5 Matthew Pre-calibration Rainfall Gauge Selection 3-7 Figure 3-6 March 31, 2017 Rainfall Event Gauge Selection 3-8 Figure 3-7 October 8-9, 2016, USGS Tidal Elevation in London Bridge Creek at Route Figure 3-8 October 8-9, 2016, USGS Tidal Elevation in West Neck Creek at Dam Neck Road 3-9 Figure 3-9 March 31, 2017, USGS Tidal Elevation in London Bridge Creek at Route Figure 3-10 March 31, 2017, USGS Tidal Elevation in West Neck Creek at Dam Neck Road 3-10 Figure 3-11 March 31, 2017 Storm, Gauge 8 Stages 3-13 Figure 3-12 March 31, 2017 Storm, Gauge 10 Stages 3-13 Figure 4-1 Simulated Flooding, Existing Conditions 10-year Design Storm 4-5 Figure 4-2 Simulated Flooding, Existing Conditions 100-year Design Storm 4-7 Figure 5-1a Conceptual Bow Creek Golf Course Storage 5-5 Figure 5-1b Model Schematic of Expanded Bow Creek Golf Course Pond 5-5 Figure 5-2 Alternative 3: Proposed Stormwater Improvements 5-7 Figure 5-3 Alternative 4: Proposed Stormwater Improvements 5-9 Figure 5-4 Alternative 3: 10-year Storm Flooding 5-13 Figure 5-5 Alternative 4: 10-year Storm Flooding 5-15 Figure 5-6 Alternative 3: 100-year Storm Flooding 5-19 Figure 5-7 Alternative 4: 100-year Storm Flooding 5-21 Figure 5-8 Alternative 3: Extreme Rainfall Event Simulated Flooding 5-25 Figure 5-9 Alternative 4: Extreme Rainfall Event Simulated Flooding 5-27 ii

5 Table of Contents Figure 6-1 Alternative 3: Effective FEMA Floodway 6-5 Figure 6-2 Alternative 4: Effective FEMA Floodway 6-7 List of Tables Table 2-1 WS 6 Model Elements 2-2 Table 2-2 Study Area Model Elements 2-2 Table 3-1 Hurricane Matthew Flood Elevations 3-12 Table 3-2 September 21, 2016 Flood Elevations at 10:00 AM 3-14 Table 4-1 Design Storm Simulations 4-2 Table 4-2 Summary of Existing Conditions, 10-year Storm Results 4-3 Table year Storm Results Alternative 3 and Alternative Table year Storm Results Study Area Simulated Flooded Addresses 5-17 Table 5-3 Summary of Improvements for Alternative 3 and Alternative Table 6-1 Planning-level Opinion of Probable Construction Cost 6-9 Appendices Appendix A Field Investigation Documentation Appendix B Stormwater Model Development Methodology Appendix C Model Results for Recommended Alternative Appendix D Planning-level Opinion of Probable Construction Costs Appendix E Summary of Alterntatives Evaluated iii

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7 Abbreviations and Acronyms 1D one dimensional 2D two dimensional BCGC Bow Creek Golf Course cfs cubic feet per second CHI Computational Hydraulics International DCR Virginia Department of Conservation and Recreation DEM digital elevation model FEMA Federal Emergency Management Agency GUI graphical user interface H/H hydrologic and hydraulic HRSD Hampton Roads Sanitation District LOS level of service MMPS-160 rainfall gauges at Pine Tree Pressure Reducing Station MMPS-171 rainfall gauges at Shipps Corner Pressure Reducing Station NAVD North American Vertical Datum of 1988 NOAA National Oceanic and Atmospheric Administration NWI National Wetlands Inventory NWP Nationwide Permit OPCC opinion of probable construction cost RCB reinforced concrete RCP reinforced concrete pipe RECP reinforced elliptical concrete pipe SMS Stormwater Management System SWMM US EPA Stormwater Management Model USACE United States Army Corps of Engineers v

8 Abbreviations and Acronyms USGS United States Geological Survey VDHR Virginia Department of Historic Resources VMRC Virginia Marine Resources Commission WS 4 City of Virginia Beach Watershed 4 WS 6 City of Virginia Beach Watershed 6 WS 10 City of Virginia Beach Watershed 10 vi

9 Executive Summary The Princess Anne Plaza, The Lakes and Northgate neighborhoods (herein, the study area are in the City of Virginia Beach Watershed 6 (WS 6, primarily located east of Rosemont Road, north of Lynnhaven Parkway and west of Carriage Hill Road Stormwater from the study area discharges to London Bridge Creek, a tributary of the Lynnhaven River The study area has experienced repeated, significant flooding during recent rainfall events, including: October 2016 Hurricane Matthew September 2016 Tropical Storm Julia October 2012 Hurricane Sandy The City of Virginia Beach retained CDM Smith to develop a hydrologic and hydraulic (H/H model calibrated to multiple storm events over a range of rainfall intensities and boundary conditions Model calibration and validation results were compared with estimated flood depths derived from photos of observed water levels H/H model simulations were performed with the 2-year, 10-year, 25-year, 50-year, and 100-year design storms with correlated tidal boundary conditions Model simulations were completed with and without sea level rise scenarios for potential future flood risk consideration Flood impacts were identified by superimposing the 100-year storm flood map on aerial photography to identify impacted areas Additionally, surveys were conducted at garages in low-lying areas of the study area (Northgate Drive, Lamplight Lane, Club House Road, and Ferry Farm Lane for confirmation of model results The H/H model provided the foundation for exploring improvement alternatives to alleviate flooding in the study area Alternatives were then evaluated on their ability to meet the minimum level of service (LOS for this project: Limit peak stages to 3 inches or less above the road crown for the 10-year design storm Prevent flooding of structures for the 100-year design storm The LOS formulated for this project is less than the City s current design standard Achieving the full design standard in existing neighborhoods is generally not practicable, and requires extensive impacts such as raising and reconstructing roads Recommended improvements to meet the LOS for this project still provides substantial protection (ie, reduced flooding from existing conditions for existing structures, and maintains emergency vehicle access to roadways for the 10-year design storm CDM Smith analyzed four alternatives, of which Alternative 3 and Alternative 4 met the LOS in the study area, with a few exceptions noted in Section 5 Both flood mitigation alternatives consist of multiple individual elements, including gated control structures, pump stations, additional storage areas, ditches, and pipes varying in size from 18 inches to 60 inches in diameter ES-1

10 Executive Summary Alternative 3 and Alternative 4 use identical Stormwater Management System (SMS mitigation options in the study area, while the proposed SMS to convey flow downstream in WS 6 is different in each alternative Alternative 3 includes two pump stations and gated control structures located on London Bridge Creek, one each at the north and south ends of the study area The northern pump station and control structure is at London Bridge Creek near South Lynnhaven Road and the southern pump station and control structure is on Lynnhaven Parkway near Fern Ridge Road This configuration protects the study area and the Green Run subdivision from tidal influences Alternative 4 includes a northern boundary pump station and gated control structure located near Potters Road, near the confluence of London Bridge Creek and Canal 2 A second, southern boundary pump station is not required under this alternative, but a gated control structure is included at Dam Neck Road south of the study area, at the boundary between WS 6 and Watershed 10 (WS 10 This alternative includes dredging 1,760 linear feet of London Bridge Creek upstream of Potters Road This alternative provides flood protection for a much larger tributary area, to include all of London Bridge Creek and Canal 2 drainage areas Summarized below, for both Alternative 3 and Alternative 4, are the key requirements for addressing capacity limitations of the SMS, which were categorized into near-term and long-term flood mitigation options Analysis of the near-term mitigation improvements alone, without the long-term solutions, showed limited benefit in meeting the LOS in the study area However, the near-term improvements are readily correctable and can be implemented immediately: Canal dredging of Northgate Ditch, scheduled to commence in late 2017, to reduce head loss along the canal and to repair the canal to a conveyance capacity close to the original design Canal dredging of the Green Run canals, also planned for late 2017 and 2018, for similar reasons Longer-term solutions should be designed and implemented in a sequence such that an option that benefits one area does not adversely impact an adjacent area (ie, do not move the problem to another area As such, the following are listed in order of preferred implementation: Construct London Bridge Creek pump station(s and gated control structures Construct approximately 85 acres of new detention storage, the majority achieved by converting half of the Bow Creek Golf Course to a lake (60 acres; the remaining storage is included at Plaza Northgate Park and two locations in the Green Run canals system ES-2

11 Executive Summary Lower stages behind the gated structures to 0 feet NAVD prior to an impending storm to maximize storage Increase the size of the SMS significantly within the study area to improve conveyance toward London Bridge Creek For the longer-term solutions, initial analyses revealed that conveyance improvements alone were not sufficient to meet the LOS Additional testing with a larger capacity pump station produced adverse impacts on London Bridge Creek and neighborhoods adjacent to the study area Therefore, the combination of conveyance improvements within the study area SMS, the addition of the recommended stormwater pump station(s and control structures along London Bridge Creek, and expanding the detention volume are all required to achieve the target LOS for the 100-year event The storage volume is further maximized in the existing canals and ponds upstream of the gated areas by lowering stages to 0 feet NAVD prior to a storm with the use of the pump station(s Alternative 3 and Alternative 4 were identified to not cause adverse impacts in London Bridge Creek or elsewhere in the watershed And, implementation of the elements for either alternative provides protection from downstream tidal conditions and future sea level rise The planning level opinion of probable construction cost for Alternative 3 and Alternative 4 are $248M and $463M, respectively The recommended alternatives are in presented in Figure ES-1 and Figure ES-2 Both Alternative 3 and Alternative 4 are recommended to be advanced to preliminary engineering and engineering design stage While both alternatives meet the LOS, the probable construction cost of Alternative 4 is higher than Alternative 3 Alternative 4 is recommended for further consideration because it provides protection to a significantly larger tributary area in WS 6 and additional evaluation is needed to quantify flood mitigation benefits With additional information for the cost, feasibility and benefits of both alternatives, the appropriate solution can be selected to advance to final engineering design and construction As documented in Section 5, approximately 117 structures would still be vulnerable to flooding when testing the Alternative 3 solution with an extreme rainfall event simulation Under Alternative 4, the extreme rainfall event identified 95 structures with simulated flooding Alternative 3 and Alternative 4 were sized based on meeting the 10-year and 100-year target LOS, and the extreme event simulation reveals that flooding risks would still exist even under the improved scenario In addition, areas draining to London Bridge Creek upstream of the pump station(s and gated control structures that have not historically experienced widespread flooding will rely on the recommended improvements in the future It should be noted that reliance on the mechanical devices, such as the recommended pump station and gated control structure, carries residual risk from larger events and potential mechanical failure Backup systems, backup power, and manual operable capability are methods to mitigate for some of the residual risk Without the recommended improvements the study area is highly vulnerable to flooding from tidal flooding and flooding from rainfall runoff With the recommended improvements the study area will continue to be vulnerable to flooding from ES-3

12 Executive Summary rainfall runoff for storms that exceed the 100-year design storm and/or from larger joint rainfalltidal events and surges The analyses performed as part of this study are conceptual, and the completed H/H models are intended to serve as planning level tools The conceptual improvements identified in this study should be advanced through engineering design The H/H models should also be refined during engineering design to evaluate additional details or modifications developed during the design process, and to confirm anticipated operation of the recommended pumps and gates Additional analysis is recommended during preliminary design to provide decision makers and the public with the information necessary to understand residual risk and vulnerability, and to provide a comprehensive basis for prioritizing actions and investments to reduce risk and provide adaptable contingency plans for emergency conditions ES-4

13 Legend 37 PS PumpStatio n C\ GatedCo ntro lstructure Sto rag eimpro vement Sto rmw aterpipeimpro vement DitchImpro vement Bo w Creek Go lfco urse 24'Φ 3 42'Φ 4 48'Φ 'Φ 'Φ 43'x68' 18'Φ 38'x60' 30'Φ 30'Φ 48'Φ 30'Φ 24'Φ 48'Φ 53'x83' 36'Φ 21'Φ 36'Φ C\ 37 PS London Bridge Creek Ü 8 PlannedDredg ing Bo xculvert PSTributary Area PlazaNo rthg ate Park 48'Φ 24'Φ 24'Φ 24'Φ 24'Φ 24'Φ 30'Φ 6 36'Φ 43'x68' 24'Φ 24'Φ 36'Φ 'Φ 24'Φ 'x68' 30'Φ 'Φ 24'Φ 30'Φ Canal 2 38'x60' 42'Φ 36'Φ 48'Φ 60'Φ 18'Φ 24'Φ 30'Φ 18'Φ 42'Φ 36'Φ 30'Φ 6'x9' 37 PS C\ GreenRun Canals West Neck Creek Miles Figure ES-1 Princess Anne Plaza and The Lakes Alternative 3: Proposed Stormwater Improvements

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15 Thalia Creek C\ 37 PS Ü Bo w Creek Go lfco urse 24'Φ 42'Φ 48'Φ 2 30'Φ 30'Φ 30'Φ 30'Φ 48'Φ 30'Φ 24'Φ 36'Φ 48'Φ 36'Φ London Bridge Creek 'x53' 18'Φ 18 21'Φ 8 10 PlazaNo rthg ate Park 48'Φ 24'Φ 24'Φ 24'Φ 24'Φ 30'Φ 6 36'Φ 43'x68' 24'Φ 24'Φ 36'Φ 'Φ 24'Φ 30'Φ 'x68' 'Φ 24'Φ 30'Φ Canal 2 38'x60' 42'Φ 18'Φ 36'Φ 48'Φ 36'Φ 60'Φ 30'Φ 18'Φ 24'Φ GreenRun Canals 6'x9' 30'Φ C\ Legend 37 PS PumpStatio n C\ GatedCo ntro lstructure Sto rag eimpro vement Sto rmw aterpipeimpro vement DitchImpro vement Canal 2 PlannedDredg ing Bo xculvert PSTributary Area West Neck Creek Miles Figure ES-2 Princess Anne Plaza and The Lakes Alternative 4: Proposed Stormwater Improvements

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17 Section 1 Introduction The Princess Anne Plaza, The Lakes and Northgate neighborhoods are in the City of Virginia Beach WS 6 Figure 1-1 displays the location of the study area and the overall WS 6 boundary The study area is approximately 790 acres, primarily located east of Rosemont Road, north of Lynnhaven Parkway and west of Carriage Hill Road Known historical flooding-prone areas include the neighborhoods north of The Lakes and Holland Road, south of Bow Creek Golf Course (BCGC, and west of the golf course near Rosemont Road and Club House Road The major hydraulic systems in the project area include: London Bridge Creek, which flows north to the Lynnhaven River and may flow south towards West Neck Creek Bow Creek, which flows east to London Bridge Creek though BCGC Northgate Ditch, which flows east as a tributary to Bow Creek at two locations in BCGC The ditch splits near the north end of Plaza Northgate Park, with a west branch flowing north into the golf course; and, an eastern branch flowing parallel to Northgate Drive, under Bow Creek Boulevard at the eastern edge of the course, near the confluence with London Bridge Creek The Lakes, which are a series of connected man-made lakes and canals tributary to London Bridge Creek from the southeast corner of the Holland Windsor Shopping Center to Lynnhaven Parkway The study area has experienced repeated, significant flooding during recent rainfall events, including: October 2016 Hurricane Matthew September 2016 Tropical Storm Julia October 2012 Hurricane Sandy To address these repeated flooding events, the City directed CDM Smith to develop a H/H model to evaluate alternatives to alleviate flooding issues The study area cannot be separated from the larger WS 6 drainage area without making extensive assumptions of downstream stages and flows Therefore, the study area modeling was conducted within a larger WS 6 model Performed under the ongoing stormwater master plan project, the WS 6 model development included the entire tributary area to London Bridge Creek and Canal 2, from Dam Neck Road (southern boundary to Virginia Beach Boulevard (northern boundary The WS 6 model enabled a more accurate prediction of flood stages throughout the system, and, as such, established the starting point for the study area evaluations 1-1

18 Section 1 Introduction CDM Smith calibrated the model to multiple storm events over a range of rainfall intensities and boundary conditions Model results were compared with estimated flood depths derived from photos of observed water levels and model parameters were adjusted based on conditions observed during site visits (see Appendix A To achieve calibration, model parameters were adjusted until a reasonable agreement between estimated flood depths and simulated flood depths was reached Additional discussion of model calibration is provided in Section 3 CDM Smith applied the calibrated model to identify areas of probable flooding for the 10-year and 100-year design storm events under existing land use conditions, as discussed in Section 4 CDM Smith then modeled potential system improvements to mitigate flooding within the study area under the 10-year and 100-year design storms Alternatives were then evaluated on their ability to meet the minimum level of service (LOS for this project: Limit peak stages to 3 inches or less above the road crown for the 10-year design storm Prevent flooding of structures for the 100-year design storm The LOS formulated for this project is less than the City s current design standard Provision of the full design standard in existing neighborhoods is generally not practicable, which often requires modifications such as raising roads Recommended improvements to meet the LOS for this project still provides substantial protection (ie, reduced flooding from existing conditions for existing structures, and maintains emergency vehicle access to roadways for the 10-year design storm The alternatives were developed to mitigate flooding in the study area while also avoiding adverse impacts to offsite areas (ie, locations outside the study area in Watershed 6 Alternative options that meet the minimum LOS, while also limiting offsite impacts include transforming approximately half (60 acres of the Bow Creek Golf Course into a lake and constructing one or more pump station/gated structure facilities to maximize the available storage volume The remainder of this report is organized as follows: Section 2 describes the development of the H/H model and supporting data Section 3 presents the calibration of the H/H model to observed flood conditions Section 4 presents model results of the SMS prior to evaluating improvements Section 5 provides alternative solution sets evaluated to achieve the established LOS Section 6 describes anticipated environmental permitting and a conceptual-level construction cost opinion for selected alternatives, and considerations to accommodate future sea level rise conditions Section 7 presents a summary of the solutions to be further evaluated 1-2

19 Section 1 Introduction Appendix A contains field investigation documentation used to calibrate the H/H model Appendix B expands on Section 2 and provides technical details of the stormwater model development methodology Appendix C contains model results for the selected alternatives Appendix D contains detailed information about the opinion of probable construction costs presented in Section 6 Appendix E contains a comprehensive summary of the alternative improvements presented in Section 5 1-3

20 Watershed 6 Eastern Branch Lynnhaven River Ü Watershed 6 Study Extent Thalia Creek London Bridge Creek Bow Creek Canal #2 The Lakes West Neck Creek Figure 1-1 Princess Anne Plaza and The Lakes Study Area Overview Miles Legend Study Boundary WS6 Boundary

21 Section 2 Model Development 21 Basin Description The study area model encompasses 790 acres of relatively flat, low-lying, urban land that includes the Bow Creek Golf Course and the Princess Anne Plaza, The Lakes and Northgate neighborhoods, among others The study area is drained through a series of ditches and canals that outfall to London Bridge Creek and West Neck Creek, with tidal influences from the north by the Lynnhaven River, and to the south by West Neck Creek Figure 2-1 shows the topographic digital elevation model (DEM for the study area extracted from LiDAR Topographic elevations range from approximately 0 feet NAVD along London Bridge Creek to 15 feet NAVD within portions of the study area The highest elevations within the natural topography is approximately 15 feet NAVD, with most stormwater inlets ranging between 5 feet and 11 feet NAVD 22 Model Overview Stormwater models were developed using PCSWMM Version 70 by Computational Hydraulics International (CHI PCSWMM uses the US EPA Stormwater Management Model (SWMM version computational engine, and includes a custom graphical user interface (GUI that offers GIS functionality, model building, calibration, and post processing tools The primary objective for developing H/H models for the City s SMS is to provide a tool suitable for evaluating the performance of the system US EPA SWMM uses physical H/H parameters based on the best available data, engineering guidelines, and judgement The model is most reliable when calibrated using observed data, such as rainfall, tide elevations, and flood elevations Once CDM Smith developed the model to match observed conditions, it was used to simulate the SMS response to other rainfall events (design storms and downstream tidal boundary conditions The detailed description of SWMM methodologies and parameters is provided in Appendix B Due to the extent of recurring flooding and the flat topography in the study area, both one dimensional (1D and two dimensional (2D models were developed While a 1D model of subsurface (pipe and surface (open channel flows typically is sufficient for flooding evaluations, the 2D model better predicts flood depths in extremely flat and flooding-prone areas (see Appendix B57 Figure 2-2 shows the entire study area, as well as an outline of the 2D model area The 2D model covers 296 acres and includes London Bridge Creek from Ferry Farm Lane to Lynnhaven Parkway West to east, the 2D model extends from Plaza Northgate Park to Kings Arm Drive 2-1

22 Section 2 Model Development 23 Hydraulic Model Setup The model SMS includes open channels, pipes of 15-inch diameter and larger, constructed stormwater facilities, and overland flows Since the portion of the study area is a subset of the larger WS 6 model, both the full WS 6 components and those within the study area are presented in Table 2-1 and Table 2-2, respectively Figure 2-3 includes the modeled SMS network for the study area Table 2-1 WS 6 Model Elements Subcatchments 1,379 1D Model Elements 2D Model Elements Junctions 2,293 Storage Nodes 1,187 Outfalls 61 Conduits Circular 3,014 Arch 3 Ellipse 46 Rectangular Open 1 Rectangular Closed 14 Irregular 1,175 2D Cells 21,372 Junctions 21,364 Outfalls 0 Conduits Open Rectangular 58,719 Table 2-2 Study Area Model Elements Subcatchments 364 1D Model Elements 2D Model Elements Junctions 800 Storage Nodes 288 Outfalls 8 Conduits Circular 933 Arch 3 Ellipse 16 Rectangular Open 1 Rectangular Closed 6 Irregular 365 2D Cells 21,372 Junctions 21,364 Outfalls 0 Conduits Open Rectangular 58,

23 Ü Lake Windsor Windsor Woods Canal Rosemont Rd Bow Creek Northgate Ditch London Bridge Creek Legend Study Boundary Figure 2-1 Princess Plaza and The Lakes Topography Miles 2D Model Boundary Topography DEM (feet NAVD High : 15 Low : 0

24 Watershed 6 Ü Rosemont Road Study Extent London Bridge Creek Figure 2-2 Princess Anne Plaza and The Lakes 2D Model Extent Miles Legend Study Boundary 2D Model Boundary WS6 Boundary

25 Legend 2D Model Princess Anne Plaza/ The Lakes Ü # # 1D Nodes Other Manhole # Storages # 1 Miles Figure 2-3 City of Virginia Beach Princess Anne Plaza and The Lakes Model Schematic Canal #2 # 05 Bow Creek Northgate Ditch Creek Outfalls London Bridge Inlet # Weirs # Rectangular 025 Overland 0 Elliptical Channel # # Circular 1D Conduits Sources: Esri, HERE, DeLorme, USGS, Intermap, Esri Korea, Esri (Thailand, MapmyIndia, NGCC, OpenStreetMap contributors, and INCREMENT P, NRCan, Esri Japan, METI, Esri China (Hong Kong, the GIS User Community

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27 Section 2 Hydrologic and Hydraulic Model Development 24 Hydrologic Model Setup The hydrologic portion of the H/H model represents the process for transforming rainfall to stormwater runoff that enters the SMS For this study, the 2017 hydrologic conditions were modeled for the calibration storms and for estimating the existing LOS The primary components of the H/H model representation of existing hydrology are soils and land use data Soils data are used to define the infiltration and stormwater runoff from pervious areas included in the model Land use data is used to estimate impervious cover, and for parameters that define the routing of stormwater runoff to the SMS Information for soils and land use data follows; additional detail is included in Appendix B 241 Soils Figure 2-4 shows the distributions of surface soil types found in the study area based on spatial data provided by the City The study area primarily consists of land complex (which is a dual class soil consisting of silty loam and sandy to silty clay In urban areas, land complex soils are often compacted with the clay layer representing a confining layer During calibration, the saturated hydraulic conductivity of the land complex soils was set to 001 inches per hour, which represents a clay soil The remaining soil types include loam and silt loams (191 percent and 188 percent, respectively and clay, sandy loam, and muck type soils (16 percent Silt Loam Mucky Peat Loamy Loam, 0 to 2 % Slopes Loam Land Complex Land Fine Sandy Loam, 6 to 35% slopes Fine Sandy loam 20% 33% 05% 67% 23% 11% 188% 191% 461% 0% 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% Figure 2-4 Existing Conditions Surface Soil Percentages 242 Land Use Figure 2-5 presents the land use percentages within the study area The majority of the land use is residential (37 percent, with other uses being commercial, light industrial and institutional (34 percent, and heavy industrial and transportation (23 percent Waterbodies account for nearly 3 percent of the area The land use coverage is not used to estimate impervious cover (see Section 243 below, since the coverage was provided by the City However, land use is still necessary to estimate additional hydrologic parameters, such as roughness and depression storage The model is more sensitive to impervious coverage than to these land use-derived hydrologic parameters 2-7

28 Section 2 Model Development Forest, Open and Parks Waterbodies 1% 2% 3% Medium Density Residential 23% High Density Residential 28% Commercial, Light Industrial, and Institutional Heavy Industrial and Transportatation Wetlands 34% 9% Figure 2-5 Existing Conditions Land Use Percentages 243 Imperviousness The existing condition impervious percentages were compiled from the City s 2017 GIS coverage layers, which are directly measured from aerial photography For modeling with SWMM, wetlands and waterbodies are also considered impervious Based on this data and the land use coverage of wetlands and waterbodies, a total of 41 percent is impervious (33 percent of directly connected impervious area, plus 8 percent of impervious area routed to pervious areas with the remaining 59 percent considered pervious Figure 2-6 presents the percent cover of each type The impervious routed to pervious designation is typically for residential rooftop gutters that are directed to a yard (ie, not directly connected to the SMS 2-8

29 Section 2 Hydrologic and Hydraulic Model Development Impervious Pervious Routed to Pervious 8% 33% 59% Figure 2-6 Existing Conditions Imperviousness Percentages The map in Figure 2-7 shows the study area s soil type composition and Figure 2-8 shows the study area s land use for existing conditions 2-9

30 Ü Lake Windsor Windsor Woods Canal London Bridge Creek Legend Figure 2-7 Princess Anne Plaza and The Lakes Soil Types Miles Study Boundary Soil Type Loam Silt Loam Land Complex (Silty, Sandy Clay Clay, Peat, Muck

31 Land Use Ü Type Forest, Open Land and Parks Waterbodies Golf Courses and Agriculture Low Density Residential Medium Density Residential High Density Residential Commercial, Light Industrial, and Institutional Heavy Industrial and Transportation Figure 2-8 Princess Anne Plaza and The Lakes Existing Land Use Miles Legend Study Boundary

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33 Section 3 Model Calibration This section describes the model calibration process for the study area model Model calibration is the adjustment of model parameters to match observed conditions The study area portion of the WS 6 model was calibrated and validated with available storm data provided by the City These data included the City s storm complaints database, photographs of flooding during actual rain events, and estimated peak flooding depths based on the photographs Areas outside of the study area tributary to London Bridge Creek were included in the analysis and revised as part of the calibration process Known flooding areas within the study area include, but are not limited to, the intersection of Rosemont Road and Club House Road and neighboring side streets; the Northgate neighborhood, from Northgate Drive to Lakecrest Road; along Bow Creek Boulevard, South Club House Road, Lamplight Lane, and many of their side streets Figure 3-1 shows reported flooding based on the City s storm complaints database The records were geo-referenced and separated by date to evaluate various storm events The call information does not give the depth of flooding at the location The model was calibrated to both Hurricane Matthew, which provided larger flow volumes and deeper flows in London Bridge Creek, and the March 31, 2017 storm, which provided runoff from a short, relatively high-intensity storm The City provided surveyed flood depths corresponding to photographs taken during Hurricane Matthew at five locations in the study area In addition, water level data measured from two temporary gauges during the March 2017 storm event were made available by the City The gauge data includes a time-stage hydrograph which allows for calibration to local runoff timing, while the photographs from Matthew provide a known time and location of flood extents The calibrated model was validated with the September 2016 Tropical Storm Julia event, using the City s storm complaints database along with observed peak stages on Rosemont Road (obtained from field investigations during the storm The locations of the calibration and validation points are presented in Figure 3-2 There are multiple boundary conditions for the WS 6 model, including outfalls in London Bridge Creek to the Lynnhaven River and West Neck Creek, south to WS 10 Two United States Geological Survey (USGS stage gauges were utilized to reveal interactions between WS 6 and WS 10 The gauge located in West Neck Creek at Dam Neck Road showed a peak stage of 70 feet NAVD during Hurricane Matthew, while the gauge in London Bridge Creek at Virginia Beach Boulevard had a peak stage of 53 feet NAVD Additionally, flow was observed to flow from London Bridge Creek both into and out of WS 10, likely driven by wind and tides during Hurricane Matthew Additionally, peak water elevations observed during Hurricane Matthew (as surveyed by the City in the upstream portions of the study area were higher than both USGS gauge readings, indicating the direction of flow was both north and south out of the study area 3-1

34 Section 3 Model Calibration The remainder of the outfalls in the WS 6 model are either in the Lynnhaven River (and given the same boundary condition as London Bridge Creek at Virginia Beach Boulevard or represent overflows to neighboring watersheds Most of these overflows allow a relatively minor amount of flow out of the system; however, at the intersection of Rosemont Road and Club House Road, flooding in WS 4 is especially deep and flow enters WS 6 at this location The results from the calibrated WS 4 Windsor Woods model was used as a time-stage boundary condition at this intersection for Hurricane Matthew 3-2

35 (( ( ( (( ((( (((((( (((( ((( ((((((((((((((((((((((( (( ( ( (((( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( (( ( ((( ((((((((((((((((((((((( ( (( (( (( Sources: Esri, HERE, DeLorme, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Hong Kong, Esri Korea, Esri (Thailand, MapmyIndia, NGCC, OpenStreetMap contributors, and the GIS User Community Figure 3-1 City of Virginia Beach Princess Anne Plaza and The Lakes City Flood Complaint Database Legend ( March 2017 (3/31/2017 ( Matthew (10/8-10/2016 ( Julia (9/19-22/2016 ( Other Dates 2D Model Boundary Study Boundary Ü Miles

36 Ü Photo Photo ^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_^_ ^_ Gauge 10 ^_ ^_ Survey/Photo 3 Survey/Photo 2 ^_ ^_ ^_ Gauge 8 Survey/Photo 4 Survey/Photo 1 ^_ Survey/Photo 5 Sources: Esri, HERE, DeLorme, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Hong Kong, Esri Korea, Esri (Thailand, MapmyIndia, NGCC, OpenStreetMap contributors, and the GIS User Community Figure 3-2 Princess Anne Plaza and The Lakes Model Calibration and Validation Locations Miles Legend ^_ Topical Storm Julia (Sep 2016 ^_ Hurricane Matthew (Oct 2016 ^_ March 31, D Model Boundary Study Boundary

37 Section 3 Model Calibration 31 Storm Events for Calibration The storms selected for model calibration are characterized below October 8 October 9, 2016: Hurricane Matthew Total precipitation ranged from 8 inches to 14 inches over a 24-hour period across Virginia Beach, with localized precipitation in the study area between 95 inches and 135 inches Maximum intensity was 035 inches in a 5-minute period (42 inches per hour, though the maximum hourly total was only 18 inches Total storm volume was much larger than the 100-year storm, but peak hourly intensity was less than a 5-year storm (see Appendix B, Table B-2 Peak stage in London Bridge Creek at Virginia Beach Boulevard reached 53 feet NAVD at 3 am on October 9, 2016 (approximately 12 hours after the storm began At West Neck Creek at Dam Neck Road, peak stage was 70 feet NAVD at 6 am on October 9, 2016 Six rainfall gauges were used to characterize rainfall spatial and temporal variation Two were from Hampton Road Sanitation District s (HRSD rainfall gauges at Pine Tree Pressure Reducing Station (MMPS-160 and Shipps Corner Pressure Reducing Station (MMPS-171, with data provided in 15-minute intervals Data from an additional gauge at City of Virginia Beach Department of Public Utilities Plaza Bow Creek Boulevard (PS-526 was available in hourly intervals The remaining three gauges were from Public Work s London Bridge, Plaza, and Green Run rainfall gauges (5-minute intervals The 5-minute distributions in the Public Works gauges were used to disaggregate the data from the other three gauges to 5-minute intervals Prior to calibration, Theissen Polygons were used to distribute the rain gauges in the model (ie, each subcatchment received rainfall from the closest gauge Changes made to the rainfall distribution during calibration are discussed in Section 33 March 31, 2017: Storm Event Total precipitation was between 20 inches and 30 inches in the study area over a 12-hour period, with the bulk of the rainfall falling within a two-hour period from 7 pm to 9 pm Maximum intensity was 041 inches in a 5-minute period (49 inches per hour Maximum hourly volume was 183 inches Total storm volume is slightly smaller than a 1-year storm, 24-hour duration event, and closer to the 1-year, 12-hour duration event The maximum intensity is similar to a 1-year storm for a 5-minute duration The peak stage in London Bridge Creek at Virginia Beach Boulevard reached 24 feet NAVD near 1 am on April 1, 2017 (approximately 12 hours after the beginning of the storm The peak stage in West Neck Creek at Dam Neck Road was 24 feet NAVD at 1:30 am on April 1,

38 Section 3 Model Calibration The same six rainfall gauges that were used for Hurricane Matthew were also used to characterize rainfall spatial and temporal variations for this storm Theissen Polygons were again used to distribute the rain gauges in the model The storm event was selected due to the availability of data from temporary water level gauges installed by the City in the study area These gauges are located on the upstream end of the Bow Creek culvert under Club House Road, in London Bridge Creek at Lynnhaven Parkway (inoperable on March 31, 2017, and in Bow Creek within Bow Creek Golf Course near the seventh hole Figure 3-3 and Figure 3-4 show the precipitation of the two selected storms at Public Works London Bridge gauge Rainfall volume is measured in five-minute increments Figure 3-5 shows the spatial distribution of the rainfall gauges and subcatchment assignment within WS 6 for Hurricane Matthew pre-calibration Figure 3-6 shows the spatial distribution of rainfall gauges and subcatchment assignment for March 31, 2017 event Figure 3-3 October 8-9, 2016 Rainfall Public Works London Bridge Figure 3-4 March 31, 2017 Rainfall Public Works London Bridge 3-6

39 Ü l? l? Sources: Esri, HERE, DeLorme, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Hong Kong, Esri Korea, Esri (Thailand, MapmyIndia, NGCC, OpenStreetMap contributors, and the GIS User Community Figure 3-5 Princess Anne Plaza and The Lakes Matthew (10/8-9/2016 Rainfall Gauge Selection Miles l? Legend USGS Gauges Study Boundary Selected Gauge Rain Gauges PW-03 LONDONBRIDGE PW-16 PLAZA PW-18 GREENRUN PU-526 HRSD MMPS-160 HRSD MMPS-171

40 Ü? l l? Sources: Esri, HERE, DeLorme, USGS, Intermap, INCREMENT P, NRCan, Esri Japan, METI, Esri China (Hong Kong, Esri Korea, Esri (Thailand, MapmyIndia, NGCC, OpenStreetMap contributors, and the GIS User Community Figure 3-6 Princess Anne Plaza and The Lakes March 31, 2017 Rainfall Event Gauge Selection Legend l? USGS Gauges Rain Gauges Study Boundary Selected Gauge PW-03 LONDONBRIDGE PW-16 PLAZA PW-18 GREENRUN PU Miles HRSD MMPS-160 HRSD MMPS-171

41 Section 3 Model Calibration 32 Tidal Boundary Conditions Tide data recorded by USGS gauges in London Bridge Creek at Virginia Beach Boulevard (Route 58 and in West Neck Creek at Dam Neck Road were used for the calibration and validation storms (shown on Figure 3-5 and Figure 3-6 The Virginia Beach Boulevard gauge coincides with the London Bridge Creek outfall of the WS 6 model, while the Dam Neck Road gauge coincides with the boundary between the WS 6 and WS 10 models Data was extracted from the USGS in feet NAVD to match the model datum and was used directly as time series boundary conditions in the model Figure 3-7 through Figure 3-10 shows the tidal boundary conditions for both USGS gauges observed during Hurricane Matthew and the March 31, 2017 storm Figure 3-7 October 8-9, 2016, USGS Tidal Elevation in London Bridge Creek at Route 58 Figure 3-8 October 8-9, 2016, USGS Tidal Elevation in West Neck Creek at Dam Neck Road 3-9

42 Section 3 Model Calibration Figure 3-9 March 31, 2017, USGS Tidal Elevation in London Bridge Creek at Route 58 Figure 3-10 March 31, 2017, USGS Tidal Elevation in West Neck Creek at Dam Neck Road 33 Calibration The WS 6 model was run for Hurricane Matthew from 12:40 pm on October 8, 2016 to 12:40 am on October 10, 2016 The start time was chosen at a relatively low tide to match the initial depths in the model setup Peak runoff in the study area occurred near 8:00 pm on October 8, 2016 though there was significant runoff through approximately 3:00 am on October 9, 2016 The model indicates that London Bridge Creek crests in the study area between 3:30 am and 4:00 am It is unknown what time London Bridge Creek crested since observed flooding photographs were taken later in the morning Appendix A provides the photographic evidence of flooding locations that were later surveyed by the City The times the photos were taken range from 7:30 am to 10:00 am Anecdotal evidence from homeowners suggest higher stages than the surveyed flood elevations occurred earlier in the storm Initial simulations indicated that model-predicted stages at photographed locations were too low, and that the flood receded much faster than the photographs showed Since flood levels were sustained near peak levels several hours after the local runoff had ended, changes to local pipe roughness, minor losses, and/or ditch roughness values had no measurable effect on the results However, the March 31, 2017 simulation was sensitive to these values, as well as the center channel roughness of London Bridge Creek and Canal 2 Therefore, the model was calibrated to 3-10

43 Section 3 Model Calibration both storms, with the parameters that affect local timing and peak flows calibrated to the smaller storm, while the parameters that affect watershed-wide flow volumes and timing were calibrated to Hurricane Matthew The following changes were made to the model to provide higher stages in the Hurricane Matthew simulation, while maintaining a match of the recorded gauge data for the March 31, 2017 storm: Increased rainfall: Rainfall gauge selection was originally performed using Theissen Polygons, which provides the nearest gauge to each subcatchment This assumes that each gauge is equally weighted However, during Hurricane Matthew, the rainfall volumes were higher in the northeastern portion of the watershed than the southwest For calibration, the extent of influence of the northeastern gauges was moved from the midpoint to approximately two-thirds northeast/one-third southwest (see Figure 3-5 The original Theissen Polygon distribution was maintained for the other storms (see Figure 3-6 Decreased infiltration: Most of the soils in the watershed are Land Complex, which are a disturbed soils type due to urbanization Land Complex soils may have a range of infiltration capacity, but are often poorly draining In this case, the soils were assigned a saturated hydraulic conductivity of 001 inch/hour, which is equivalent to a clay soil (or Type D soil Additionally, infiltration rates for all other subbasins were divided by two, which moves the sand loam types to silt loams, and the silt loams types to silty clay loams Floodplain roughness: Values of London Bridge Creek and Canal 2 were increased based on field reconnaissance (see Appendix A The floodplains have thick, tall brush which is unlikely to significantly bend in the direction of flow, therefore the roughness values in about a dozen locations were increased to 02, which is at the top of the range provided in the channel roughness guidance provided in Appendix B (Table B-7 The center channel roughness values for both London Bridge Creek and Canal 2 were tested at higher values as well, but reset to 004, due to the March 31, 2017 storm calibration Roughness of ditches: Throughout the model, values were increased, where appropriate, based on aerial and street view imagery, to delay flow and therefore delay the time of the flood crest in London Bridge Creek Roughness modifications ranged from 0035 to 01 for the center channel and from 004 to 02 for the banks, while others increased from 01 to 012 (or 015, and 015 to 018 (or 02 For the March 31, 2017 storm, calibration adjustments included changing the center channel roughness of London Bridge Creek and Canal 2 (see above, refining the roughness of the Northgate Ditch and Bow Creek, and estimating sedimentation and local losses in the Bow Creek culverts In addition, the Northgate ditch section transects in the study area were modified based on survey data provided by the City 3-11

44 Section 3 Model Calibration 331 Calibration Results With the aforementioned H/H model revisions, the model produced the results shown in Table 3-1, for the five surveyed flood depths for Hurricane Matthew The model provides identical or slightly higher peak stages (01 feet to 03 feet higher as compared to observed water levels in photographs provided by the City, but the model recedes more quickly than the observed levels The time of the photographs ranged from about 3 hours to 5 hours after the peak of the storm Possible explanations for the model not replicating the sustained, hours-long duration of flooding include: Debris: As the floodwater receded, debris likely clogged the inlets, and possibly even constricted capacity in bridges or culverts downstream Evidence of floating debris was noted in the photographs Wind: Hurricane Matthew produced high winds from the northeast, which may have created wind stress on London Bridge Creek during the flood recession limb Baseflow: The precipitation from Hurricane Matthew started 16 days after the conclusion of Tropical Storm Julia, which was another 14-inch event It is possible that baseflow from Julia was still flowing through the underlying aquifer to London Bridge Creek and Canal 2 These specific potential onsite conditions are not reflected in the model Since these onsite conditions represent unique events, they would not necessarily be present during future storm events Hence, the difference between the model and observed flooding duration was considered to have minimal effect on model calibration accuracy for the alternatives analysis simulations Table 3-1 Hurricane Matthew Flood Elevations ID Location Date/Time of Observation (TO Observed Flood Elevations (feet NAVD Model Peak Lakecrest Road 10:00 am October 9, Bow Creek Boulevard 07:45 am October 9, Forrest Trail 08:10 am October 9, Deer Park Drive 08:30 am October 9, Dixie Drive 07:30 am October 9, TO For the March 31, 2017 storm, stages in Bow Creek were compared to two of the City s temporary gauge locations shown in Figure 3-2 The other temporary gauge was not operational on the date of the storm Additionally, the time stamp on the temporary gauges did not match the USGS gauge time values An analysis of recorded high tides between the USGS gauges and the temporary gauges from December 2016 through May 2017 revealed a time slippage in the temporary gauges that was contributed to instrument error However, the temporary gauge depth values were determined to be valid; therefore, only the time was adjusted for model calibration The amplitudes and the relative time from the start of the simulation remained unchanged for the temporary gauges Figure 3-11 and Figure 3-12 present the model results at Gauge 8 and Gauge 10, respectively for the March 31, 2017 storm As shown, the modeled peak stages are within 01 feet of observed levels; hence the model was considered reasonably calibrated 3-12

45 Section 3 Model Calibration Figure 3-11 March 31, 2017 Storm, Gauge 8 Stages Figure 3-12 March 31, 2017 Storm, Gauge 10 Stages 3-13

46 Section 3 Model Calibration 34 Validation The calibrated model was validated by conducting simulations of Tropical Storm Julia and comparing the results with field observations Tropical Storm Julia produced between 90 inches and 140 inches of rain over a 72-hour period USGS gauge data was used for the time-stage hydrograph, with a peak of 26 feet NAVD in London Bridge Creek at Virginia Beach Boulevard, and 45 feet NAVD at Dam Neck Road A time-stage boundary condition at Rosemont Road and Club House Road from the Calibrated WS 4 Windsor Woods model was used for Tropical Storm Julia A single location, with an estimated high-water mark, was available along Rosemont Road near Club House Road The remainder of the area was compared to the City complaints map shown in Figure 3-1 However, most of the complaints, except for Rosemont Road, were due to inlet clogging, and thus the locations would not necessarily show as flooding in the model Table 3-2 September 21, 2016 Flood Elevations at 10:00 AM Location Northbound Rosemont Road between Old Forge Road and Club House Road Southbound Rosemont Road between Old Forge Road and Club House Road Flood Elevations (feet NAVD Observed Model Delta (feet The results provide flood elevations at the time of the field visit The model indicates peak flood elevations of 65 feet NAVD for both sides of Rosemont Road These results validate the model to the extent possible with the data provided 3-14

47 Section 4 Existing Conditions Model Results The calibrated model was run for the 2-year, 10-year, 25-year, 50-year, and 100-year National Oceanic and Atmospheric Administration (NOAA locally normalized Type C, 24-hour design storm events under existing land use conditions The methodologies used to develop the design storm hyetographs are presented in Appendix B, Section B4 The design storm depths range from 37 inches with a 6-minute peak intensity of 41 inches per hour for the 2-year storm to 95 inches with a peak 6-minute intensity of 107 inches per hour for the 100-year storm (the hyetographs are discretized in 6-minute intervals Appendix B provides the volume and intensity data for all storms presented in this report To perform the design storm simulations, the model was setup with boundary conditions for the various tailwater conditions and initial depths associated with each storm event The hydrologic conditions (eg, impervious cover and hydraulic conditions (eg, pipes, culverts and ditches remained unchanged from the calibrated model The northern boundary condition (Lynnhaven River at I-264/Potters Road, utilized rainfall-correlated tailwater conditions for the 50-year and 100-year design storms, and used the King Tide stage for all other design storm recurrences, as shown in Table 4-1 The King Tide stage is more conservative since the rainfall-correlated tailwater stages are lower (see Appendix B58 for more information The southern boundary of WS 6 (West Neck Creek at Dam Neck Road depends not only on rainfall intensity/volumes, but also wind conditions During model calibration, and as observed by USGS gauges during Hurricane Matthew, flow traveled both directions at the Dam Neck Road gauge (from WS 6 into/from WS 10 However, for the design storms analysis, the southern boundary was modeled as a no-flow boundary This is because, at the time of this study, the WS 10 master plan model was under development and the expected flow direction at the watershed boundary was unknown during the design storms For the sea level rise scenarios, fixed increases of 15 feet and 30 feet are added to each boundary condition, respectively Since the models are not highly sensitive to tenth of a foot differences in tailwater elevation, the base condition plus 30 feet sea level rise is expected to have the same result as the 25-year storm surge models for the 2-year, 10-year, 25-year, and 50-year design storms All boundary conditions are set to fixed stage if the tailwater condition is higher than the outfall, otherwise normal depth boundary conditions are set for the outfall Initial depths in the model are set to match the corresponding fixed tailwater condition, unless there is a structure in the SMS, in which case the higher of the tailwater condition or the structure invert is used to set the initial condition 4-1

48 Section 4 Existing Conditions Model Results Table 4-1 Design Storm Simulations Tailwater Condition Tailwater Stage at Sewells Point Corresponding to Design Storm Recurrence Interval (feet NAVD 2-year 10-year 25-year 50-year 100-year Base condition* Base plus 15 feet sea level rise Base plus 30 feet sea level rise year storm surge year storm surge + 15 feet sea level rise 25-year storm surge + 30 feet sea level rise * Base condition represents rainfall correlated tailwater with a minimum stage set to the King Tide 41 Target Level of Service (LOS The target LOS for the 10-year and 100-year 24-hour design storms were determined from multiple meetings between CDM Smith and the City Discussions centered on public concern and project feasibility, among other topics, leading to the following project-specific LOS: Limit peak stages to 3 inches or less above the road crown for the 10-year design storm Prevent flooding of structures for the 100-year design storm The LOS formulated for this project is less than the City s current design standard Provision of the full design standard in existing neighborhoods is generally not feasible (cost-effective since they are often a retrofit of existing infrastructure The formulated LOS provides substantial protection for existing structures, and maintains access to roadways for the 10-year design storm To model the 10-year LOS for this project, indicator elevations were estimated from LiDAR information at all potential roadway intersections with simulated flooding within the study area For the 100-year storm, the edge of the building footprint was intersected with the simulated flood extent to determine if the structure would be counted as flooded In some cases, incidental overlap between the edge of the building and the flood extent was dismissed (and not counted as a flooded structure, if the overlap was within the uncertainty of the LiDAR interpolation The count of simulated flooded structures is approximate for the following reasons: The City provided surveyed finished garage elevations for 300 structures located within the study area; in locations where survey data was not available, the structure was assumed to be equal to the adjacent ground elevation from LiDAR For non-surveyed structures, the extent of structures containing front steps or raised floors was not known; therefore, a 6-inch or 12-inch buffer (ie, increase from LiDAR elevations was not applied in the analysis 4-2

49 Section 4 Existing Conditions Model Results Multi-family residences (ie, townhomes or duplexes were counted as a single structure, with the lowest garage elevation used in the analysis; in such cases, the entire structure was counted as one flooded structure Sheds and outbuildings were not included in the flooding analysis To provide a more accurate count of flooded structures, CDM Smith recommends obtaining addition survey information to advance the project through engineering design 42 Results for 10-year Design Storm The results of the existing condition simulations indicate that 116 intersections do not meet the LOS described above for the 10-year storm Of these 116 locations, Table 4-2 presents the locations where flooding is expected to be more than 10 feet above the road crown; the maximum flood depth is 16 feet above the road crown at Club House Road and Hannibal Street Figure 4-1 shows the 10-year storm flood map for the study area and Appendix C contains the complete list of flood depths and indicator elevations Table 4-2 Summary of Existing Conditions, 10-year Storm Results Model Node Location Estimated Crown (feet NAVD 10-year Existing (feet NAVD 06_PAP-81 Club House Road and Hannibal Street _PAP-78 Club House Road and Granite Trail _PAP-320 Valient Avenue and Garrison Place _PAP-87 Rosemont Road and Old Forge Road _PAP-163 Club House Road and Duplin Street _PAP-98 Hannibal Street and Country Club Circle _PAP-197 Skipper Drive and Barcelona Lane Windsor Oaks Boulevard and Cainhoy Lane _PAP-294 Lark Street and Dauphin Lane _PAP-154 Club House Road and Crosset Street _P-573 Fern Ridge Road and Lakecrest Road _PAP-137 Silina Drive and Lineberry Road _PAP-103 Skipper Drive and Bethune Drive McKinley Avenue and Faraday Lane _PAP-166 Duplin Street and Olympia Lane McKinley Avenue and Good Hope Road _PAP-211 Club House Road and Dauphin Lane Campion Avenue and Campion Court South Pine Grove Road and Pine Grove Lane Delta (feet McKinley Avenue and Dana Lane _PAP-314 Silina Drive and Barcelona Lane

50 Section 4 Existing Conditions Model Results 43 Results for 100-year Design Storm The results of the existing condition simulations for the 100-year design storm are shown in Figure 4-2 This figure represents a flood extent map, superimposed on an aerial photograph of the study area The simulated flood extent intersects the building footprint for a total of 600 structures for the 100-year design storm The count of simulated flooded structures is approximate; as noted in Section 41, completing surveys for additional structures in the study area would provide a more accurate count of the flooded structures Areas of concern are concentrated along Bow Creek Boulevard, Northgate Drive, South Club House Road, Lamplight Lane, and many of their side streets; additional areas include the northwest corner of Bow Creek Golf Course near Rosemont Road and Club House Road 44 Results for All Storms The results of the existing condition simulations for all design storms, with and without sea level rise are presented in Appendix C Table C-1 through Table C-9 present the peak stage compared to the road crown elevations at intersections within the study area for each design storm/ sea level rise scenario 4-4

51 Legend Ü 264 Rd Boundary Ly nn ha ve n Hydrography ft Silina Dr Rosem ft S ont Rd Flood Depth ft ft ft > 20 ft Bow Cree Blvd k en av h n y w Pk Rosem d ont Rd n Ly R nd lla Ho Win dso ro aks B lvd Bow Creek Blvd d In S e nc de en ep vd Bl 0 Figure 4-1 Princess Anne Plaza and The Lakes Simulated Flooding, Existing Conditions 10-year Design Storm Miles

52 Section 4 Existing Conditions Model Results This page intentionally left blank 4-6

53 Legend Boundary Hydrography Flood Extent 264 Rosemont Rd Silina Dr S Lynnhaven Rd Ü Bow Creek Blvd Windsor Oaks Blvd Holland Rd Bow Creek Blvd Lynnhaven Pkwy Rosemont Rd S Independence Blvd Miles Figure 4-2 Princess Anne Plaza and The Lakes Simulated Flooding, Existing Conditions 100-year Design Storm

54 Section 4 Existing Conditions Model Results This page intentionally left blank 4-8

55 Section 5 Alternative Solution Sets 51 Approach The results presented in Section 4 indicate that the existing study area SMS is not sufficiently sized to meet the 10-year design storm LOS nor the 100-year design storm LOS The modeled SMS lacks adequate capacity to convey flows away from the Princess Anne Plaza, The Lakes and Northgate neighborhoods, leading to simulated flood depths too deep for normal traffic to pass through, and with the flooding duration lasting for many hours The City s objective with this master plan is to develop a phased Capital Improvement Program that identifies both short-term and long-term solutions The short-term improvements will address immediate, readily correctible issues; the more comprehensive, longer-term solutions will enhance the overall LOS provided by stormwater infrastructure in the area The model provides a conceptual-level evaluation with sufficient detail to identify SMS capacity improvement requirements Implementation of any identified alternative should be advanced from planning-level through engineering design, as the location of improvements may vary to account for siting, permitting and construction constraints For example, parallel pipes were typically added until water levels decreased sufficiently to meet the target LOS Considerations during engineering design may include evaluating whether a single, equivalent larger system is preferred, instead of parallel system improvements, and identifying potential utility conflicts A critical aspect of evaluating improvement alternatives was the potential for adverse impacts downstream in London Bridge Creek and in Canal 2 The capacity and operation of the pump stations evaluated were sized to limit downstream increases in water surface elevation, and to prevent increases in flooding above existing condition elevations To analyze structure flooding, the 100-year storm flood map was superimposed on aerial photography and inspected for overlap Surveys were conducted at garages in low-lying areas of the study area (at a total of 300 structures to confirm this methodology For the 10-year storm LOS, peak stages were compared to road crown indicator elevations at intersections throughout the study area Section 52 presents discussion of the alternatives evaluated to mitigate flooding in the study area and the significant findings of each alternative 52 Alternatives Analysis The concept behind the alternatives approach was to provide additional conveyance within the study area and increase storage capacity to mitigate simulated flooding under existing conditions The alternatives were combined and built upon one another until the target LOS was met and adverse offsite impacts were removed 5-1

56 Section 5 Alternative Solution Sets 521 Alternative 1 Alternative 1 included incorporating planned or under construction ditch and canal dredging projects within the study area In addition, isolation of London Bridge Creek from tidal influences was evaluated by constructing a pump station and gated control structure at both the northern and southern extents of the study area The following main components are included in Alternative 1: 1,000 cubic feet per second (cfs pump station and gated structure at South Lynnhaven Road and London Bridge Creek the pump is operated in stages to not increase offsite peak water levels 600 cfs pump station and gated structure at Lynnhaven Parkway and London Bridge Creek, south of the confluence with The Lakes and Green Run Canal the pump station is operated in stages to not increase offsite peak water levels This location is upstream of Canal 2 Dredge 4,750 liner feet of Northgate Ditch (east of McKinley Avenue to reduce head loss and to repair the canal to a conveyance capacity close to the original design Construction is planned to commence in late 2017 Dredge additional 2,900 linear feet of Northgate Ditch (west of McKinley Avenue CDM Smith identified the need to extend dredging further west beyond the planned dredging limits The modeled ditch bank slopes are similar to existing slopes and match the planned east Northgate Ditch slopes Improved capacity of Bow Creek and tributary ditches and culverts in BCGC to include clearing of overgrown vegetation and debris Canal dredging for all six Green Run canals, four of which are planned to start in late 2017 and 2018, to restore conveyance capacity close to the original design 1,330 linear feet of improved ditch east of Bethune Drive, along South Lynnhaven Road This ditch, which drains portions of Princess Anne Plaza near Silina Drive, connects to London Bridge Creek downstream of the northern pump station location Approximately 39,500 linear feet of pipe and culvert improvements in the study area Design plans for the improved Northgate Ditch and Green Run canals were provided by the City and incorporated into the model by CDM Smith Additionally, the initial depth in London Bridge Creek, for the portions of the system behind the northern and southern gated structures, was lowered to 0 feet NAVD prior to an impending storm, creating a storage volume to be filled by the storm The results of Alternative 1 showed that many of the existing system restrictions for the 10-year event (identified in Section 42 were resolved, but not all For the half-dozen locations in Alternative 1 that remained higher than 3 inches above the road crown, improvements other than larger-sized pipes are required, which are addressed under Alternative 2, Alternative 3 and Alternative 4 For the 100-year design storm, Alternative 1 resulted in 264 flooded structures, down from the 600 simulated with the existing system, providing a reduction of 56 percent 5-2

57 Section 5 Alternative Solution Sets As for lowering the pre-storm initial depth in London Bridge Creek to 0 feet NAVD, the model showed the additional storage resulted in lowering peak 100-year stages by 03 feet to 04 feet in most areas Alternative 1 is not recommended and was removed from further consideration because it did not sufficiently meet the 10-year and the 100-year LOS requirements 522 Alternative 2: Addition of Moderate Storage Alternative 2 included all Alternative 1 improvements with additional options to mitigate the remaining flooding locations The northern and southern pump stations remained along London Bridge Creek, as well as the identified neighborhood pipe improvements The Alternative 2 configuration includes the following main elements: All elements of Alternative 1 Widening ditches within BCGC to an extent to maintain playability of the existing 18 holes and driving range Converting 16 acres of Plaza Northgate park to a pond Two additional storage locations on vacant lots in the Green Run Canal system, adjacent to Green Run Lake 1 and Cinnamon Ridge Lake Based on discussions with City Parks and Recreation Department staff, several baseball fields at Plaza Northgate park are underused In this analysis, the northeastern field and the wooded portion of the park (located at the entrance off Northgate Drive were converted to a pond Between the pond at Plaza Northgate Park, and the Green Run ponds, a total footprint of 25 acres was added, for a volume of 204 acre-feet (0 feet NAVD to the top of bank CDM Smith also considered converting a 10-acre vacant parcel east of Windsor Oaks Boulevard to a pond, but the City indicated this parcel is under development and could not be used for storage As with Alternative 1, the initial depth in London Bridge Creek was lowered to 0 feet NAVD prior to the rain event However, Alternative 2 also includes 560 linear feet of bleeder pipe improvements in the Green Run system to allow lowering pre-storm levels to 0 feet NAVD Existing culverts in Green Run are generally at inverts between 05 feet to 15 feet NAVD, preventing the upstream system from being lowered without the addition of the lower bleeder pipes The results of this alternative resolved the remaining 10-year LOS issues at the Rosemont Road and Club House Road areas, but did not significantly address meeting the 100-year LOS The number of simulated flooded structures was reduced from 264 (under Alternative 1 to 200 under Alternative 2, providing a total reduction of 66 percent from existing conditions (600 flooded structures To mitigate potential offsite impacts, the northern and southern pump stations are operated in stages to match the existing conditions flow in London Bridge Creek To test the sensitivity in increased offsite water levels, CDM Smith performed an evaluation of Alternative 2 by allowing the northern pump station capacity to be increased (from 1,000 cfs to 1,400 cfs 5-3

58 Section 5 Alternative Solution Sets The results showed an increase in peak levels of less than 05 feet downstream of the northern pump station Though within the bank of the creek, tailwater conditions led to peak stage increases in locations adjacent to London Bridge Creek In most cases, the affected areas appeared to impact parking lots near London Bridge Creek, not structures A more detailed analysis would be required, however, to consider the impacts of offsite increases versus flood mitigation benefits of maximizing the pump station capacity Though Alternative 2 met the target 10-year LOS within the study area, this option failed to significantly reduce flooding for the 100-year design storm Based on the previous tests and alternative simulations, additional storage would be necessary to meet the target 100-year LOS As such, Alternative 2 is not recommended and was removed from further consideration 523 Alternative 3: Expanded Storage at Bow Creek Golf Course Alternative 3 continued to build upon the evaluations performed for Alternative 1 and Alternative 2, with the goal to mitigate the remaining flooded locations under the 100-year design storm, while maintaining the 10-year LOS performance The Alternative 3 configuration includes the following main elements: All elements of Alternative 2, except the canal widening and culvert replacement in BCGC Conversion of approximately half of the BCGC to a storage facility (60-acre pond footprint Removal of existing weir between The Lakes and Green Run Canal systems confluence on London Bridge Creek at Lynnhaven Parkway and the addition of 2 acres of storage at the southern pump station location behind the Title Max property Based on discussions with City staff, the conceptual lake at the golf course was envisioned to lie within the inner portion of the property, leaving in place the outer portions along structures and the driving range, as shown in Figure 5-1a A model schematic is provided in Figure 5-1b, where a single storage node represents the new lake, with the various Northgate ditches connecting to the new node The existing BCGC ponds and ditches have been removed and all pipes, culverts, and overflows are connected to the node representing the lake Figure 5-2 presents a map of the study area with all identified Alternative 3 improvements Recommended conveyance improvements identified in The Lakes neighborhood were found to be less extensive than improvements in the Princess Anne Plaza and Northgate neighborhoods, further to the north, and in the Windsor Woods neighborhood (per the December 2017 Windsor Woods Stormwater Management System Flood Mitigation Plan Report Differences in design and construction standards between older neighborhoods versus newer neighborhoods is the likely explanation in the lesser improvements identified in The Lakes portion of the SMS 5-4

59 Section 5 Alternative Solution Sets Figure 5-1a Conceptual Bow Creek Golf Course Storage Figure 5-1b Model Schematic of Expanded Bow Creek Golf Course Pond The existing weir between The Lakes and Green Run Canal is no longer needed because the new southern pump station maintains lake levels The three existing parallel box culverts under Lynnhaven Parkway (4 feet by 9 feet each have inverts at 03 feet NAVD, which is too high to drain The Lakes to 0 feet NAVD To lower The Lakes stages pre-storm, a deeper box configuration was modeled at this location (at -17 feet NAVD 5-5

60 Section 5 Alternative Solution Sets The additional storage at BCGC provided significant improvement over Alternative 2, reducing the simulated flooded structures from over 200 structures down to 58 structures for the 100-year storm A second iteration, which removed the weir at London Bridge Creek and added 2 acres of storage behind the Title Max property at 1092 Lynnhaven Parkway, provided the benefit of lower head loss from The Lakes These secondary improvements further reduced the simulated 100-year flooded structures from 58 structures to 11 structures (a 98 percent reduction from the 600 structures simulated as flooding under existing conditions 524 Alternative 4: Potters Road Pump Station Under Alternative 4, the proposed northern gate structure and pump station was moved further downstream in London Bridge Creek to the crossing of Potters Road, just south of I-264 Canal 2 is located behind the gate structure under this alternative, significantly expanding the service area located behind the gate structure Alternative 4 is shown in Figure 5-3 and includes the following main elements: 5,000 cfs pump station and gated structure at Potters Road, downstream of the crossing of London Bridge Creek and Canal 2 Dredge 1,760 linear feet of the center channel of London Bridge Creek from the confluence with Canal 2 to the new pump station Dredging to occur from elevation -1 feet to -10 feet NAVD (to avoid floodplain wetlands and to match the surveyed bottom Install a new gated structure in southern London Bridge Creek at the transition from WS 6 to WS 10, south of Canal 2 All elements of Alternative 3, except removing the southern pump station and gate structure at Lynnhaven Parkway and London Bridge Creek, since the southern portion of the study area is now protected via Canal 2 The benefit of moving the pump station includes providing more available storage volume prior to the storm and enabling a higher pump station discharge rate since London Bridge Creek has higher banks near Potters Road Additionally, Alternative 4 provides flood protection for a much larger service area The increase in total, summed pump station capacity from 1,600 cfs to 5,000 cfs (from Alternative 3 to Alternative 4 was required due to pumping flow from both the entire London Bridge Creek and Canal 2 drainage systems Under the 100-year design storm, Alternative 4 provided similar results to Alternative 3, with flood mitigation benefits provided to 97 percent of structures within the same study area extent (19 structures remained flooded While Alternative 4 provides flood protection to a larger service area than Alternative 3, the potential additional flood mitigation benefits were not quantified in this study The model utilizes a lower-level detail for portions outside the study area; as such, a model with a higher-level detail in Canal 2 and in London Bridge Creek near I-264 is required to evaluate additional flood mitigation benefits of Alternative 4 5-6

61 Legend 37 PS PumpStatio n C\ GatedCo ntro lstructure Sto rag eimpro vement Sto rmw aterpipeimpro vement DitchImpro vement Bo w Creek Go lfco urse 24'Φ 3 42'Φ 4 48'Φ 'Φ 'Φ 43'x68' 18'Φ 38'x60' 30'Φ 30'Φ 48'Φ 30'Φ 24'Φ 48'Φ 53'x83' 36'Φ 21'Φ 36'Φ C\ 37 PS London Bridge Creek Ü 8 PlannedDredg ing Bo xculvert PSTributary Area PlazaNo rthg ate Park 48'Φ 24'Φ 24'Φ 24'Φ 24'Φ 24'Φ 30'Φ 6 36'Φ 43'x68' 24'Φ 24'Φ 36'Φ 'Φ 24'Φ 'x68' 30'Φ 'Φ 24'Φ 30'Φ Canal 2 38'x60' 42'Φ 36'Φ 48'Φ 60'Φ 18'Φ 24'Φ 30'Φ 18'Φ 42'Φ 36'Φ 30'Φ 6'x9' 37 PS C\ GreenRun Canals West Neck Creek Miles Figure 5-2 Princess Anne Plaza and The Lakes Alternative 3: Proposed Stormwater Improvements

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63 Thalia Creek C\ 37 PS Ü Bo w Creek Go lfco urse 24'Φ 42'Φ 48'Φ 2 30'Φ 30'Φ 30'Φ 30'Φ 48'Φ 30'Φ 24'Φ 36'Φ 48'Φ 36'Φ London Bridge Creek 'x53' 18'Φ 18 21'Φ 8 10 PlazaNo rthg ate Park 48'Φ 24'Φ 24'Φ 24'Φ 24'Φ 30'Φ 6 36'Φ 43'x68' 24'Φ 24'Φ 36'Φ 'Φ 24'Φ 30'Φ 'x68' 'Φ 24'Φ 30'Φ Canal 2 38'x60' 42'Φ 18'Φ 36'Φ 48'Φ 36'Φ 60'Φ 30'Φ 18'Φ 24'Φ GreenRun Canals 6'x9' 30'Φ C\ Legend 37 PS PumpStatio n C\ GatedCo ntro lstructure Sto rag eimpro vement Sto rmw aterpipeimpro vement DitchImpro vement Canal 2 PlannedDredg ing Bo xculvert PSTributary Area West Neck Creek Miles Figure 5-3 Princess Anne Plaza and The Lakes Alternative 4: Proposed Stormwater Improvements

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65 Section 5 Alternative Solution Sets 53 Recommended Alternatives Based upon the findings presented in Section 52, Alternative 3 and Alternative 4 provide suitable options to be considered since both alternatives meet the target 10-year LOS and nearly resolve 100 percent of the simulated flooding locations for the 100-year LOS Conversely, Alternative 1 and Alternative 2 did not sufficiently meet the target 10-year and 100-year LOS requirements and therefore, are not presented in this section Appendix E-1 provides a comprehensive list of system improvements, which are also identified on Figure 5-2 and Figure 5-3 for Alternative 3 and Alternative 4, respectively Appendix E-2 summarizes the model results of each alternative year Storm Table 5-1 displays the 10-year storm results of Alternative 3 and Alternative 4 for the locations provided in Table 4-1 As shown, each of these locations meets the 10-year target LOS, which is 3 inches or less (<025 feet above the road crown The 10-year flood maps are shown in Figure 5-4 and Figure 5-5 for Alternative 3 and Alternative 4, respectively Appendix C contains results for all indicator elevation locations in the study area 5-11

66 Section 5 Alternative Solution Sets Table year Storm Results Alternative 3 and Alternative 4 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 3 (feet NAVD 10-year Alternative 4 (feet NAVD 06_PAP-81 Club House Road and Hannibal Street _PAP-78 Club House Road and Granite Trail _PAP-320 Valient Avenue and Garrison Place _PAP-87 Rosemont Road and Old Forge Road _PAP-163 Club House Road and Duplin Street _PAP-98 Hannibal Street and Country Club Circle _PAP-197 Skipper Drive and Barcelona Lane Windsor Oaks Boulevard and Cainhoy Lane _PAP-294 Lark Street and Dauphin Lane _PAP-154 Club House Road and Crosset Street _P-573 Fern Ridge Road and Lakecrest Road _PAP-137 Silina Drive and Lineberry Road _PAP-103 Skipper Drive and Bethune Drive McKinley Avenue and Faraday Lane _PAP-166 Duplin Street and Olympia Lane McKinley Avenue and Good Hope Road _PAP-211 Club House Road and Dauphin Lane Campion Avenue and Campion Court S Pine Grove Road and Pine Grove Lane McKinley Avenue and Dana Lane _PAP-314 Silina Drive and Barcelona Lane

67 Legend Boundary Hydrography Flood Depth ft ft ft 264 Rosemont Rd Silina Dr S Lynnhaven Rd Ü ft ft > 20 ft Bow Creek Blvd Windsor Oaks Blvd Holland Rd Bow Creek Blvd Lynnhaven Pkwy Rosemont Rd S Independence Blvd Miles Figure 5-4 Princess Anne Plaza and The Lakes Alternative 3: 10-year Storm Flooding

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69 Legend Boundary Hydrography Flood Depth ft ft ft 264 Rosemont Rd Silina Dr S Lynnhaven Rd Ü ft ft > 20 ft Bow Creek Blvd Windsor Oaks Blvd Holland Rd Bow Creek Blvd Lynnhaven Pkwy Rosemont Rd S Independence Blvd Miles Figure 5-5 Princess Anne Plaza and The Lakes Alternative 4: 10-year Storm Flooding

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71 Section 5 Alternative Solution Sets year Storm The simulated flood extents for Alternative 3 and Alternative 4 are presented in Figure 5-6 and Figure 5-7, respectively, for the 100-year design storm Both alternatives mitigate flooding for nearly all 600 structures where flooding was simulated under existing conditions As shown in Table 5-2, Alternative 3 shows 11 remaining structures with simulated flooding (98 percent mitigated, while Alternative 4 results in 19 remaining flooded structures (97 percent mitigated While Alternative 4 provides flood protection to a larger service area than Alternative 3, the potential additional flood mitigation benefits outside the study area were not quantified in this report Table year Storm Results Study Area Simulated Flooded Addresses Simulated Flooding, Alternative 4 Count Address Simulated Flooding, Alternative 3 Finished Garage Elevation (confirmed by survey feet NAVD Ashaway Road Ashaway Road Bethune Drive Coggeshall Court Holly Hedge Avenue Holly Hedge Avenue Lamplight Lane Lamplight Lane Lamplight Lane Lamplight Lane Lamplight Lane Northgate Drive Northgate Drive Northgate Drive Northgate Court Northgate Court Woodburne Drive Woodburne Drive Woodburne Drive Total = The simulated flooding locations are primarily dependent on peak stage in London Bridge Creek and therefore pump station capacity Increasing SMS conveyance in localized locations will not mitigate these flood elevations For Alternative 3, the northern pump capacity cannot be increased without causing adverse offsite impacts immediately downstream of the station Under Alternative 4, due to roughness in London Bridge Creek south of Potters Road, the maximum modeled flow conveyed to the station was 5,000 cfs Therefore, without additional improvements to London Bridge Creek, these addresses will not meet the target LOS for the 100-year storm 5-17

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73 Legend Boundary Hydrography Flood Extent 264 Rosemont Rd Silina Dr S Lynnhaven Rd Ü ( 297 BETHUNE DR Bow Creek Blvd ( 661 LAMPLIGHT LN Windsor Oaks Blvd Holland Rd ( ( 909 COGGESHALL CT 807 HOLLY HEDGE AVE 868 LAMPLIGHT LN ( ( 704 NORTHGATE CT 716 NORTHGATE CT 3344 ASHAWAY RD 872 LAMPLIGHT LN ( ( ( ( 876 LAMPLIGHT LN Bow Creek Blvd Lynnhaven Pkwy Rosemont Rd ( 900 LAMPLIGHT LN S Independence Blvd Miles Figure 5-6 Princess Anne Plaza and The Lakes Alternative 3: 100-year Storm Flooding

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75 Legend Boundary Hydrography Flood Extent 264 Rosemont Rd Silina Dr S Lynnhaven Rd Ü ( 297 BETHUNE DR Bow Creek Blvd ( 661 LAMPLIGHT LN Windsor Oaks Blvd Rosemont Rd Holland Rd 3216 NORTHGATE DR 704 NORTHGATE CT ( ( ( ( ( ( 3381 WOODBURNE DR ( ( ( ( ( 3365 ASHAWAY RD 909 COGGESHALL CT 3344 ASHAWAY RD 716 NORTHGATE CT 3357 WOODBURNE DR 3333 WOODBURNE DR 3212 NORTHGATE DR 3217 NORTHGATE DR 868 LAMPLIGHT LN 872 LAMPLIGHT LN ( ( ( ( ( ( 807 HOLLY HEDGE AVE 876 LAMPLIGHT LN 835 HOLLY HEDGE AVE 900 LAMPLIGHT LN Bow Creek Blvd Lynnhaven Pkwy S Independence Blvd Miles Figure 5-7 Princess Anne Plaza and The Lakes Alternative 4: 100-year Storm Flooding

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77 Section 5 Alternative Solution Sets 533 Extreme Rainfall Event Simulation While the goal of this report was to identify system improvements to mitigate flooding for the 10-year and 100-year design storm, this section discusses the performance of Alternative 3 and Alternative 4 for an extreme rainfall event where the design frequency is exceeded To evaluate the improved stormwater system performance under these conditions, the rainfall and boundary conditions were obtained from those observed during the October 2016 Hurricane Matthew storm Hurricane Matthew led to significant flooding in the study area and other areas within Virginia Beach Based on a 24-hour duration intensity, Hurricane Matthew was between a 500-year to 1000-year recurrence interval (Analysis of Historical and Future Heavy Precipitation, Dewberry 2017 Model assumptions for this analysis include: Utilized the same model inputs to match the analysis for all other alternatives Utilized observed October 9, 2016 rainfall from various gauges; utilized tidal conditions based on USGS gauges (as discussed in Section 3, in particular, for the gauge located at Dam Neck Road in West Neck Creek, at the boundary between WS 6 and WS 10 Applied a starting water surface elevation of 0 feet NAVD for portions of the study area located upstream of the recommended pump station(s and gate structures The Alternative 3 and Alternative 4 configurations were not resized or modified for the extreme event analysis The simulated extreme event flood maps are shown in Figure 5-8 (for Alternative 3 and Figure 5-9 (for Alternative 4 The number of simulated flooded structures are reduced from over 950 structures under existing model conditions (with the extreme event down to 117 structures under the Alternative 3 model and 95 structures under the Alternative 4 model, meaning close to 90 percent of structures are protected under either improvement scenario Compared to the 100-year design event results presented in Section 532, the extreme event simulation led to an additional 2,000 acre-feet of runoff volume, corresponding to peak stage increases in London Bridge Creek, immediately south of Bow Creek Boulevard, of 09 feet higher (Alternative 3 and 05 feet higher (Alternative 4 To further mitigate flooding under the simulated extreme conditions, the northern and southern pump station capacities would need to be increased beyond those specified in Section 52 to lower stages in London Bridge Creek However, this change would potentially cause offsite adverse impacts downstream of the pump station(s, which were not evaluated in this analysis The results for the extreme storm event are provided in Appendix C 534 Other Storms The results for the remaining design storm simulations (2-year, 25-year and 50-year are provided in Appendix C Included are 15 feet and 30 feet sea level rise scenario results 5-23

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79 Legend ( Flooded Structure Study Area Boundary Hydrography Flood Extent 264 Rosemont Rd Silina Dr S Lynnhaven Rd Ü ( ((( Bow Creek Blvd Windsor Oaks Blvd Rosemont Rd Holland Rd ( ( ( ( ( ( ( ( (( ( ( ((( ( ( ( ( ( ( ( ( ( ( ( ( ((((((( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( (( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( (( ( (( ( ( ( ( ( ( ( Bow Creek Blvd Lynnhaven Pkwy S Independence Blvd Miles Figure 5-8 Princess Anne Plaza and The Lakes Alternative 3: Extreme Rainfall Event Simulated Flooding

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81 Legend ( Flooded Structure Study Area Boundary Hydrography Flood Extent 264 Rosemont Rd Silina Dr S Lynnhaven Rd Ü ( ((( Bow Creek Blvd Windsor Oaks Blvd Rosemont Rd Holland Rd ( (( ( ( ( ( ( (( ( ( ( ( ( ( ( (( ((( ( ( ( ( ( (( (((( (( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( (( ( ( ( ( ( ( ( ( ( ( ( ( ( ( Bow Creek Blvd Lynnhaven Pkwy S Independence Blvd Miles Figure 5-9 Princess Anne Plaza and The Lakes Alternative 4: Extreme Rainfall Event Simulated Flooding

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83 Section 5 Alternative Solution Sets 54 Summary 541 Alternatives Improvements The Alternative 3 and Alternative 4 system improvements provide results that meet the target 10-year LOS and nearly resolve 100 percent of the simulated flooding locations under the 100-year design event Conversely, Alternative 1 and Alternative 2 did not sufficiently meet the target 10-year and 100-year LOS requirements Two critical aspects are required to be resolved within the study area SMS to meet the target LOS for both the 10-year and 100-year design storms: Increasing Conveyance Capacity: As determined in initial tests, improving the canals/ditches alone provides lower peak stages, but the existing SMS does not provide adequate capacity to meet the volume and intensity of the SCS locally normalized Type-C 10-year design storm As such, larger capacity pipes and structures are needed to improve conveyance from neighborhoods to the receiving ditches and canals to meet the 10-year LOS Expanding Storage Capacity and New Pump Station(s: Increasing SMS conveyance capacity does not, by itself, provide LOS improvement for the 100-year storm The simulated tributary volume to London Bridge Creek contributes to peak flood elevation, which occurs well after the peak local runoff Therefore, to meet the 100-year LOS, a significant amount of storage volume needs to be created This is achieved by a two-part approach: 1 adding new detention storage, mainly at BCGC, and, 2 lowering peak stages prior to the storm by constructing a gated structure and pump station(s Appendix E contains a comprehensive list of system improvements and summarizes results of each alternative The Alternative 3 and Alternative 4 flood mitigation improvements are nearly identical, as both alternatives were designed to first mitigate the head losses in the SMS With the resultant flattening of the system HGL, peak stages in the upstream portions of the study area were lowered, thereby utilizing the maximum storage volume Table 5-3 provides a comparison of the improvement quantities identified for Alternative 3 and Alternative 4 Table 5-3 Summary of Improvements for Alternative 3 and Alternative 4 Improvement Category Quantity Alternative 3 Pipe and culvert improvements (linear feet 40,600 40,600 Ditch/canal dredging (linear feet 7,150 8,900 Ditch/canal dredging (cubic yard 89, ,000 New pond footprint (acre New pond volume 1 (acre-feet Pump station and gate structure 2 locations: 1,000 cfs north 600 cfs south Note: 1 pond storage volumes are calculated from 0 feet NAVD to the top of bank Quantity Alternative 4 1 location: 5,000 cfs north Gate only south 5-29

84 Section 5 Alternative Solution Sets 542 Mitigating Adverse Offsite Impacts A comparison of peak stages within the entire WS 6 model was conducted to analyze potential offsite impacts at various locations upstream and downstream of the proposed alternatives To meet the target LOS, pump station(s are necessary to move water out of the upstream neighborhoods and into London Bridge Creek; however, pumping water from one neighborhood to another can cause adverse offsite impacts This is partially mitigated by lowering the stage in London Bridge Creek prior to the storm and creating a storage volume to be filled by the storm However, to avoid adverse offsite impacts, the pumped flows cannot be greater than the existing flows in London Bridge Creek The pump capacity necessary to meet the target LOS under Alternative 1 and Alternative 2 was much higher than the existing peak flows to London Bridge Creek Only after expanding the storage volume under Alternative 3 and Alternative 4, was there enough storage to meet LOS with a pump capacity that matched the existing storm flows Peak stages would increase for all storms if the full capacity of the northern and southern pump stations were implemented at the peak of the storm As such, the elevation triggers for the pump stations were set to match the existing condition flows through London Bridge Creek 543 Phasing of System Improvements The identified system improvements can be categorized as short-term and long-term solutions for Capital Improvement Program planning The short-term solutions will address immediate, readily correctible issues; the more comprehensive, longer-term solutions will enhance the overall LOS provided by stormwater infrastructure in the area The following near-term improvements would restore the system closer to its original capacity: Canal dredging of Northgate Ditch, scheduled to commence in late 2017, to reduce head loss along the canal and to repair the canal to a conveyance capacity close to the original design Canal dredging of the Green Run canals, also planned for late 2017, for similar reasons Longer-term solutions are required to fully meet the target LOS, and should be designed and implemented in a sequence such that an option that benefits one area does not adversely impact an adjacent area (ie, do not move the problem to another area As such, the following are listed in order of preferred implementation: Construct London Bridge Creek pump station(s and gated control structures Construct approximately 85 acres of new detention storage, the majority achieved by converting half of the Bow Creek Golf Course to a lake (60 acres; the remaining storage is included at Plaza Northgate Park and two locations in the Green Run canals system Lower stages behind the gated structures to 0 feet NAVD prior to an impending storm to maximize storage Increase the size of the SMS significantly within the study area neighborhoods to improve conveyance toward London Bridge Creek 5-30

85 Section 5 Alternative Solution Sets The proposed gate structures are modeled as an operable gate, that open and close based on upstream and downstream stages and only remain open when the upstream stage is greater than the downstream stage 544 Recommended Further Evaluations While Alternative 4 provides flood protection to a larger service area than Alternative 3, the potential additional flood mitigation benefits were not quantified in this study The model utilizes a lower-level detail for portions outside the study area; as such, a model with a higher-level detail in Canal 2 and downstream in London Bridge Creek is required to evaluate any additional flood mitigation benefits of Alternative 4 Furthermore, discussions with the City revealed that there does not appear to be significant residential areas outside the study area with known, reported flooding issues CDM Smith also analyzed a scenario in which the Potters Road Pump Station capacity (under Alternative 4 was increased from 5,000 cfs to 6,000 cfs and then to 7,000 cfs The purpose of this evaluation was to identify whether a larger-sized station would lower peak stages sufficiently to further reduce the remaining 19 structures simulated as flooded However, the model showed that London Bridge Creek does not provide capacity to route more than 5,000 cfs to the suction-side of the pump station, without increasing the conveyance capacity of the creek While not considered in this study, modifying London Bridge Creek would be required to evaluate a pump station capacity beyond 5,000 cfs Several additional analyses are recommended for further evaluation during preliminary engineering and engineering design, to further build upon the findings of Alternative 3 and Alternative 4, including: Evaluate the feasibility and cost-effectiveness of constructing improvements in the London Bridge Creek floodplain downstream of South Lynnhaven Road towards Potters Road or acquiring property and/or easements to allow increased discharges Evaluate environmental permitting requirements, land acquisition, liability and legal exposure for modifying London Bridge Creek, including a benefit-impact analysis for increased pumping capacity (ie, lower peak water levels versus environmental impacts 5-31

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87 Section 6 Regulatory Feasibility and Probable Cost 61 Feasibility The Princess Anne Plaza and The Lakes SMS improvements identified in Section 5 would impact wetlands and require wetland permitting and consultation with governing regulatory agencies During the permitting process, an early pre-application meeting should be scheduled with the United States Army Corps of Engineers (USACE, the Virginia Department of Environmental Quality (DEQ, and the Virginia Marine Resources Commission (VMRC to introduce the project and its objectives and identify agency concerns A wetland delineation (preliminary jurisdictional determination and wetland confirmation from USACE should be performed prior to this meeting to identify jurisdictional areas For this assessment, the National Wetlands Inventory (NWI maintained by the US Fish and Wildlife Service was consulted However, while NWI maps can be used as a planning tool, they are not sufficient for permitting Wetland boundaries likely vary from those shown on NWI mapping, and considerable wetland areas can be found outside those identified on the NWI maps Permits from the USACE, DEQ, and the VMRC would be applied for through a single Joint Permit Application, available at the USACE Norfolk District Regulatory Branch website ( For USACE and DEQ permitting, the following permits apply: Nationwide Permit (NWP 43 impacts result in the loss of ½ acre or less of wetlands, stream impacts are 300 linear feet or less, and all impacts are non-tidal USACE State Programmatic General Permit (12-SPGP-01 wetland losses equal one acre or less and stream impacts 2,000 linear feet or less, and all impacts are non-tidal This permit can also be used for the DEQ Virginia Water Protection Compliance Program USACE and DEQ individual permits impacts exceed 1 acre, exceed 2,000 linear feet of stream, or are tidal General conditions placed on NWP 43 and 12-SPGP-01 regarding federal lands, protected species, or historic resources may preclude their use, even if wetland impacts are below the maximum amounts allowed In all cases, an individual VMRC permit would be required if applicable The permitting agencies (USACE, DEQ, and VMRC will require the submitted application be for a single and complete project with independent utility This means the project for which the application is submitted cannot be part of a larger project that has been split into smaller projects to lessen the apparent impacts During the pre-application process, the City should communicate the extent of project s size and scope to the agencies 6-1

88 Section 6 Regulatory Feasibility and Probable Cost The permitting agencies will also require that the impacts proposed represent those of the least environmentally damaging practicable alternative As a result, changes in the exact location of the drainage ditches, pump stations, and gated control structures may be required during design to minimize wetland impacts Permanent wetland impacts will require compensatory mitigation by regulatory agencies Compensatory mitigation, through the purchase of credits at a mitigation bank, is typically the most feasible option and preferred by the regulatory agencies The ratio of compensatory mitigation is by habitat types due to the difficulty in re-creating some habitats and the time required for them to reach maturity Although project-specific ratios can be considered, typical mitigation ratios are as follows: 2:1 for forested wetland impacts (2 acres compensation per 1 acre of impact 15:1 for scrub-shrub wetland impacts 1:1 for emergent wetland impacts The permitting process will further assess impacts to protected species through consultation with the Virginia Department of Conservation and Recreation and the Virginia Natural Heritage Program Impacts to historic and cultural resources would also need to be determined by consulting with the Virginia Department of Historic Resources (VDHR A preliminary examination of the National Register of Historic Places spatial data (last updated by the National Parks Service in 2014 does not indicate any historic properties within the area However, not all properties are shown on this database, nor are any archaeological resources Consultation with VDHR will be required to avoid impacts to regulated resources The City has applied for NWP 3, NWP 18, NWP 19, and NWP 31 for the ongoing dredging operations along the Windsor Woods Canal (also known as Presidential Canal The NWP 3 is applicable for outlet protection against future scour and erosion at each of the existing stormwater outfalls NWP 18 and NWP 19 are applicable for new forebay construction, and NWP 31 covers maintenance of existing flood control activities to a baseline approved by USACE Since this permit does not have a limit on the maximum area, volume, or length of channel that can be authorized under it, the City should coordinate with USACE to establish the baseline condition for canals proposed to be dredged and the applicability of the existing permit for proposed dredging operations during the design stage Proposed dredging of Northgate ditch for both Alternative 3 and Alternative 4 would potentially impact riverine wetlands along 4,400 linear feet of the ditch Impacts to tidal wetlands are possible at the outfall structures along London Bridge Creek The proposed new ponds in the golf course, Plaza Northgate park, next to Green Run, and adjacent to the Title Max property, are adjacent to locations identified by NWI mapping as wetlands; this must be confirmed in the field 6-2

89 Section 6 Regulatory Feasibility and Probable Cost For both Alternative 3 and Alternative 4, since pump station(s are proposed adjacent to London Bridge Creek, an evaluation to estimate the effect of maximum flow during low tide on the salinity in the creek should be performed at the preliminary design stage In addition, implications of locating the proposed gated control structures along tidally-influenced London Bridge Creek will need to be evaluated in coordination with USACE and DEQ For Alternative 4, proposed dredging of 1760 linear feet of London Bridge Creek would potentially impact Estuarine and Marine Deepwater wetlands Compensatory mitigation may be required due to the loss of vegetated wetlands from dredging of London Bridge Creek, as well as conversion of vegetated wetlands to open water, and other areas if jurisdictional wetlands are present The amount of compensatory mitigation would depend on the actual impacts in jurisdictional areas, as well as mitigation ratios required by the regulatory agencies Agency coordination regarding protected species and historical resources will be required as described above As shown in Figure 6-1 and Figure 6-2, study area SMS improvements for Alternative 3 and Alternative 4 are in the effective Federal Emergency Management Agency (FEMA floodway Construction of improvements in the floodway require approval from FEMA Construction activities in the floodway and modification of hydraulic conditions will require analyses to demonstrate that the project will not increase flood elevations and documents revisions to the regulatory floodway A Conditional Letter of Map Revision will be required to obtain FEMA s concurrence that the proposed project will, upon construction, result in acceptable revisions to the floodway After construction of the project, a Letter of Map Revision will be required to officially modify the effective Flood Insurance Rate Map, Base Flood Elevations, and the Special Flood Hazard Area 6-3

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91 Legend 37 PS Pump Station C\ GatedControlStructure Storag eimp rovement StormwaterPip eimp rovement DitchImp rovement 24'Φ 42'Φ 48'Φ 'Φ 'Φ 43'x 68' 18'Φ 38'x 60' 30'Φ 30'Φ 48'Φ 30'Φ 24'Φ 48'Φ 53'x 83' 36'Φ 21'Φ 36'Φ C\ London Bridge Creek 37 PS 24'Φ 'Φ Canal 2 Ü PlannedDredg ing Box Culvert PSTributaryArea Bow Creek Golf Course GolfCourseProp erty 48'Φ 24'Φ 24'Φ 24'Φ 24'Φ 30'Φ 36'Φ 43'x 68' 24'Φ 24'Φ 36'Φ 11 24'Φ 24'Φ 'x 68' 30'Φ 14 24'Φ 30'Φ EfectiveFEMAFloodway 38'x 60' 42'Φ 36'Φ 48'Φ 60'Φ 18'Φ 24'Φ 30'Φ 18'Φ 42'Φ 36'Φ 30'Φ 6'x 9' 37 PS C\ West Neck Creek Miles Figure 6-1 Princess Anne Plaza and The Lakes Alternative 3: Effective FEMA Floodway

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93 Thalia Creek C\ 37 PS Ü 30'Φ 24'Φ 42'Φ 48'Φ 2 30'Φ 30'Φ 30'Φ 30'Φ 48'Φ 24'Φ 36'Φ 48'Φ 36'Φ London Bridge Creek 'x 53' 18'Φ 18 21'Φ 'Φ 'Φ 42'Φ Canal 2 38'x 60' 48'Φ 36'Φ 24'Φ 24'Φ 48'Φ 60'Φ 24'Φ 30'Φ 36'Φ 43'x 68' 24'Φ 24'Φ 36'Φ 18'Φ 24'Φ 11 24'Φ 24'Φ 'x 68' 'Φ 14 24'Φ 30'Φ 30'Φ Legend 37 PS Pump Station C\ GatedControlStructure Storag eimp rovement 18'Φ 36'Φ 30'Φ 6'x 9' StormwaterPip eimp rovement DitchImp rovement C\ PlannedDredg ing Box Culvert PSTributaryArea Bow Creek Golf Course GolfCourseProp erty EfectiveFEMAFloodway West Neck Creek Miles Figure 6-2 Princess Anne Plaza and The Lakes Alternative 4: Effective FEMA Floodway

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95 Section 6 Regulatory Feasibility and Probable Cost 62 Opinion of Probable Construction Cost The planning-level opinion of probable construction cost (OPCC developed for this study is at an Estimate Class 4 level as defined by AACE International (formerly the Association for the Advancement of Cost Engineering This estimate class is typical for projects at the concept or feasibility phase of project definition Quantities and unit costs to develop the OPPC are presented in Appendix D Unit costs were developed from major stormwater pumping station projects in the southeast and local stormwater and roadway construction bid tabulations Where available, costs were compared with industry standard unit cost references and adjusted using engineering judgement The expected accuracy of Class 4 estimates can range from -30% to +50% Actual construction costs are largely dictated by market conditions at the time of bidding CDM Smith cannot guarantee that bids and actual costs will not vary from the OPCC presented in this section In addition, the OPCC does not include costs for change orders, the City s administrative costs, land acquisition, easement acquisition, financing or funding costs, legal fees, or any other costs that would not be specifically part of a contractor s scope of work Table 6-1 summarizes the Alternative 3 and Alternative 4 estimated cost estimates, while additional details and quantities are provided in Appendix D Table 6-1 Planning-level Opinion of Probable Construction Cost Category Cost Alternative 3 Alternative 4 Culverts $ 934,561 $ 934,561 Ditch $ 6,397,200 $ 8,063,649 Erosion and Sediment Control $ 5,720,000 $ 5,720,000 Miscellaneous $ 4,525,000 $ 4,525,000 Pavement $ 4,940,535 $ 4,940,535 Stormwater Piping $ 10,607,124 $ 10,607,124 Pond $ 21,913,026 $ 21,913,026 Pond and Ditch $ 1,370,225 $ 1,444,485 Pump Station(s $ 40,000,000 $ 125,000,000 Tide Gate Control Structures $ 6,000,000 $ 8,000,000 Subtotal $ 102,407,671 $ 191,148,380 Mobilization (10% $ 10,240,767 $ 19,114,838 Contingency (40% $ 45,059,375 $ 84,105,287 Engineering, Survey & Permitting (15% $ 23,656,172 $ 44,155,276 Subtotal Cost, Today's dollars $ 181,363,985 $ 338,523,781 Escalation to midpoint of construction (FY 26 based on inflation rate of 4% per year $ 66,845,152 $ 124,769,389 Total Construction Cost $ 248,209,137 $ 463,293,170 USE $ 248,000,000 $ 463,000,

96 Section 6 Regulatory Feasibility and Probable Cost 63 Sea Level Rise Virginia Beach and the Hampton Roads area have been experiencing sea level rise at a slow, gradual pace since tide levels have been recorded Future increases in sea level will increase water levels in the Lynnhaven River and West Neck Creek, which will impact the performance of the existing WS 6 SMS For planning purposes, two sea level rise conditions were evaluated: 15 feet and 30 feet of increase in water levels above the existing tailwater condition for each storm Boundary conditions were included in the H/H model to represent tailwater conditions in London Bridge Creek The existing conditions water elevation in London Bridge Creek at Virginia Beach Boulevard was defined at 362 feet NAVD for the 100-year rainfall event The two sea level rise scenarios included downstream water elevations of 512 feet NAVD and 662 feet NAVD, respectively Model results for the two sea level rise scenarios are presented in Appendix C The recommended alternatives were specifically formulated to provide sea level rise resiliency All proposed elements of the recommended alternatives lie upstream of gated structures, and are protected from downstream tidal conditions and sea level rise Future performance with sea level rise will require appropriate design of the gated structures on London Bridge Creek to protect against increased water elevations Similarly, the operation of the pump station in combination with proposed improvements for Alternative 3 and Alternative 4 in the study area prevent downstream impacts for both existing sea level conditions and with sea level rise 6-10

97 Section 7 Summary and Conclusions 71 Recommended Solutions The Princess Anne Plaza, The Lakes and Northgate neighborhoods have experienced repeated, significant flooding during recent extreme rainfall events The City of Virginia Beach retained CDM Smith to develop a H/H model calibrated to multiple storm events and analyze flood risk to structures in the study area The H/H model provided the foundation for exploring improvement alternatives to alleviate flooding in the study area The model identified several key requirements for SMS to meet the LOS, which were categorized as near-term and long-term flood mitigation options Alternative 3 and Alternative 4 are being recommended to advance to preliminary engineering design Each alternative meets the desired LOS in the study area, with noted exceptions, while preventing adverse impacts in the downstream system in London Bridge Creek or elsewhere in the watershed The planning level opinion of probable construction cost for Alternative 3 is $248M and $463M for Alternative 4 Analysis of the near-term mitigation improvements alone, without the long-term solutions, showed limited benefit in meeting the LOS However, these near-term improvements can be performed immediately: Canal dredging of Northgate Ditch, scheduled to commence in late 2017, to reduce head loss along the canal and to repair the canal to a conveyance capacity close to the original design Canal dredging of the Green Run canals, also planned for late 2017 and 2018, for similar reasons Longer-term solutions should be designed and implemented in a sequence such that an option that benefits one area does not adversely impact an adjacent area (ie, do not move the problem to another area As such, the following are listed in order of preferred implementation: Construct London Bridge Creek pump station(s and gated control structures Construct approximately 85 acres of new detention storage, the majority achieved by converting half of the Bow Creek Golf Course to a lake (60 acres; the remaining storage is included at Plaza Northgate Park and two locations in the Green Run canals system Lower stages behind the gated structures to 0 feet NAVD prior to an impending storm to maximize storage Increase the size of the SMS significantly within the study area to improve conveyance toward London Bridge Creek 7-1

98 Section 7 Summary and Conclusions While both alternatives meet the LOS, the probable construction cost of Alternative 4 is higher than Alternative 3 Alternative 4 is recommended for further consideration because it provides protection to a significantly larger tributary area in WS 6 and additional evaluation is needed to quantify flood mitigation benefits With additional information for the cost, feasibility and benefits of both alternatives, the appropriate solution can be selected to advance to final engineering design and construction Several additional analyses are recommended for further evaluation during preliminary engineering and engineering design: Evaluate the feasibility and cost-effectiveness of constructing improvements in the London Bridge Creek floodplain, downstream (north of South Lynnhaven Road to Lynnhaven Parkway, or acquiring property and/or easements to allow increased discharges Evaluate alternative scenarios for operating the gate and pump station The objective of additional evaluations is intended to clarify operating conditions, reduce operational complexity, and confirm environmental permit requirements For Alternative 4, evaluate flood mitigation benefits to residential areas outside the study by preparing appropriately detailed H/H modeling for Canal 2 and downstream portions in London Bridge Creek towards I-264 Evaluate additional improvement options in the study area to reduce project cost and community impacts during construction It should be noted that reliance on the mechanical devices such as the recommended pump station and gated control structure carries residual risk from larger events and potential mechanical failure Backup systems, backup power, and manual operable capability are methods to mitigate for some of the residual risk Without the recommended improvements the study area is highly vulnerable to flooding from tidal flooding and flooding from rainfall runoff With the recommended improvements the study area will continue to be vulnerable to flooding from rainfall runoff for storms that exceed the 100-year design storm and/or from larger joint rainfall-tidal events and surges The analyses performed as part of this study are conceptual, and the completed H/H models are intended to serve as planning-level tools The conceptual improvements identified in this study should be advanced through engineering design The H/H models should also be refined during engineering design to evaluate additional details or modifications that are revealed during the design process 7-2

99 Appendix A Field Investigation Documentation A-1

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101 Memorandum To: File From: Tom Nye/Jay Gopal Date: August 9, 2017 Subject: Pictures from Hurricane Matthew The City of Virginia Beach provided pictures showing flooding at locations shown in Figure A1 in the Princess Anne Plaza and the Lakes area during Hurricane Matthew Figure A 1 Map showing calibration points for Hurricane Matthew Appendix Adocx

102 Memorandum August 9, 2017 Page 2 The City also got field survey to estimate flood depths corresponding to each of these pictures The table below summarizes surveyed flood depths at each location: Item Address Date/Time Flood Depth (feet NAVD Lakecrest Rd 10:00 AM October 9, Bow Creek Blvd 07:45 AM October 9, Forrest Trail 08:10 AM October 9, Deer Park Drive 08:30 AM October 9, Dixie Drive 07:30 AM October 9, Location 1 Address: 3209 Lakecrest Rd Date/Time: ~10:00 AM October 9, 2016 Flood Depth: 724 ft NAVD

103 Memorandum August 9, 2017 Page 3 Location 2 Address: 3308 Bow Creek Blvd Date/Time: ~07:45 AM October 9, 2016 Flood Depth: 709 ft NAVD

104 Memorandum August 9, 2017 Page 4

105 Memorandum August 9, 2017 Page 5

106 Memorandum August 9, 2017 Page 6 Location 3 Address: 701 Forrest Trail Date/Time: ~08:10 AM October 9, 2016 Flood Depth: ~ 73 ft NAVD 782 ft NAVD, but likely off by half a foot

107 Memorandum August 9, 2017 Page 7

108 Memorandum August 9, 2017 Page 8 Location 4 Address: 3329 Deer Park Drive Date/Time: ~08:30 AM October 9, 2016 Flood Depth: 715 ft NAVD

109 Memorandum August 9, 2017 Page 9

110 Memorandum August 9, 2017 Page 10 Location 5 Address: 905 Dixie Drive Date/Time: ~07:30 AM October 9, 2016 Flood Depth: 715 ft NAVD

111 Memorandum August 9, 2017 Page 11

112 Memorandum To: File From: Jay Gopal Date: August 9, 2017 Subject: Field Visit on July 26, 2017 On July 26, 2017, Tom Nye, Jay Gopal, and CJ Bodnar (City of Virginia Beach inspected the Princess Anne Plaza study area between 9:00 AM and 11:30 AM This visit revealed that the floodplains have thick, tall brush This finding was used to estimate a higher roughness in the model See photos below: Figure A 2 Map showing site visit locations Appendix Adocx

113 Memorandum August 9, 2017 Page 2 Location 1

114 Memorandum August 9, 2017 Page 3 Location 2

115 Memorandum August 9, 2017 Page 4 Location 3

116

117 Appendix B Stormwater Model Development Methodology B-1

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119 Appendix B Stormwater Model Development Methodology B1 Introduction The model used to investigate the flooding in the Princess Anne Plaza and the Lakes neighborhoods was derived from the Watershed 6 model, originally developed for the City of Virginia Beach as one of the City s stormwater management planning tools The methodology described below generically covers all the City s planning models Parameters and methodologies specific to the Princess Anne Plaza Project are noted in the main body of this report The primary objective for developing hydrologic and hydraulic (H/H models of the City of Virginia Beach watersheds was to provide a tool suitable for evaluating the performance of the City s stormwater management system as well as alternative improvements to meet the desired level of service for flood control This appendix describes the approach used to develop and apply the H/H model for this purpose, including data collection and evaluation, general model development considerations, summary of the modeling process, and development of H/H model parameters To support the planning level analysis described herein, the models developed have focused on the primary stormwater management system (PSMS for multiple size rainfall events and downstream tidal boundary conditions The PSMS includes constructed stormwater facilities and overland flow paths that drain to the downstream waterbody (ie, boundary condition In general, the PSMS includes open channels and pipes of 24-inch diameter and larger Furthermore, in some areas two dimensional (2D overland flow modeling was considered necessary to achieve modeling objectives In general, 2D overland flow modeling is preferable in flat areas where overland flows paths become complicated and variable over the course of a period of simulation However, 2D modeling comes at a cost in terms of data storage and computational time Therefore, the areas of 2D modeling were limited to those areas considered critical based on knowledge of the system performance B2 Model Development and Application B21 Stormwater Model Software Stormwater models were developed using PCSWMM Version 70 by CHI (Computational Hydraulics International PCSWMM uses the US Environmental Protection Agency (EPA Stormwater Management Model (SWMM Version computational engine, and includes a custom graphical user interface (GUI, which offers GIS functionality, model building, calibration, and post processing tools PCSWMM also includes a pseudo-2d development tool that allows gridded overland flow modeling using fully compliant EPA SWMM input B-1

120 Appendix B Stormwater Model Development Methodology B22 Levels of Detail, Temporal Scales, and Numerical Time Steps The levels of detail must be adequate to accurately define and characterize flooding problems and represent the local and sub watershed-wide effects of each alternative and/or series of alternatives to cost-effectively size projects to solve existing problems Typically, an adequate level of detail was considered to include the PSMS as defined by 24-inch diameter pipes and larger In general, this means that pipes smaller than this size are considered secondary and were not explicitly modeled However, where areas of reasonably large size are served by pipes smaller than 24-inches in diameter, these pipes were included in the model In the 1D portion of the model, all pipes in the primary system have runoff loaded to the upstream terminus and thus help define subbasin delineation It should be noted that if pipes as small as 15 inches in diameter (or smaller were included in the model, the inlets providing runoff to these pipes would require loading from the hydrologic analysis and, therefore, further subbasin refinement This would require a level of delineation that could be as much as an order of magnitude more detailed, and well beyond the scope of the SMP Typically, the larger pipes will accurately represent the limitations and the level of service (LOS for a given intersection s stormwater system However, there may be some instances where the smaller pipes and/or the inlet capacities are the constraints on LOS In these cases, the model may underestimate flooding To accurately represent watershed hydrology, the rainfall interval should be less than the travel times within the smallest subbasin For this project, 5-minute and 6-minute rainfall intervals were used With respect to hydraulics, it is also important that time steps be considered that provide appropriate computational iterations within the shortest travel time associated with system hydraulic conveyances, thereby maintaining continuity (shortest travel times are typically associated with relatively short sections of large-diameter pipes For 2D modeling, relatively small time steps have been utilized to most directly simulate stormwater features and maintain continuity (05 seconds B3 Model Development Process This section presents an overview of the modeling development process that was applied for the watersheds included in this study B31 Collect Data and Characterize Watershed Table B-1 provides a list of data sources that were used for model development Table B-1 Virginia Beach Stormwater Master Plan Available GIS Data Data Type Name Description Topographic Data LiDAR_DSM_ feet DEM This digital surface model (DSM has been prepared from LiDAR data acquired in the spring of 2013 The fundamental vertical accuracy for bare earth elevations is 042 feet, with a consolidated vertical accuracy of 064 feet The bare earth DSM excludes buildings, trees, etc Soil Data Soils (2010 Soil information provided by City of Virginia Beach Department of Communications and Information Technology Center for Geospatial Information Services (ComIT/CGIS and generated by the USDA (United States Department of Agriculture B-2

121 Appendix B Stormwater Model Development Methodology Data Type Name Description Impervious Area Land Use Stormwater Infrastructure Structures and Physical Features (2017 LandTablemdb (2015 Stormwater (2017 Planimetric data including building footprints, road edges, hydrography edges, transportation surfaces such as driveways, parking lots, sidewalks, trails, poles, walls Land Use database provided based on City of Virginia Beach tax assessor information Public Works Stormwater Infrastructure data including drop inlets, manholes, outfalls, dam and spillways, pipes, pump stations, BMPs, ditches provided by City of Virginia Beach ComIT/CGIS Following review of the data, data gaps were identified and strategies were formulated to obtain the missing data City staff gathered and provided to CDM Smith the data necessary for model development The Public Works Stormwater infrastructure data included pipe invert elevations based on the date of implementation The elevation datum has changed from NGVD 1929, to NGVD 1929 datum with 1972 adjustment, to NAVD 88 during the lifetime of the current stormwater system The date of implementation attribute was used to adjust the invert elevation based on the following: pipes built before January 1, 1978, were considered to be on the NGVD 1929 datum, those built between January 1, 1978, and Dec 1, 1997, were considered to be on the NGVD 1929 datum with 1972 adjustment, and those built after January 1, 1998, were considered to be on the NAVD 88 datum Elevation adjustments were applied to the GIS pipe inverts so all expressed in the NAVD 88 datum The adjustment values vary by watershed and were provided by the City for each; however, the adjustment is generally NAVD = NGVD -11 ft for the earlier dates and NAVD = NGVD 08 ft for the latter The GIS data was converted to model input parameters using GIS and the methodologies below Critical attributes in the stormwater layers, aside from coordinates, include Unit Identification (UNITID in the junction shapefiles (Storm_Inletshp, Storm_Manholeshp, Storm_Nodeshp, and Storm_Miscshp For the conduit shapefile (Storm_Mainshp, critical attributes include UNITID, pipe shape (PIPESHP, pipe type (PIPETYPE, pipe height (PIPEHT, pipe diameter for circular pipes or pipe width for elliptical (PIPEDIAM, and upstream (US_INV and downstream (DS_INV inverts For model naming, the UNITID is used for all junctions and outfalls, and most storages (the exception being named lakes For pipes, the naming convention is USN:DSN, where USN is the upstream node and DSN is the downstream node B32 Model Preparation Model preparation involved the following steps: The first step was to prepare a model schematic based on the defined levels of detail The figures depict the subbasins, nodes (junctions, links (conduits, pipes and channels, and identification codes (alphanumeric on an aerial photogrammetric base map In the preparation of the model schematic, model node placement helps define the level of detail for the overall stormwater model Model node placement was primarily based on the locations of inlets, manholes, and other miscellaneous nodes in the GIS layers Nodes have B-3

122 Appendix B Stormwater Model Development Methodology been added to the model, which were not in the GIS, at topographic low points and locations of hydraulic elements along the stormwater system (eg, storage, confluence of ditches, locations of changes in stream cross-section Note that since the level of detail chosen for this project is a 24-inch diameter pipe, feeder (collector pipes from inlets to the primary system are often not modeled Therefore, each modeled inlet may represent multiple real inlets For example, an intersection may include eight curb inlets that all feed to a 24-inch diameter pipe At a stormwater management plan level of detail, it is expected that the critical element (the control point in the system is the 24-inch diameter pipe and not the curb/gutter inlets or the feeder pipes It is possible to redefine the level of refinement of any watershed model for future work Smaller pipes can be added to the SMP models and the subbasins redelineated at finer levels In models which include 2D modeling, the modeled 2D area typically has a higher level of refinement, allowing more access points between the 1D PSMS and the 2D grid Inlet control may also be applied to the orifice connection between the 1D PSMS and the 2D grid After the model network was defined, subbasins were delineated based on available topographic data and local stormwater system maps, and the hydrograph load points were assigned to each In general, subbasins were delineated for the area tributary to each node Occasionally, nodes were adjusted and/or added to define subbasins with relatively uniform hydrologic properties and/or to properly distribute the runoff from the subbasin to the modeled stormwater systems Each subbasin defines a model node as load point to route the corresponding hydrograph along the modeled network The load point generally corresponds to a node nearest to the lowest elevations in the subbasins Flat topography generally determined the areas where 2D overland flow routing was used During high-intensity storms, including the 100-year storm, it is expected that many roads and low-lying areas will be the first locations to flood If the topography shows a general downhill direction for this flooding to flow, the area may be represented with 1D modeling There are still above-ground model elements, including stage-storage area nodes and hydraulic overland flow links; but a full 2D model is not necessary In areas that are generally flat and the ponded areas may flow in more than one direction, 2D modeling is used to more accurately identify the flood elevations Boundary Conditions and Sea Level Rise (SLR for the stormwater system evaluation include design-frequency tailwater elevation in combination with coincident rainfall applicable to tidal flood conditions Section B59 below provides a more detailed discussion of the development of model boundary conditions B33 Model Validation Following initial model development, the simulation results were compared against known flooding conditions within the watershed Adjustments were made to model parameters to obtain a reasonable fit with available data Refer to the body of the report for the individual model validations storms, historical flood data, and results B-4

123 Appendix B Stormwater Model Development Methodology B4 Hydrologic Data and Parameters B41 Rainfall Data Rainfall data was used to generate stormwater runoff hydrographs for each subbasin represented in the design storm event hydrologic model Design storm rainfall data are generally characterized by a depth (measured in inches, intensity (inches per hour, return period (years, event duration (hours, spatial distribution (locational variance, and temporal distribution (time variance Design storm events are usually designated to reflect the return period of the rainfall depth and the event duration For example, a 25 year, 24 hour design event describes a rainfall depth over a 24-hour period that has a 4 percent (1/25 chance of occurring at a particular location in any given year Figure B-1 below presents rainfall data utilized for this study A normalized (unit rainfall hyetograph was adopted from the localized Type C distribution for the 25-year storm obtained from the NRCS The 25-year normalized distribution was used for storms of all recurrence periods by multiplying the unit hydrograph with the given return period volume Total storm volumes for the 2-, 10-, 25-, 50-, and 100-year storms are shown in Table B-2 These total storm volumes were also obtained from the NRCS based on the location of the centroid of the City 12 Normalized Rainfall Intensity (hr Time (min Figure B-1 Design Storm Rainfall Data B-5

124 Appendix B Stormwater Model Development Methodology Table B-2 Design Storm Volumes Return Period/ Duration Rainfall Depth (inches Peak Intensity (inches/hour 2 Year Year Year Year Year Year B42 Topography and Vertical Datum Topographic data are used to define hydrologic boundaries, runoff flow paths and slopes, out ofbank channel cross sections, overland hydraulic links, stage area storage relationships, and critical flood elevations For this study, the principle source of topographic data was the Digital Elevation Model (DEM provided by the City A DEM is a 2D surface with elevation values at discrete points on the surface These discrete points are tiles each having a specific elevation value and a resolution of 25 feet in length and width The fundamental vertical accuracy for bare earth elevations is 042 feet, with a consolidated vertical accuracy of 064 feet This DEM was used to determine hydrologic and hydraulic overland flow paths, stage areastorage relationships, and subbasin boundaries The DEM was also used to develop the 2D model The elevation data used in the computer models and provided in this report are referenced to the North American Vertical Datum of 1988 (NAVD88 B43 Subbasin Delineation Subbasins are defined by natural physical features, and by constructed stormwater conveyance systems that control and direct stormwater runoff to a common outfall Delineation of the study area subbasins was based primarily on the DEM, land use, and stormwater collection system data Once established, each subbasin is given specific hydrologic values that describe the area in terms of key hydrologic characteristics These values are among the critical inputs to the model The hydrologic parameters assigned to each subbasin included area, flow width, slope, impervious area, roughness, initial abstraction, and Green-Ampt soil parameters of saturated hydraulic conductivity, capillary suction, and initial moisture deficit Additionally, not all of the impervious surface is directly connected to the hydraulic system The percent of the impervious surface routed to pervious surfaces is an additional input parameter (see Section B441 and was estimated by land use Subbasin roughness and initial abstraction were also assigned according to the land use within the subbasin The total impervious area was directly estimated from the impervious coverage in GIS, while the soils parameters are estimated from the soils coverage Section B47 below describes how these data were reduced for use in the model B-6

125 Appendix B Stormwater Model Development Methodology B44 Land Use Parameters and Impervious Areas The City provided planimetric impervious coverage layers (see Table B-1 in Section B3 that were used to directly calculate the percentage of impervious area within each subbasin using GIS tools The land use coverage also provided by the City was used to estimate secondary hydrologic parameters, such as impervious to pervious routing, surface friction factors, and initial abstractions for each subbasin Aside from pervious area roughness, the models are not as sensitive to these secondary land use derived parameters as to impervious coverage and soil infiltration capacities Therefore, typical values derived from engineering experience and/or SWMM defaults were used for these parameters Local analysis of the secondary runoff parameters was not performed for the SMP as per scope Pervious area roughness was derived from land use type, as discussed below, and was often adjusted during calibration For this project, the land uses were grouped into 10 categories of relatively homogeneous geophysical parameters Present land uses within the study area include: Forests, Open Land, and Parks (Open Pasture (Past Golf Courses and Agriculture (Ag/GC Low Density Residential (LDR Medium Density Residential (MDR High Density Residential and Mixed Use (HDR Commercial, Light Industrial, and Institutional (Comm Heavy Industrial and Transportation (HInd Wetlands; and (Wetlnd Waterbodies (Water B441 Land Use Dependent Parameters Land cover was used to characterize the percent of the impervious area routed to pervious areas (the Routed parameter The infiltration and runoff routing parameters for the directly connected impervious area (DCIA differs from the non-dcia areas Non-DCIA areas may include roof surfaces that are routed to pervious yards as opposed to directly to the stormwater system, for example Some roads and minor parking lots all may runoff to grassy swales prior to loading to the PSMS Typically, about one-third of medium density residential impervious surfaces are routed to pervious; while only 10 percent of commercial surfaces are routed to pervious Land cover was also used to characterize the surface roughness (Manning n of the overland flow path and the depression storage within the subbasin Each modeled subbasin requires values defined for the following land cover model parameters: B-7

126 Appendix B Stormwater Model Development Methodology Surface Roughness (Pervious n and Impervious n The Manning n Roughness Coefficient along the representative overland flow Depression Storage (Pervious Ia and Impervious Ia Depression storage is the amount of rainfall at the beginning of a precipitation event that is trapped within areas (usually small and does not become surface runoff (in SWMM, generally a portion of the impervious area is given no (zero depression storage The impervious surface roughness represents the composite roughness of rooftops, sidewalks, streets, gutters, inlets, and collector pipes, if these are not modeled explicitly in the hydraulic model The pervious roughness is the composite roughness of sheet flow over pervious surfaces such as lawns and open areas Table B-3 lists ranges by land cover type Note the values are higher than Manning s n values for channel flow (for example, because the depth of flow is much less Table B-3 Published Values of Manning n Roughness Coefficients for Overland Flow Source Ground Cover Manning n Range Crawford and Linsley Smooth asphalt 0012 ( Asphalt of concrete paving Packed clay 0014 Packed clay 003 Light turf 02 Dense turf 035 Dense shrubbery and forest litter 04 Engman ( Concrete or asphalt Bare sand Graveled Surface Bare clay-loam (eroded Range (natural Bluegrass sod Short grass prairie Bermuda grass Depression storage characterizes the interception of runoff before it reaches the inlets of the collection system In SWMM, depression storage is treated as an initial abstraction, such that the depression storage volume must be filled prior to surface runoff Depression storage is expressed as a depth (in inches over the entire subbasin and values are required for both impervious and pervious areas The volume of depression storage within a subbasin represents the sum of depression areas including small cracks and voids in paved surfaces, puddles, sags in street profiles, rooftops, and interception due to vegetation In SWMM, water that ponds in these depression areas either evaporates from the impervious surface area or infiltrates into the soil from pervious surface areas The portion of the impervious area given zero depression storage is 1 Crawford, NH and Linsley, RK, Digital Simulation in Hydrology: Stanford Watershed Model IV, Tech Report No 39, Civil Engineering Department, Stanford University, Palo Alto, CA, July Engman, ET, Roughness Coefficients for Routing Surface Runoff, Journal of Irrigation and Drainage Engineering, ASCE, Vol 112, No 1, February 1986, pp B-8

127 Appendix B Stormwater Model Development Methodology set to 25 percent, unless adjusted during model calibration This SWMM default value was used to simulate impervious areas that are sloped and/or smooth enough to not allow ponding Typical depression storage values range from 005 to 05 inches and vary by subbasin and land cover Global values of land use dependent variables are compiled in Table B-4 Table B-4 Global Land Use Dependent Parameters Parameter Open Past Ag/GC LDR MDR HDR Comm HInd WetLnd Water Impervious n Pervious n Impervious Ia Pervious Ia Routed 80% 80% 80% 50% 34% 21% 10% 10% 0% 0% Note: Refer to Section B44 above for heading land use definitions The parameters in Table B-4 were incorporated into the model by intersecting the land use coverage with the subbasin polygons in GIS, and the resulting values were area weighted by subbasin to develop parameter values for inclusion in the model B442 Impervious Area Any rainfall that occurs on impervious area becomes surface runoff once its depression storage is filled The City provided multiple surface coverages in GIS: polygon features in plan view of buildings, streets, parking lots, sidewalks, waterbodies, and miscellaneous features such as pools and sheds These layers of impervious surfaces were combined in GIS to form one impervious surface layer of the City The combined impervious coverage was verified using recent aerial photography of the City and corrected, where necessary The impervious layer was then intersected with the subbasin polygons to provide the impervious area per subbasin B45 Runoff Parameters For this project, kinematic wave flow routing techniques (EPA SWMM RUNOFF methodology were used as opposed to more traditional unit hydrograph techniques for the following reasons (note: kinematic wave routing applies only to the hydrologic or runoff layer, hydraulic routing is performed with dynamic wave methodologies: Unit hydrograph techniques have primary applicability on mid-size subbasins, on the order of 1 to 400 square miles, whereas kinematic wave techniques become more accurate with decreasing subbasin size One of the model selection criteria was the ability to run continuous simulations The Green-Ampt infiltration methodology is more applicable to continuous simulations than curve number methodologies B-9

128 Appendix B Stormwater Model Development Methodology SWMM runoff is a rigorous, parameter-based methodology that more readily lends itself to local, physical parameter changes (through calibration and/or detailed modeling of a watershed subset The time of concentration calculation in SCS methodology does not vary by storm depth; however, real travel times are shorter in larger storms due to increasing depth of flow, which is estimated in SWMM With the SWMM methodology, runoff parameters that affect the timing and shape of the stormwater runoff hydrograph are defined as opposed to a unit hydrograph Each model subbasin requires the following runoff parameters: B-10

129 Appendix B Stormwater Model Development Methodology Subbasin Area The total subbasin area calculated in GIS Representative runoff flow paths, which are developed within each subbasin that characterize the route runoff takes to the modeled stormwater network (to estimate the subbasin width and slope parameters below Subbasin widths The subbasin area divided by the average length of the runoff flow paths within the subbasin Average surface slopes The average slope of the subbasin along representative runoff flow paths The timing of the runoff is dependent on the subbasin geometry (average slope and average length, roughness of both the impervious and pervious surfaces, and total flow (developed from rainfall minus infiltration and initial abstraction Therefore, times of concentration are not calculated or input directly in SWMM To develop representative parameters for modeling, up to three flow paths were developed for each subbasin Each flow path was used to characterize routing of flow through an associated percentage of the subbasin Each of the portions of the subbasins are idealized as a rectangular runoff area of length equal to the flow path and width equal to the area divided by the flow path length Area weighted average of the flow path parameters is then used in the model These parameters, together with surface roughness and rainfall are used to calculate runoff hydrographs for each subbasin The formulation of each model parameter is further discussed in the paragraphs below B451 Length and Slope The length (L parameter is the average area-weighted travel length to the hydraulic model load point The load point is typically located at the lowest inlet within the subbasin For ponded or detention storage areas, the hydraulic model load point is typically represented by the edge of the pond or basin For areas where ponding does not occur, the hydraulic model load point is typically the downstream extent of the subbasin area The slope parameter is the average slope over the flowpath length and is calculated by dividing the difference in elevation by the length Length and slope information was obtained using the LiDAR topographic data (DEM Up to three flow paths were evaluated within the irregular shaped subbasins to develop the average area-weighted path, slope, width, and Manning n roughness Each flow path was defined in GIS and assigned an associated length, slope, and weight (ie, the percentage of the subbasin area that the individual flow path represents Flow paths begin at a high elevation located along the subbasin boundary and end at the hydraulic load point discussed above The average surface slope of each subbasin was determined as the area-weighted average of the representative flow paths Area-weighted flow path lengths and slopes were initially determined utilizing available topographic data B-11

130 Appendix B Stormwater Model Development Methodology B46 Soils and Geotechnical Data Soils in the pervious part of the subbasin affect the rate and volume of water infiltration The hydrologic model uses the Green-Ampt equations to determine infiltration and soil moisture accounting In PCSWMM, the Modified Green-Ampt option was chosen, because the original EPA SWMM Green-Ampt option may cause an inadvertent loss of infiltration capacity under certain hydrologic conditions and therefore may cause erroneous results The Green-Ampt equation was chosen because it may be adopted for continuous simulation of weeks, months, and years since it provides a more accurate recovery of soil storage for multiple events over a long time period This method for modeling infiltration assumes that a sharp wetting front exists in the soil column, separating soil with some initial moisture content below from saturated soil above The input parameters required are the initial moisture deficit of the soil, the soil's hydraulic conductivity, and the suction head at the wetting front The recovery rate of moisture deficit during dry periods is empirically related to the hydraulic conductivity The initial deficit for a completely drained soil is the difference between the soil's porosity and its field capacity Estimated values for all of these parameters can be found in Table B-6 Characteristics of various soils for the Green-Ampt Method were applied from EPA SWMM 5 Help, Green-Ampt Infiltration Parameters, Soil Characteristics Table; which in turn was developed from Rawls, Brakensiek, and Miller, Green Ampt Infiltration Parameters from Soils Data, Journal of Hydraulic Engineering, 109:1316 (1983 Table B-6 Soil Parameter Estimates Soil Texture Hydraulic Conductivity (inches/hour Initial Moisture Deficit (fraction Suction Head (inches Sand Loamy Sand Sandy Loam Loam Silt Loam Sandy Clay Loam Clay Loam Silty Clay Loam Sandy Clay Silty Clay Clay B-12

131 Appendix B Stormwater Model Development Methodology B47 Hydrologic Model Input The hydrologic parameters described above were either developed within a subbasin (area, width, slope, or developed from city-wide coverages of impervious surface, land-use, and soils (impervious percent, percent routed to pervious, impervious and pervious depression storage, impervious and pervious roughness values, saturated hydraulic conductivity, initial moisture deficit, and suction head These latter parameters may vary over a given subbasin; however, a single value must be entered in the model Therefore, the coverages and the subbasin polygons were intersected and the various parameters area weighted to find the representative value over each subbasin area For saturated hydraulic conductivity, because the values may vary by nearly two orders of magnitude, logarithmic area-weighting is performed The values within intersected sub-areas were converted to a log scale, area-weighted, then inverse logs were calculated Once the area-weighted parameters were set for each subbasins, the values were moved to PCSWMM using the PCSWMM import tools B5 Hydraulic Data and Parameters The H/H model uses a node/link (junction/conduit representation of the PSMS For this study, the PSMS links were primarily circular pipes ranging in size from 24 to 72 inches in diameter In some cases, smaller pipes down to 15 inches were included Links also may represent culverts, pumps, weirs, orifices, ditches, streams, canals, and overland flows (dual links, which are described below Nodes are located at: The ends of pipes or culverts Locations of inlets where the subbasin runoff is loaded Manholes Locations where the stormwater pipes change diameter Points representing the subbasin low surface elevations (storage units For nodes in the 2D modeling area (see Section B57, node rim elevations were set 30 ft above ground elevation by default because the 2D model builds overland conduits 30 ft deep To not have water flood out of the model, the node rim elevation needs to be as high as the highest linked conduit Similar methodology was used in the 1D modeling areas, where above ground features, such as stage-storage junctions and overland links, were used to not only keep flooding within model elements, but also to provide a relatively accurate estimate of flood depth Again, the node rim elevations need to be as high as the highest connecting link, and in the case of storage junctions, as high as the stage-storage curve All nodes in the 1D modeling area were set at least 10 ft above ground elevation, to provide a consistent offset In the case of manholes, adding 10 ft to the rim effectively seals the manhole, by not allowing water to flood out of the model at that location In the case of nodes representing points on ditches, stream, and canals, the rim may be set more than 10 ft above ground elevation because the connecting links may be more than 10 ft deep (and the node invert in this case is generally at ground elevation A column has been added to the model files to include the actual ground elevation at each node, labeled as Ground B-13

132 Appendix B Stormwater Model Development Methodology B51 Stage Area Relationships In the 1D portion of models, storage is accounted for explicitly in nodes as stage area storage relationships and in open (irregular conduits Actual starting water levels are also considered for dead storage accounting Stage storage area relationships are necessary for lakes and low lying areas that are not part of overland and street flow conduits An accurate accounting of the storage and conduit volumes is needed for accurate peak flood stage, flow, and velocity estimates Stage storage area relationships were computed for each storage node using the topography from LiDAR and GIS The plan areas for stages of depth above node invert were calculated from the surface as appropriate In general, the area attributed to each storage node is limited by the subbasin boundary around that node, though in practice, the maximum stage in the curve is not always deep enough to extend to the subbasin boundary Storage volume is calculated internally in the model Not all subbasins have related storage junctions, as some subbasins have no storage beyond that which is represented in the model links It is also important to avoid double counting surface area and conveyance system storage in the hydraulic models The project team verified that each conduit s storage capacity was not included in the stage storage relationships by removing an equivalent amount of storage from the LiDARbased stage storage relationships of the adjacent junctions Where appropriate, this was done by clipping the area within the storage node s subbasin, to remove the areal footprint of the overland conduits (ditches, streams, street flow conduits, see Section B56 Each subbasin was reviewed and areas of surface conduits were identified For each surface conduit, profiles at key locations were inspected to determine the top of bank locations These locations were then used as the basis of the area clipped from the area of storage analysis B52 Culverts and Pipes Survey data obtained for the project provided the necessary data for the stormwater pipes and structures that defined the PSMS The City provided GIS data files which contained culvert and pipe location, shape, size, material, upstream and downstream invert elevations, and date of installation The date of installation was used to determine the likely datum of the invert elevations as described in Section B31 Where size or invert elevations were missing from the City s GIS inventory, additional survey was performed For the purposes of initial model development, pipe roughness values in the model assume a clean, maintained system Therefore, reinforced concrete pipes (RCP were assigned a Manning s roughness value of 0013 and corrugated metal pipe (CMP roughness values were set to 0024 For HDPE and PVC pipes, a value of 0011 was used These values were used as a calibration parameter as the model was further refined with available calibration data Pipe lengths were determined using the survey and the GIS database Local losses were developed from the Virginia Department of Transportation (VDOT Drainage Manual, Section 9493 (Conservation of Energy and Energy Losses Head losses at each model junction are calculated internally in SWMM, using the user supplied loss coefficients Entrance loss coefficienst (Ke were set to 035 Exit loss coefficients (Ko were set to 025 for manholes, 05 for pipes and culverts discharging into moving water, and 10 for pipes and culverts discharging into still water Under the additional minor loss in SWMM, bend losses were added B-14

133 Appendix B Stormwater Model Development Methodology based on VDOT Table 9-9, Losses in Junction Due to Change in Direction of Flow Lateral For example, a K value 07 is used for 90-degree bends and T s, a value of approximately 05 is used for 45-degree bends and a value of approximately 025 is used for 20-degree bends The VDOT drainage manual includes adjustments to the total local losses (entrance plus exit plus bend losses based on plunging losses at upstream inlets and inlet shaping in manholes Due to the scope of the masterplan models, these loss corrections are ignored Plunging losses occur at the upstream elements in a drainage system, where the inflows are larger than 20 percent of the pipe flow For the masterplan models, these upstream elements are considered part of the secondary system, which are combined with the runoff elements and not explicitly modeled In general, the primary (modeled system contains 24-inch pipes and larger For City-wide master planning purposes, the larger system is tested for potential deficiencies and local inlets and feeder pipes are expected to convey the subbasin runoff to the PSMS Since these secondary systems are not included in the model, plunging losses are not included Inlet shaping refers to manhole design and the ability of flow to smoothly move through the structure VDOT allows for a 50 percent reduction in local losses for smooth flow conditions The City-wide masterplan models do not include this adjustment due to the tens of thousands of junctions in the model, the unknown condition of most of these, and to be conservative B53 Open Streams and Open Channels Open channels and streams typically consist of an incised or main channel surrounding the stream centerline that is capable of passing flows with return periods ranging from a few months to a few years, and a floodplain that stores and/or conveys flows that are greater than what the main channel can carry In SWMM, open channels are represented as prismatic channels, meaning that the hydraulic properties defined for the transect provided to each link are applied consistently throughout the length of the modeled link Open channel ditch, stream, and canal segments were modeled as irregular cross-sections with a center channel representing the ditch or canal, and left and right overbank areas representing the floodplain Roughness values for center channels ranged from 003 to 010 based on vegetation and engineering judgment Roughness in the overbanks ranged from 004 to 02 based on vegetation and engineering judgment Additional guidance on channel roughness is provided in Table B-7 (from Chow, VT, Open Channel Hydraulics, McGraw-Hill, 1959 B54 Stormwater Control Structures Review CDM Smith reviewed storm water control structure related information provided by the City for inclusion within the models, primarily in CAD drawings These represent weir or drop structures that regulate lake levels within the watersheds and allow for overflow B55 Outfalls Based on project specific survey and the GIS coverage of stormwater pipes provided by the City, stormwater points of discharge were identified and simulated as outfalls that discharge to water bodies These points of discharge or outfalls are modeled in SWMM as streams and pipes discharging into receiving waters with fixed boundary conditions For calibration and validation, time series boundary conditions were used where data was available For subbasins discharging B-15

134 Appendix B Stormwater Model Development Methodology to a water body without pipes, these may be simulated as sheet flow to the waterbodies from the subbasins along the shore Table B-7 Open Channel Manning s Roughness Values Channel Type Minimum Normal Maximum Main Channels a Clean, straight, full stage, no rifts or deep pools b Same as above, but more stones and weeds c Clean, winding, some pools and shoals d Same as above, but some weeds and stones e Same as above, lower stages, more ineffective slopes and sections f Same as d with more stones g Sluggish reaches, weedy, deep pools h Very weedy reaches, deep pools, or floodways with heavy stand of timber and underbrush Mountain streams, no vegetation in channel, banks usually steep, trees and brush along banks submerged at high stages a Bottom: gravels, cobbles, and few boulders b Bottom: cobbles with large boulders a Pasture, no brush Floodplains 1 Short grass High grass b Cultivated areas 1 No crop Mature row crops Mature field crops c Brush 1 Scattered brush, heavy weeds Light brush and trees, in winter Light brush and trees, in summer Medium to dense brush, in winter Medium to dense brush, in summer d Trees 1 Dense willows, summer, straight Cleared land with tree stumps, no sprouts Same as above, but with heavy growth of sprouts Heavy stand of timber, a few down trees, little undergrowth, flood stage below branches Same as 4 with flood stage reaching branches B-16

135 Appendix B Stormwater Model Development Methodology Channel Type Minimum Normal Maximum Excavated or Dredged a Earth, straight, and uniform 1 Clean, recently completed Clean, after weathering Gravel, uniform section, clean With short grass, few weeds b Earth winding and sluggish 1 No vegetation Grass, some weeds Dense weeds or aquatic plants in deep channels Earth bottom and rubble sides Stony bottom and weedy banks Cobble bottom and clean sides c Dragline-excavated or dredged 1 No vegetation Light brush on banks d Rock cuts 1 Smooth and uniform Jagged and irregular e Channels not maintained, weeds and brush uncut 1 Dense weeds, high as flow depth Clean bottom, brush on sides Same as above, highest stage of flow Dense brush, high stage Lined or Constructed a Cement 1 Neat surface Mortar b Wood 1 Planed, untreated Planed, creosoted Unplanned Plank with battens Lined with roofing paper c Concrete 1 Trowel finish Float finish Finished, with gravel on bottom B-17

136 Appendix B Stormwater Model Development Methodology Channel Type Minimum Normal Maximum Lined or Constructed 4 Unfinished Gunite, good section Gunite, wavy section On good excavated rock On irregular excavated rock d Concrete bottom float finish with sides of: 1 Dressed stone in mortar Random stone in mortar Cement rubble masonry, plastered Cement rubble masonry Dry rubble or riprap e Gravel bottom with sides of: 1 Formed concrete Random stone mortar Dry rubble or riprap f Brick 1 Glazed In cement mortar g Masonry 1 Cemented rubble Dry rubble h Dressed ashlar/stone paving i Asphalt 1 Smooth Rough j Vegetal lining B56 Overland Flow Conduits In the 1D portion of the model, depth of flooding, and movement of flood waters is controlled by two methods: stage storage curves in the storage nodes (see Section B51, and overland flow conduits (also known as multi-links, dual links, or major/minor systems Overland flow conduits serve two purposes, storage of floodwaters and conveyance of flood waters; both generally occurring when the subsurface stormwater system is overwhelmed by a large volume and/or a high intensity storm In some municipalities, the storage and conveyance in streets is a feature of the stormwater system and therefore needs to be included in the model There are two basic types of overland flow conduits: a weir type and a channel type B-18

137 Appendix B Stormwater Model Development Methodology B561 Weir-type Overland Flow The weir-type overland flow link represents an equalizer between two low areas For this overland flow, there is generally a hydraulic boundary between two subbasins, such as a (relative high point in a road, a major road crossing, or a berm between two ponds A weir may be used to represent the boundary between the subbasins (and hence the storage node areas, but typically the boundary is irregular and therefore, the overland flow link is a cross section representative of the street or other defined boundary between the two subbasins The length of these channels is typically short (20 to 50 feet to minimize additional storage while maintaining computational stability The cross-section widths are on the order of 50 to 300 feet (though some may be much wider The Manning s roughness values range from 0015 to 015 and there may be different roughness values in the center channel (representing the street and overbank, although many have constant roughness values These values were selected based on published values included in Table B-7 and engineering judgement considering other obstructions within the street would result in a slightly high averaged roughness value Flow occurs in these links when ponding on either side of the link reaches the height of the topographic boundary (eg, road crown, curb, and landscape berm During high-intensity storm events, surface ponding is prevalent and flow transfer can occur from one subbasin to another B562 Channel-type Overland Flow (Street Flow The channel-type overland flow link represents flow in a street, typically parallel to the subsurface pipe Street flows are similar to the weir-type overflows in that they represent overland flow conduits linking two low-lying areas However, in a street flow link, flow is parallel to the direction of the street and the linked subbasins do not have a hydraulic boundary (high point separating them Street flows are implemented with the full length between nodes and account for both storage and conveyance It is critical to not double count this storage in the stage storage curves of the connecting nodes; therefore, the footprint of the conduit is removed from the subbasin area prior to the calculation of the storage curve (see above Flow occurs in these links as a very shallow channel connecting street inlets If the next inlet down the street has sufficient capacity, the flow may re-enter the subsurface system at that point The Manning s roughness values range from 0015 to 015 and there usually are different roughness values in the center channel (representing the street and overbank, representing yards, etc It is a judgment call by the modeler which of these types of overflows should be used in a given situation It is important to note that the stage-storage curves are a more accurate representation of storage than the conduit storage This is because the conduit has an accurate cross-section in only one location (where it is extracted from the DEM, and this cross-section is extrapolated along the length of the conduit, with the only other control being upstream and downstream inverts Though in general, this extrapolation should be close to the correct storage, it is not as accurate as the stage-area calculation performed in GIS Therefore, most of the overland links are weir-types, with channel types used, as necessary, where there is no high point in the roads between subbasins and/or where there is significant slope in the road B-19

138 Appendix B Stormwater Model Development Methodology B57 2D Model Area The two dimensional model areas were initially selected as those areas with low lying, flat topography as well as known areas with local flooding problems provide by the City Trial simulations were then performed to determine where significant overland flow was occurring that would benefit from the use of 2D modeling In PCSWMM, 2D flow is performed using similar methods as the overland flows in Section B56, though performed at a very fine resolution No storage areas are required, because the overland links cover all the area within the 2D model boundary PCSWMM internally calculates the length and widths of the overland conduits to maintain accurate calculations of storage while adding connectivity in up to 6 directions (for the hexagonal grid The grid spacing may be variable, depending on the resolution needed to meet the level of service for a given area For this project, a 10-foot or 15-ft spacing is used in the streets, while a 50-foot spacing is used in open fields and higher areas, where flooding is less likely Where flow is channelized, a directional mesh may be used so that the overland links are directed parallel to flow The steps to develop the 2D model were: 1 Set up the 1D model in all areas where 2D is unnecessary using the overland components described above in Section B51 and Section B56 The 2D model cannot be used everywhere in the watershed at the resolution necessary to see flooding in roads (for example, so a determination of where to use 1D with overflows and stage-storage areas versus where to use 2D must be made 2 In the 2D model area, 1D nodes and conduits (subsurface pipes, pumps, and force mains are built as for a 1D model The maximum depth in each node is set 30 feet higher than ground elevation (in SWMM, the maximum depth needs to be higher than the crown of the highest connecting link; in the 2D model, the highest links are the 2D conduits representing overflows 3 A 2D boundary polygon(s is built, outlining the 2D area, but also separating areas of differing resolution and/ or differing mesh shape (hexagonal, direction, etc 4 A grid of 2D nodes is automatically built in PCSWMM based on the resolution and mesh design within each boundary polygon At this step in the process, the 2D grid may be transferred from PCSWMM to GIS to edit the mesh along the roads GIS tools may be used to build the mesh so that the flowpaths would follow the road flowlines on each side, while also including the road crown as a potential barrier Once the 2D node grid was edited, the revised file may be ported back to PCSWMM and the normal process was continued 5 Once, the 2D nodes are added, 2D conduits are added between these nodes based on the elevation in the DEM PCSWMM internally calculates the geometry of the conduits to maintain total storage 6 Once the 2D conduits are built, the 1D nodes representing inlets are connected to the nearest 2D node on the 2D grid with orifices, such that once the subsurface system can no longer handle the flow, the 1D node floods onto the 2D grid The default orifices are large enough to not obstruct flow; however, they may be reduced in size to model inlet control B-20

139 Appendix B Stormwater Model Development Methodology 7 Finally, the 2D mesh is connected to the 1D model in locations other than inlets, as appropriate For example, both ends of a culvert would be directly connected to the 2D grid, at the elevation of the grid Also, a 1D overflow that connects to a storage node outside of the mesh may be connected to a 2D node along a road flowline B58 Boundary Data and Conditions Boundary conditions for the model are necessary to represent the influence from downstream water levels in the downstream receiving water body When the receiving water body is low, the existing stormwater drainage system will be able to provide maximum conveyance; however, when the receiving water body is high, there will be portions of the existing drainage system that have reduced conveyance capacity The modeling software (both 1D and 2D provides flexibility for defining boundary conditions, and can adopt fixed stage boundaries or time series boundaries as necessary The City supplied raster datasets of rainfall-correlated tidal boundary conditions in the Lynnhaven River for the 2-year, 10-year, 25-year, 50-year, and 100-year storm events These 2-D surfaces represent the expected peak tidal surge that corresponds to the given storm s recurrence interval However, the 100-year rainfall event is not paired with a 100-year tidal surge, because the chance of the 100-year rainfall occurring at the same time as the 100-year tide is much less than 1 percent There is some correlation between the rainfall and tidal events and therefore the peak stages in the 100-year boundary condition take this into account See the main report for information on the boundary condition used for each model For each design storm, the boundary condition data was spatially joined to the model outfall locations in GIS to estimate the fixed stage at each model outfall Where the PSMS is below the fixed stage, initial depths were set in the model to match the boundary condition for each storm A spreadsheet was developed that linked each model node to a given outfall location For lakes and ponds with outfall structures, and all model junctions upstream of these elements, the initial depths were set using the maximum of the connecting outfall boundary condition and the structure s bleeder elevation (ie, normal pool elevation B-21

140

141 Appendix C Model Results for Recommended Alternative C-1

142 Appendix C Model Results for Recommended Alternative This page intentionally left blank C-2

143 Table C-1: 10-Year Existing Conditions Model Node Location Estimated Crown (feet NAVD 10-yr Base (feet NAVD Delta 10-year (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Table C-1 10 Year Existing C-1 1

144 Model Node Location Estimated Crown (feet NAVD 10-yr Base (feet NAVD Delta 10-year (feet Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Table C-1 10 Year Existing C-1 2

145 Model Node Location Estimated Crown (feet NAVD 10-yr Base (feet NAVD Delta 10-year (feet 4247 Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Table C-1 10 Year Existing C-1 3

146 Model Node Location Estimated Crown (feet NAVD 10-yr Base (feet NAVD Delta 10-year (feet Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-1 10 Year Existing C-1 4

147 Table C-2: 10-Year Existing Conditions with 15 Feet Sea Level Rise Model Node Location Estimated Crown (feet NAVD 10-yr Base +15 feet SLR (feet NAVD Delta 10-year (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Table C-2 10 Year Existing with 15 ft SLR C-2 1

148 Model Node Location Estimated Crown (feet NAVD 10-yr Base +15 feet SLR (feet NAVD Delta 10-year (feet Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Table C-2 10 Year Existing with 15 ft SLR C-2 2

149 Model Node Location Estimated Crown (feet NAVD 10-yr Base +15 feet SLR (feet NAVD Delta 10-year (feet 4247 Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Table C-2 10 Year Existing with 15 ft SLR C-2 3

150 Model Node Location Estimated Crown (feet NAVD 10-yr Base +15 feet SLR (feet NAVD Delta 10-year (feet Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-2 10 Year Existing with 15 ft SLR C-2 4

151 Table C-3: 10-Year Alternative 3 Model Node Location Estimated Crown (feet NAVD 10-yr Alternative 3 (feet NAVD Delta 10-year (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Table C-3 10 Year Alternative 3 C-3 1

152 Model Node Location Estimated Crown (feet NAVD 10-yr Alternative 3 (feet NAVD Delta 10-year (feet Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Table C-3 10 Year Alternative 3 C-3 2

153 Model Node Location Estimated Crown (feet NAVD 10-yr Alternative 3 (feet NAVD Delta 10-year (feet 4247 Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Table C-3 10 Year Alternative 3 C-3 3

154 Model Node Location Estimated Crown (feet NAVD 10-yr Alternative 3 (feet NAVD Delta 10-year (feet Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-3 10 Year Alternative 3 C-3 4

155 Table C-4: 10-Year Alternative 3 with 15 Feet Sea Level Rise Model Node Location Estimated Crown (feet NAVD 10-year Alternative feet SLR (feet NAVD Delta 10-year (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr Table C-4 10 Year Alternative 3 with 15 ft SLR C-4 1

156 Model Node Location Estimated Crown (feet NAVD 10-year Alternative feet SLR (feet NAVD Delta 10-year (feet 06_P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Table C-4 10 Year Alternative 3 with 15 ft SLR C-4 2

157 Model Node Location Estimated Crown (feet NAVD 10-year Alternative feet SLR (feet NAVD Delta 10-year (feet Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir Table C-4 10 Year Alternative 3 with 15 ft SLR C-4 3

158 Model Node Location Estimated Crown (feet NAVD 10-year Alternative feet SLR (feet NAVD Delta 10-year (feet 06_P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-4 10 Year Alternative 3 with 15 ft SLR C-4 4

159 Table C-5: 10-Year Alternative 3 with 3 Feet Sea Level Rise Model Node Location Estimated Crown (feet NAVD 10-year Alternative 3 +3 feet SLR (feet NAVD Delta 10-year (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr Table C-5 10 Year Alternative 3 with 3 ft SLR C-5 1

160 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 3 +3 feet SLR (feet NAVD Delta 10-year (feet 06_P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Table C-5 10 Year Alternative 3 with 3 ft SLR C-5 2

161 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 3 +3 feet SLR (feet NAVD Delta 10-year (feet Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir Table C-5 10 Year Alternative 3 with 3 ft SLR C-5 3

162 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 3 +3 feet SLR (feet NAVD Delta 10-year (feet 06_P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-5 10 Year Alternative 3 with 3 ft SLR C-5 4

163 Table C-6: 10-Year Alternative 4 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 4 (feet NAVD Delta 10-year (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Table C-6 10 Year Alternative 4 C-6 1

164 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 4 (feet NAVD Delta 10-year (feet Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Table C-6 10 Year Alternative 4 C-6 2

165 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 4 (feet NAVD Delta 10-year (feet 4247 Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Table C-6 10 Year Alternative 4 C-6 3

166 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 4 (feet NAVD Delta 10-year (feet Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-6 10 Year Alternative 4 C-6 4

167 Table C-7: 10-Year Alternative 4 with 15 Feet Sea Level Rise Model Node Location Estimated Crown (feet NAVD 10-year Alternative feet SLR (feet NAVD Delta 10-year (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr Table C-7 10 Year Alternative 4 with 15 ft SLR C-7 1

168 Model Node Location Estimated Crown (feet NAVD 10-year Alternative feet SLR (feet NAVD Delta 10-year (feet 06_P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Table C-7 10 Year Alternative 4 with 15 ft SLR C-7 2

169 Model Node Location Estimated Crown (feet NAVD 10-year Alternative feet SLR (feet NAVD Delta 10-year (feet Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir Table C-7 10 Year Alternative 4 with 15 ft SLR C-7 3

170 Model Node Location Estimated Crown (feet NAVD 10-year Alternative feet SLR (feet NAVD Delta 10-year (feet 06_P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-7 10 Year Alternative 4 with 15 ft SLR C-7 4

171 Table C-8: 10-Year Alternative 4 with 3 Feet Sea Level Rise Model Node Location Estimated Crown (feet NAVD 10-year Alternative 4 +3 feet SLR (feet NAVD Delta 10-year (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr Table C-8 10 Year Alternative 4 with 3 ft SLR C-8 1

172 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 4 +3 feet SLR (feet NAVD Delta 10-year (feet 06_P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Table C-8 10 Year Alternative 4 with 3 ft SLR C-8 2

173 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 4 +3 feet SLR (feet NAVD Delta 10-year (feet Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir Table C-8 10 Year Alternative 4 with 3 ft SLR C-8 3

174 Model Node Location Estimated Crown (feet NAVD 10-year Alternative 4 +3 feet SLR (feet NAVD Delta 10-year (feet 06_P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-8 10 Year Alternative 4 with 3 ft SLR C-8 4

175 Table C-9: Existing Conditions Model Node Location Estimated Crown (feet NAVD 2-year Existing (feet NAVD 25-year Existing (feet NAVD 50-year Existing (feet NAVD 100-year Existing (feet NAVD 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Table C-9 Existing Conditions C-9 1

176 Model Node Location Estimated Crown (feet NAVD 2-year Existing (feet NAVD 25-year Existing (feet NAVD 50-year Existing (feet NAVD 100-year Existing (feet NAVD Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St Table C-9 Existing Conditions C-9 2

177 Model Node Location Estimated Crown (feet NAVD 2-year Existing (feet NAVD 25-year Existing (feet NAVD 50-year Existing (feet NAVD 100-year Existing (feet NAVD 06_P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Table C-9 Existing Conditions C-9 3

178 Model Node Location Estimated Crown (feet NAVD 2-year Existing (feet NAVD 25-year Existing (feet NAVD 50-year Existing (feet NAVD 100-year Existing (feet NAVD Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Table C-9 Existing Conditions C-9 4

179 Model Node Location Estimated Crown (feet NAVD 2-year Existing (feet NAVD 25-year Existing (feet NAVD 50-year Existing (feet NAVD 100-year Existing (feet NAVD Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct Table C-9 Existing Conditions C-9 5

180 Model Node Location Estimated Crown (feet NAVD 2-year Existing (feet NAVD 25-year Existing (feet NAVD 50-year Existing (feet NAVD 100-year Existing (feet NAVD 06_PAP-73 Club House Rd & Elmont Rd Table C-9 Existing Conditions C-9 6

181 Table C-10: Existing Conditions with 15 feet Sea Level Rise Model Node Location Estimated Crown (feet NAVD 2-year Existing +15 feet SLR (feet NAVD 25-year Existing +15 feet SLR (feet NAVD 50-year Existing +15 feet SLR (feet NAVD 100-year Existing +15 feet SLR (feet NAVD 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln Table C-10 Existing Condition with 15 ft SLR C-10 1

182 Model Node Location Estimated Crown (feet NAVD 2-year Existing +15 feet SLR (feet NAVD 25-year Existing +15 feet SLR (feet NAVD 50-year Existing +15 feet SLR (feet NAVD 100-year Existing +15 feet SLR (feet NAVD S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln Table C-10 Existing Condition with 15 ft SLR C-10 2

183 Model Node Location Estimated Crown (feet NAVD 2-year Existing +15 feet SLR (feet NAVD 25-year Existing +15 feet SLR (feet NAVD 50-year Existing +15 feet SLR (feet NAVD 100-year Existing +15 feet SLR (feet NAVD 06_PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd Table C-10 Existing Condition with 15 ft SLR C-10 3

184 Model Node Location Estimated Crown (feet NAVD 2-year Existing +15 feet SLR (feet NAVD 25-year Existing +15 feet SLR (feet NAVD 50-year Existing +15 feet SLR (feet NAVD 100-year Existing +15 feet SLR (feet NAVD McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Table C-10 Existing Condition with 15 ft SLR C-10 4

185 Model Node Location Estimated Crown (feet NAVD 2-year Existing +15 feet SLR (feet NAVD 25-year Existing +15 feet SLR (feet NAVD 50-year Existing +15 feet SLR (feet NAVD 100-year Existing +15 feet SLR (feet NAVD Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl Table C-10 Existing Condition with 15 ft SLR C-10 5

186 Model Node Location Estimated Crown (feet NAVD 2-year Existing +15 feet SLR (feet NAVD 25-year Existing +15 feet SLR (feet NAVD 50-year Existing +15 feet SLR (feet NAVD 100-year Existing +15 feet SLR (feet NAVD 06_P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-10 Existing Condition with 15 ft SLR C-10 6

187 Table C-11: Alternative 3 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 3 (feet NAVD 50-year Alternative 3 (feet NAVD 100-year Alternative 3 (feet NAVD 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Table C-11 Alternative 3 C-11 1

188 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 3 (feet NAVD 50-year Alternative 3 (feet NAVD 100-year Alternative 3 (feet NAVD Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St Table C-11 Alternative 3 C-11 2

189 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 3 (feet NAVD 50-year Alternative 3 (feet NAVD 100-year Alternative 3 (feet NAVD 06_P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Table C-11 Alternative 3 C-11 3

190 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 3 (feet NAVD 50-year Alternative 3 (feet NAVD 100-year Alternative 3 (feet NAVD Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Table C-11 Alternative 3 C-11 4

191 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 3 (feet NAVD 50-year Alternative 3 (feet NAVD 100-year Alternative 3 (feet NAVD Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct Table C-11 Alternative 3 C-11 5

192 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 3 (feet NAVD 50-year Alternative 3 (feet NAVD 100-year Alternative 3 (feet NAVD 06_PAP-73 Club House Rd & Elmont Rd Table C-11 Alternative 3 C-11 6

193 Table C-12: Alternative 3 with 15 Feet Sea Level Rise Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 3 +3 feet SLR (feet NAVD 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln Table C-12 Alternative 3 with 15 feet SLR C-12 1

194 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 3 +3 feet SLR (feet NAVD S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln Table C-12 Alternative 3 with 15 feet SLR C-12 2

195 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 3 +3 feet SLR (feet NAVD 06_PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd Table C-12 Alternative 3 with 15 feet SLR C-12 3

196 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 3 +3 feet SLR (feet NAVD McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Table C-12 Alternative 3 with 15 feet SLR C-12 4

197 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 3 +3 feet SLR (feet NAVD Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl Table C-12 Alternative 3 with 15 feet SLR C-12 5

198 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 3 +3 feet SLR (feet NAVD 06_P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-12 Alternative 3 with 15 feet SLR C-12 6

199 Table C-13: Alternative 4 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 4 (feet NAVD 50-year Alternative 4 (feet NAVD 100-year Alternative 4 (feet NAVD 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Table C-13 Alternative 4 C-13 1

200 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 4 (feet NAVD 50-year Alternative 4 (feet NAVD 100-year Alternative 4 (feet NAVD Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St Table C-13 Alternative 4 C-13 2

201 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 4 (feet NAVD 50-year Alternative 4 (feet NAVD 100-year Alternative 4 (feet NAVD 06_P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Table C-13 Alternative 4 C-13 3

202 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 4 (feet NAVD 50-year Alternative 4 (feet NAVD 100-year Alternative 4 (feet NAVD Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Table C-13 Alternative 4 C-13 4

203 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 4 (feet NAVD 50-year Alternative 4 (feet NAVD 100-year Alternative 4 (feet NAVD Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct Table C-13 Alternative 4 C-13 5

204 Model Node Location Estimated Crown (feet NAVD 2-year Alternative 4 (feet NAVD 50-year Alternative 4 (feet NAVD 100-year Alternative 4 (feet NAVD 06_PAP-73 Club House Rd & Elmont Rd Table C-13 Alternative 4 C-13 6

205 Table C-14: Alternative 4 with 15 Feet Sea Level Rise Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 4 +3 feet SLR (feet NAVD 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln Table C-14 Alternative 4 with 15 ft SLR C-14 1

206 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 4 +3 feet SLR (feet NAVD S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln Table C-14 Alternative 4 with 15 ft SLR C-14 2

207 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 4 +3 feet SLR (feet NAVD 06_PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd Table C-14 Alternative 4 with 15 ft SLR C-14 3

208 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 4 +3 feet SLR (feet NAVD McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Table C-14 Alternative 4 with 15 ft SLR C-14 4

209 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 4 +3 feet SLR (feet NAVD Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl Table C-14 Alternative 4 with 15 ft SLR C-14 5

210 Model Node Location Estimated Crown (feet NAVD 2-year Alternative feet SLR (feet NAVD 50-year Alternative feet SLR (feet NAVD 100-year Alternative feet SLR (feet NAVD 100-year Alternative 4 +3 feet SLR (feet NAVD 06_P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-14 Alternative 4 with 15 ft SLR C-14 6

211 Table C-15: Alternative 3 with Hurricane Matthew Event Model Node Location Estimated Crown (feet NAVD Alt 3 Matthew (feet NAVD Delta Alt 3 Matthew (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Table C-15 Alt 3 Matthew C-15 1

212 Model Node Location Estimated Crown (feet NAVD Alt 3 Matthew (feet NAVD Delta Alt 3 Matthew (feet Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Table C-15 Alt 3 Matthew C-15 2

213 Model Node Location Estimated Crown (feet NAVD Alt 3 Matthew (feet NAVD Delta Alt 3 Matthew (feet 4247 Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Table C-15 Alt 3 Matthew C-15 3

214 Model Node Location Estimated Crown (feet NAVD Alt 3 Matthew (feet NAVD Delta Alt 3 Matthew (feet Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-15 Alt 3 Matthew C-15 4

215 Table C-16: Alternative 4 with Hurricane Matthew Event Model Node Location Estimated Crown (feet NAVD Alt 4 Matthew (feet NAVD Delta Alt 4 Matthew (feet 06_P-447 S Club House Rd & Bow Creek Blvd _PAP-211 Club House Rd & Dauphin Ln _PAP Block of Lee Highlands Blvd _PAP-87 Rosemont Rd & Old Forge Rd/ Club House Rd Rainey Ct Cul de Sac _PAP-81 Club House Rd & Hannibal St _P-412 Forrest Trl & Burnt Mill Rd Campion Ave & Campion Ct _PAP-320 Valient Ave & Garrison Pl Woodburne Dr & Boyton Ct _P-501 Kings Arms Dr & John Hancock Dr _PAPLAZA-6 S Plaza Trl & Waverly Dr _P-556 Kings Arms Dr & Carriage Hill Rd Waltham Cir & Chantry Dr _PAP-238 Carnegie Rd & Lee Highlands Blvd Block Crashaw St _PAP Block of Bethune Dr _P-568 Fern Ridge Rd & Brookbridge Rd _PAP-55 Silina Dr & Hannibal St Ashaway Rd & Coggeshall Ct Campion Ave & Crashaw St _PAP-166 Duplin St & Olympia Ln Windsor Oaks Blvd & Cainhoy Ln Cedar Bridge Rd & Fox Run Rd Holland Rd & Grant Ave Haley Dr & Chantry Dr Bancroft Dr & Taft Ave _PAP-254 Dillon Dr & Lineberry Rd Garfield Ave & Faraday Ln S Club House Rd & Creech Ln Block of Holland Rd McKinley Ave & Forrest Glen Rd _PAP-34 Raintree Rd & Waverly Dr Presidential Blvd & Stepping Stone Ln _P-428 Cedar Bridge Rd & Cedar Bridge Cir Sedley Rd & Forestwood Dr _P-507 Kings Arms Dr & Capital Hill Ln Campion Ave & Chimney Hill Pkwy _PAP-126 Appian Ave & Stancil St _P-545 Lamplight Ln & Gossman Dr _PAP-103 Skipper Dr & Bethune Dr _P-407 Northgate Ct Cul de Sac Table C-16 Alt 4 Matthew C-16 1

216 Model Node Location Estimated Crown (feet NAVD Alt 4 Matthew (feet NAVD Delta Alt 4 Matthew (feet Donne Ct & Campion Ave _P-488 Coach House Ln & Burgesses Ln Plainsman Trl & Woodsman Ln E _P Block Brookbridge Rd _P-416 Cedar Bridge Rd & Burnt Mill Rd Bancroft Dr & Grant Ave McKinley Ave & Dana Ln _PAP-272 Hospital Dr & Corvette Ln B Meadowood Dr & Oak Creek Ct _P Block of Stratford Hall Dr _P-573 Fern Ridge Rd & Lakecrest Rd Ashaway Rd & Woodburne Dr Stoneshore Rd & Woodhue Ct Fox Run Rd & Burnt Mill Rd Sedley Rd & Medwin Ct _P-468 Lamplight Ln & Ashlawn Ter _P-532 Kings Arms Dr & Old Towne Ln _PAP-197 Skipper Dr & Barcelona Ln Campion Ave & Dryden St _P-535 Stratford Hall Dr & Old Towne Ln Woodburne Dr & Craig Ct _P Block of Fox Run Rd Woodsman Ln & Plainsman Ct S Club House Rd & Dixie Dr Northgate Dr & Fox Run Rd Riverbend Rd & Sugar Creek Dr S Club House Rd & Farm Ridge Rd Spring Garden Ln & Corrine Ln _PAP-60 Silina Dr & Terrazzo Trl Northgate Dr & Bow Creek Blvd _PAP-42 Cassady Ave & Stancil St Woodburne Dr & Meadowburm Cir S Pine Grove Rd & Pine Grove Ln Block of Roosevelt Ave _PAP-306 Silina Dr & Corvette Ln _PAP-78 Club House Rd & Granite Trl _P-405 Northgate Dr & Northgate Ct _P-511 Stratford Hall Dr & John Hancock Dr Sedley Rd & Campion Ave _PAP-130 Appian Ave & Terrazzo Trl _PAP-225 Bethune Dr & Drake Ct _P-408 Forrest Trl & Northgate Dr Presidential Blvd & Forrester Ln _PAP-261 Dillon Dr & Dauphin Ln Table C-16 Alt 4 Matthew C-16 2

217 Model Node Location Estimated Crown (feet NAVD Alt 4 Matthew (feet NAVD Delta Alt 4 Matthew (feet 4247 Forrest Trl & Brookbridge Rd Garfield Ave & Bancroft Dr McKinley Ave & Good Hope Rd McKinley Ave & Champlain Ln _P-552 Gossman Dr & Gossman Ct Riverbend Rd & Tiverton Dr Sandy Springs Ln Cul de Sac _P-492 Coach House Ct & Coach House Ln _P-478 Lamplight Ln & Bowling Green Dr _P-563 Fern Ridge Rd & Burnt Mill Rd Ashaway Rd & Dixie Dr _P-514 Lamplight Ln & John Hancock Dr _PAP-181 Club House Rd & Barcelona Ln _P-616 Riverbend Rd & Ashaway Rd _PAP-46 Cassady Ave & Terrazzo Trl _P Block of Bow Creek Blvd _P-464 Lamplight Ln & Bow Creek Blvd _PAP-137 Silina Dr & Lineberry Rd Sedley Rd & Tealwood Dr McKinley Ave & Faraday Ln _PAP-193 Skipper Dr & Garrison Pl Riverbend Rd & Crabapple Rd _PAP-270 Dodge Dr & Corvette Ln _P-537 Holly Hedge Ave & Old Towne Ln Block Marvell Rd Blue Spring Ln & Blue Spring Ct _P-518 Lamplight Ln & Holly Hedge Ave _P-402 Northgate Dr & Deer Park Dr _PAP-301 Silina Dr & Dauphin Ln _PAP-67 Silina Dr & Elmont Rd _PAP-316 Silina Dr & Garrison Pl _PAP-216 Club House Rd & Lee Highlands Blvd _PAP-314 Silina Dr & Barcelona Ln Riverbend Rd & Stoneshore Rd _PAP-132 Silina Dr & Appian Ave Dryden St & Dryden Ct _PAP-146 Silina Dr & Crossett St Roosevelt Ave & Bancroft Dr Forrest Trl & Lakecrest Rd _P-540 Lamplight Ln & Old Towne Ln _PAP-98 Hannibal St & Country Club Cir _P-436 Deer Park Dr & Burnt Mill Rd _PAP-154 Club House Rd & Crosset St Sedley Rd & Housman Ct Table C-16 Alt 4 Matthew C-16 3

218 Model Node Location Estimated Crown (feet NAVD Alt 4 Matthew (feet NAVD Delta Alt 4 Matthew (feet Fern Ridge Rd near The Lakes _P-439 Deer Park Dr &Brookbridge Rd _P-594 Plainsman Trl & Plainsman Ct _PAP-384 S Lynnhaven Rd & Lee Highlands Blvd Ashaway Rd & Wareham Ct _P-620 Riverbend Rd & Woodburne Dr Woodburne Dr Cul de Sac Plainsman Trl & Plainsman Cir S Club House Rd & Lakecrest Rd Marvell Rd & Carew Rd _PAP-163 Club House Rd & Duplin St Plainsman Trl & Woodsman Ln W _PAP-294 Lark St & Dauphin Ln Ashaway Rd & Blue Spring Ln S _PAP-240 Bethune Dr & Lee Highlands Blvd b Lamplight Ln & Ferry Farm Ln _P-471 Lamplight Ln & Coach House Ln _P-652 Bow Creek Blvd & Northgate Ditch Culvert E _PAP-75 Club House Rd & Kingswood Pl _P-399 Bow Creek Blvd & Plainsman Trl W Riverbend Rd near The Lakes Culvert _P-914 S Club House Rd near Bow Creek Culvert _P-930 Bow Creek Blvd & Northgate Ditch Culvert W _P-454 S Club House Rd & Clubhouse Cir/Ct _PAP-73 Club House Rd & Elmont Rd Table C-16 Alt 4 Matthew C-16 4

219 Appendix D Planning-level Opinion of Probable Construction Costs D-1

220 Appendix D Model Results for all Analyzed Alternatives This page intentionally left blank D-2

221 Table D-1 Princess Anne Plaza/The Lakes Stormwater Management System Flood Mitigation Plan Planning-Level Opinion of Probable Construction Cost Item Alternative 3 Alternative 4 Item Description Category Unit Unit Cost No Qty Total Cost Qty Total Cost 1 18 RCP, Class III Piping LF $ 80 1,434 $ 114,743 1,434 $ 114, RCP, Class III Piping LF $ $ 60, $ 60, RCP, Class III Piping LF $ 99 8,446 $ 836,153 8,446 $ 836, RCP, Class III Piping LF $ 121 7,483 $ 905,502 7,483 $ 905, RCP, Class III Piping LF $ 162 5,444 $ 881,933 5,444 $ 881, RCP, Class III Piping LF $ 239 2,447 $ 584,795 2,447 $ 584, RCP, Class III Piping LF $ 265 5,493 $ 1,455,544 5,493 $ 1,455, RCP, Class III Piping LF $ $ 343, $ 343, ' X 4' Box Culvert Culverts LF $ 1, $ 195, $ 195, ' x 6' Box Culvert Culverts LF $ 1, $ 739, $ 739, x 19 RECP Piping LF $ $ 22, $ 22, x 34 RECP Piping LF $ 312 1,151 $ 359,056 1,151 $ 359, x 38 RECP Piping LF $ 430 2,051 $ 881,200 2,051 $ 881, x 43 RECP Piping LF $ 550 2,782 $ 1,530,073 2,782 $ 1,530, x 48 RECP Piping LF $ $ 312, $ 312, x 53 RECP Piping LF $ $ 570, $ 570, x 58 RECP Piping LF $ $ 403, $ 403, Dredge Existing Ditch Ditch CY $ 60 89,163 $ 5,349, ,960 $ 6,177, Clearing and Grubbing Ditch SY $ 35 29,926 $ 1,047,423 53,888 $ 1,886, Ponds Pond CY $ 16 1,369,564 $ 21,913,026 1,369,564 $ 21,913, Bank Stabilization Pond and Ditch AC $ 15, $ 1,370, $ 1,444, Pavement Demolition Pavement SY $ 16 33,263 $ 532,206 33,263 $ 532, Select Material Pavement CY $ 36 11,088 $ 399,155 11,088 $ 399, Aggregate Base Material No 21A Pavement TON $ 52 11,975 $ 622,681 11,975 $ 622, Asphalt Concrete Base Course (Type BM-25 Pavement TON $ 115 5,488 $ 631,163 5,488 $ 631, Asphalt Concrete (Type SM-95 Pavement TON $ 115 6,283 $ 722,530 6,283 $ 722, Pump Station (5000 cfs Pump Station LS $ 125,000,000 - $ - 1 $ 125,000, Pump Station (1000 cfs Pump Station LS $ 25,000,000 1 $ 25,000,000 - $ - 29 Pump Station (600 cfs Pump Station LS $ 15,000,000 1 $ 15,000,000 - $ - 30 Sluice Gate (Potters Road Control Structure LS $ 5,000,000 - $ - 1 $ 5,000, Sluice Gate (London Bridge Creek and Canal 2 Control Structure LS $ 3,000,000 2 $ 6,000,000 1 $ 3,000, Erosion and Sediment Control E&SC LS $ 5,720,000 1 $ 5,720,000 1 $ 5,720, Maintenance of Traffic Miscellaneous LS $ 824,000 1 $ 824,000 1 $ 824, Utility Relocation Miscellaneous LS $ 2,061,000 1 $ 2,061,000 1 $ 2,061, Mill and Overlay Pavement SY $ ,400 $ 2,032, ,400 $ 2,032, Water Quality Treatment Miscellaneous EA $ 40, $ 1,640, $ 1,640, Replace Drop Inlets Piping EA $ 6, $ 1,026, $ 1,026, Manholes Piping EA $ 13, $ 319, $ 319,200 Subtotal $ 102,407,671 $ 191,148,380 Mobilization (10% $ 10,240,767 $ 19,114,838 Contingency (40% $ 45,059,375 $ 84,105,287 Engineering, Survey, & Permitting (15% $ 23,656,172 $ 44,155,276 Subtotal Cost, Today's dollars $ 181,363,985 $ 338,523,781 Escalation to midpoint of construction (FY 26 based on inflation rate of 4% per year $ 66,845,152 $ 124,769,389 Total Project Cost $ 248,209,137 $ 463,293,170 SAY $ 248,000,000 $ 463,000,000 Page 1 of 1

222 Appendix E Summary of Alternative Improvements E-1

223 Appendix E Summary of Alternatives Evaluated This page intentionally left blank E-2

224 Appendix E Summary of Alternatives Improvements Table E-1 Summary of Improvements Elements for Alternative 3 and Alternative 4 Item Feature Type 1 Pump station and control structure Addition Description (Refer to Figure 5-2 or Figure 5-3 Construct a new 5000 cfs pump station and gated control structure 2 Ditch Improve Dredge 1760 linear feet of London Bridge Creek Comments The gated control structure closes when the water surface elevations in upstream of the Creek along Potters Road are higher than the downstream tailwater elevation Improve to a trapezoidal channel, with a bottom width of 38 feet by 86 feet wide bottom at -10 feet NAVD and 3:1 side slopes to meet existing grade; the center channel Manning s roughness value = 0035 The gated control structure closes when the water surface elevations in upstream of the Creek along South Lynnhaven Road are higher than the downstream tailwater elevation The pump station is proposed to operate at seven levels: Alternative 3 No No Alternative 4 3 Pump station and control structure Addition Construct a new 1000-cfs pump station and gated control structure cfs pump that is triggered on at elevation 05 feet NAVD and shuts off at 0 feet NAVD cfs pump that is triggered on at elevation 2 feet NAVD and shuts off at 0 feet NAVD cfs pump that is triggered on at elevation 3 feet NAVD and shuts off at 0 feet NAVD cfs pump that is triggered on at elevation 35 feet NAVD and shuts off at 1 feet NAVD cfs pump that is triggered on at elevation 4 feet NAVD and shuts off at 1 feet NAVD cfs pump that is triggered on at elevation 45 feet NAVD and shuts off at 2 feet NAVD cfs pump that is triggered on at elevation 5 feet NAVD and shuts off at 2 feet NAVD No 4 Ditch Improve Maintain 276 linear feet of existing ditch northwest of South Lynnhaven Road and Lee Highland Boulevard 5 Ditch Improve Dredge 236 linear feet of existing ditch upstream of item 4 6 Pipe Addition Construct 200 linear feet of 91-inch by 58-inch RECP across South Lynnhaven Road that outfalls to the ditch described in item 5 7 Ditch Improve Dredge 815 linear feet of existing ditch upstream of item 6 8 Pipe Addition 9 Pipe Addition 10 Pipe Addition 11 Pipe Addition 12 Pipe Addition 13 Pipe Addition 14 Pipe Addition 15 Pipe Addition 16 Pipe Addition 17 Pipe Addition 18 Pipe Addition 19 Pipe Addition Construct 652 linear feet of 36-inch RCP along Bethune Drive that outfalls to the ditch described in item 7 Construct 537 linear feet of 24-inch RCP along Bethune Drive from Skipper Drive that connects to the pipe described in item 8 Construct 207 linear feet of 91-inch by 58-inch RECP along Presidential Boulevard from Lark Street that outfalls to the ditch described in item 6 Construct 1558 linear feet 48-inch RCP from the intersection of Corvette Lane and Dillon Drive to Lark Street along Dillon Drive and Dodge Drive Construct 258 linear feet of 36-inch RCP along Dillon Drive from Dauphin Lane to Corvette Lane Construct 690 linear feet of 30-inch RCP along Dillon Drive from Lineberry Road to Dauphin Lane Construct 690 linear feet of 83-inch by 53-inch RECP along Lark Street from Dauphin Lane to connect to the pipe described in item 7 Construct 724 linear feet of 48-inch RCP from the intersection of Silina Drive and Barcelona Lane along Barcelona Lane to the intersection of Lark Street and Dauphin Lane Construct 495 linear feet of 24-inch RCP along Silina Drive from Corvette Lane to Barcelona Lane Construct 253 linear feet of 36-inch RCP along Silina Drive from Garrison Place to Barcelona Lane Construct 717 linear feet of 30-inch RCP along Garrison Place from Valiant Avenue to Silina Drive Construct 160 linear feet of 36-inch RCP from Lee Highlands Boulevard that outfalls to London Bridge Creek Once the water surface elevations upstream of South Lynnhaven Road are less than the downstream tailwater elevation in London Bridge Creek, the control structure will fully open Improve center channel to lower roughness Improve to a trapezoidal channel, with a 16-feet wide bottom at -05 feet NAVD and 3:1 side slopes to meet existing grade; the center channel Manning s roughness value = 0035 Improve to a trapezoidal channel, with a 16-feet wide bottom at 0 feet NAVD and 3:1 side slopes to meet existing grade; the center channel Manning s roughness value = 0035 E-3

225 Appendix E Summary of Alternatives Improvements Item Feature Type Description Alternative Alternative Comments (Refer to Figure 5-2 or Figure Pipe Addition Construct 340 linear feet of 30-inch RCP along Lee Highlands Boulevard from Carnegie Road that connects to the pipe described in item Pipe Addition Construct 653 linear feet of 21-inch RCP along Lee Highlands Boulevard east of Flint Court to Carnegie Road 22 Ditch Improve Improve 360 linear feet of existing ditch downstream of Bow Creek golf course east of South Club House Road Improve center channel to lower roughness 23 Pond Addition Construct a pond in the Bow Creek Golf Course The pond has a 61-acre footprint at 7 feet NAVD 24 Pipe Addition Construct 285 linear feet of 30-inch RCP in Bow Creek Golf Course south east of Club House Road that outfalls to the pond described in item Pipe Addition Construct 142 linear feet of 36-inch RCP south of Club House Road that outfalls to the existing ditch along Bow Creek golf course 26 Pipe Addition Construct 50 linear feet of 30-inch RCP across Club House Road from Dauphin Lane that connects to the pipe described in item Culvert Addition Construct 50 linear feet of 8 feet by 4 feet RCB from Bow Creek recreation center that outfalls to the pond described in item Ditch Improve Dredge 325 linear feet of existing ditch upstream of item 27 Improve to a trapezoidal channel, with a 15-feet wide bottom at -3 feet NAVD and 3:1 side slopes to meet existing grade; the center channel Manning s roughness value = Pipe Addition Construct 350 linear feet of 53-inch by 34-inch RECP from Club House Road along Bow Creek Recreation Center that outfalls to the ditch described in item Pipe Addition Construct 140 linear feet of 68-inch by 43-inch RECP from Club House Road that outfalls to the ditch described in item Pipe Addition Construct 219 linear feet of 18-inch RCP along Club House Road from Garrison Place to Lineberry Road 32 Pipe Addition Construct 960 linear feet of 68-inch by 43-inch RECP along Lineberry Road from Skipper Drive to Club House Road 33 Pipe Addition Construct 455 linear feet of 30-inch RCP across Skipper Drive from Barcelona Lane to Lineberry Road 34 Pipe Addition Construct 1007 linear feet of 60-inch by 38-inch RECP along Lineberry Road from Silina Drive to Skipper Drive 35 Pipe Addition Construct 224 linear feet of 60-inch by 38-inch RECP along Silina Drive from Appian Avenue to Lineberry Road 36 Pipe Addition Construct 593 linear feet of 36-inch RCP along Appian Avenue from Terrazzo Trail to Silina Drive 37 Pipe Addition Construct 251 linear feet of 30-inch RCP along Apian Avenue from Stancil Street to Terrazzo Trail 38 Bridge Addition Construct 30 linear feet of 5 feet by 6 feet bridge across the proposed pond in the Bow Creek Golf Course 39 Ditch Improve Dredge 87 linear feet of existing ditch upstream of item 38 Improve to a trapezoidal channel, with a 16-feet wide bottom at -2 feet NAVD and 3:1 side slopes to meet existing grade; the center channel Manning s roughness value = Pipe Addition Construct 136 linear feet of 76-inch by 48-inch RECP south of Club House Road upstream of item Pipe Addition Construct 1085 linear feet of 48-inch RCP along Club House Road from South Rosemont Road to Elmont Road 42 Pipe Addition Construct 906 linear feet of 48-inch RCP along Elmont Road from Silina Drive to Club House Road 43 Pipe Addition Construct 659 linear feet of 30-inch RCP along Cassidy Avenue south of Terrazzo Trail, and Silina Drive to Elmont Road 44 Pipe Addition Construct 762 linear feet of 42-inch RCP along Silina Drive from Hannibal Street to Elmont Road E-4

226 Appendix E Summary of Alternatives Improvements Item Feature Type Description Alternative Alternative Comments (Refer to Figure 5-2 or Figure Pipe Addition Construct 660 linear feet of 30-inch RCP south of Raintree Road along Stancil Street, Hannibal Street to Silina Drive 46 Pipe Addition Construct 248 linear feet of 24-inch RCP along Waverly Drive to Raintree Road 47 Ditch Improve Improve 279 linear feet of existing ditch north of Bow Creek Boulevard that outfalls to the golf course Improve center channel to lower roughness 48 Pond Addition Construct a pond in the northwestern portion of Northgate Park The pond has a 16-acre footprint at 10 feet NAVD 49 Ditch Improve Dredge 4,767 linear feet of existing Northgate Ditch Dredging this portion of the ditch is planned in Pipe Addition Construct 414 linear feet of 68-inch by 43-inch RECP from Woodsman Lane along Plainsman Trail that outfalls to the ditch described in item Pipe Addition Construct 340 linear feet of 36-inch RCP along Faraday Lane from McKinley Avenue that outfalls to the ditch described in item Pipe Addition Construct 670 linear feet of 24-inch RCP along Faraday Lane from Garfield Avenue to McKinley Avenue 53 Pipe Addition Construct 591 linear feet of 24-inch RCP along Bow Creek Boulevard, north west of Plainsman Trail that outfalls to Northgate ditch 54 Pipe Addition Construct 155 linear feet of 30-inch RCP across Northgate Drive that outfalls to Northgate ditch 55 Pipe Addition Construct 151 linear feet of 24-inch RCP south east of Plainsman Trail and Woodsman Lane that outfalls to Northgate ditch 56 Pipe Addition Construct 150 linear feet of 24-inch RCP from Plainsman Court that outfalls to Northgate ditch 57 Pipe Addition Construct 194 linear feet of 24-inch RCP from Plainsman Circle that outfalls to the pond described in item Pipe Addition Construct 163 linear feet of 18-inch RCP along Plainsman Circle that connects to the pipe described in item Pipe Addition Construct 200 linear feet of 36-inch RCP along Lakecrest Road that outfalls into the pond described in item Pipe Addition Construct 505 linear feet of 30-inch RCP along Forest Trail from Burnt Mill Road to Lakecrest Road 61 Pipe Addition Construct 50 linear feet of 18-inch RCP across Forest Trail south of Burnt Mill Road 62 Pipe Addition Construct 518 linear feet of 30-inch RCP west of Waltham Circle that outfalls to Northgate ditch 63 Pipe Addition Construct 8 linear feet of 24-inch RCP west of Waltham Circle that connects to the pipe described in item Pipe Addition Construct 642 linear feet of 24-inch RCP along Holland Road from Grant Avenue, across South Rosemont Road, to Garfield Avenue 65 Ditch Improve Dredge 450 linear feet of Northgate ditch from Garfield Avenue to south of Improve to a trapezoidal channel, with a 25-feet wide bottom at -11 feet NAVD and 3:1 side slopes to meet existing McKinley Avenue grade; the center channel Manning s roughness value = Ditch Improve Dredge 438 linear feet of Northgate ditch from South Rosemont Road to Improve to a trapezoidal channel, with a 35-feet wide bottom at 05 feet NAVD and 3:1 side slopes to meet existing Garfield Avenue grade; the center channel Manning s roughness value = Ditch Improve Dredge 549 linear feet of Northgate ditch from south of Forest Glen Court to Improve to a trapezoidal channel, with a 39-feet wide bottom at 17 feet NAVD and 3:1 side slopes to meet existing South Rosemont Road grade; the center channel Manning s roughness value = Ditch Improve Dredge 495 linear feet of Northgate ditch from north of Arthur Avenue to Improve to a trapezoidal channel, with a 46-feet wide bottom at 23 feet NAVD and 3:1 side slopes to meet existing south of Forest Glen Court grade; the center channel Manning s roughness value = Ditch Improve Dredge 212 linear feet of Northgate ditch from south of Forest Glen Road to Improve to a trapezoidal channel, with a 77-feet wide bottom at 26 feet NAVD and 3:1 side slopes to meet existing north of Arthur Avenue grade; the center channel Manning s roughness value = Pipe Addition Construct 70 linear feet of 60-inch by 38-inch RECP across Forest Glen Road west of Presidential Boulevard E-5

227 Appendix E Summary of Alternatives Improvements Item Feature Type Description Alternative Alternative Comments (Refer to Figure 5-2 or Figure Ditch Improve Dredge 279 linear feet of Northgate ditch west of Presidential Boulevard and Improve to a trapezoidal channel, with a 152-feet wide bottom at 3 feet NAVD and 3:1 side slopes to meet existing Stepping Stone Lane grade; the center channel Manning s roughness value = Pipe Addition Construct 126 linear feet of 24-inch RCP west of Presidential Boulevard that outfalls to the Northgate ditch 73 Ditch Improve Dredge 466 linear feet of Northgate ditch west of Presidential Boulevard and Improve to a trapezoidal channel, with a 102-feet wide bottom at 34 feet NAVD and 3:1 side slopes to meet existing Walnut Hill Road grade; the center channel Manning s roughness value = Pipe Addition Construct 110 linear feet of 24-inch RCP east of Sandy Springs Lane that outfalls to Northgate ditch 75 Pipe Addition Construct 96 linear feet of 24-inch RCP south of Forrester Lane that outfalls to Northgate ditch 76 Pipe Addition Construct 100 linear feet of 24-inch RCP east of South Pine Grove Road that outfalls to Northgate ditch 77 Ditch Improve Improve 466 linear feet of Northgate ditch from west of Bancroft Drive to Arthur Avenue Improve center channel to lower roughness 78 Pipe Addition Construct 143 linear feet of 30-inch by 19-inch RECP east of South Pine Grove Road that outfalls to Northgate ditch 79 Ditch Improve Improve 397 linear feet of Northgate ditch from west of Taft Avenue to Bancroft Drive Improve center channel to lower roughness 80 Ditch Improve Improve 191 linear feet of Northgate ditch north of Holland Road Improve center channel to lower roughness 81 Pipe Addition 82 Pipe Addition 83 Pipe Addition 84 Pipe Addition 85 Pipe Addition 86 Pipe Addition 87 Pipe Addition 88 Pipe Addition 89 Pipe Addition 90 Pipe Addition 91 Culvert Addition 92 Pipe Addition 93 Pipe Addition 94 Pipe Addition 95 Pipe Addition Construct 497 linear feet of 24-inch RCP along Holland Road from west of Arthur Avenue that outfalls to Northgate ditch Construct 801 linear feet of 53-inch by 34-inch RECP along Coach House Lane east of Coach House Court, along Lamplight Lane that outfalls to London Bridge Creek Construct 324 linear feet of 30-inch RCP along Coach House Lane from Burgesses Lane to east of Coach House Court Construct 185 linear feet of 36-inch RCP east of South Club House Road that outfalls to London Bridge Creek Construct 442 linear feet of 30-inch RCP along Deer Park Drive from Brookbridge Road to South Club House Road Construct 1034 linear feet of 24-inch RCP along Burnt Mill Road from Fern Ridge Road, and Deer Park Drive to Brookbridge Road Construct 320 linear feet of 76-inch by 48-inch RECP along South Club House Road that outfalls to London Bridge Creek Construct 1268 linear feet of 68-inch by 43-inch RECP across Riverbend Road, along Lakecrest Road to South Club House Road Construct 354 linear feet of 30-inch RCP along John Hancock Drive from Stratford Hall Drive to Lamplight Lane Construct 304 linear feet of 24-inch RCP along John Hancock Drive from Kings Arms Drive to Stratford Hall Drive Construct 100 linear feet of 8 feet by 4 feet RCB across the Riverbend Road connecting The Lakes Construct 688 linear feet of 24-inch RCP along Stoneshore Road that outfalls to The Lakes Construct 293 linear feet of 18-inch RCP along Ashaway Road that outfalls to The Lakes Construct 358 linear feet of 24-inch RCP across Woodburne Drive that connects to the existing pipe system north of Boynton Court Construct 70 linear feet of 36-inch RCP along Ole Towne Lane, across Stratford Hall Drive E-6

228 Appendix E Summary of Alternatives Improvements Item Feature Type 96 Pipe Addition 97 Pipe Addition 98 Pipe Addition 99 Pipe Addition 100 Pipe Addition 101 Pipe Addition 102 Pipe Addition 103 Pipe Addition 104 Pipe Addition 105 Pipe Addition 106 Pipe Addition 107 Pipe Addition 108 Pipe Addition 109 Pipe Addition 110 Pipe Addition Description (Refer to Figure 5-2 or Figure 5-3 Construct 204 linear feet of 30-inch RCP along Ole Towne Lane from Kings Arms Drive to Stratford Hall Drive Construct 82 linear feet of 24-inch RCP across Kings Arms Drive north west of Carriage Hill Road Construct 538 linear feet of 30-inch RCP along Stoneshore Road from Meadowood Road to Larkhaven Lane Construct 317 linear feet of 24-inch RCP along Meadowood Road from Oak Creek Court to Stoneshore Road Construct 794 linear feet of 60-inch RCP along Carew Road from Sedley Road that outfalls to the Lakes Construct 1682 linear feet of 36-inch RCP along Sedley Road from Campion Avenue to Carew Road Construct 329 linear feet of 30-inch RCP along Campion Avenue from Prior Court to Sedley Road Construct 709 linear feet of 18-inch RCP along Sedley Road from Westwood Drive to Campion Avenue Construct 638 linear feet of 48-inch RCP along Carew Road from Marvell Road to Sedley Road Construct 464 linear feet of 42-inch RCP along Carew Road from Campion Avenue to Marvell Road Construct 768 linear feet of 36-inch RCP along Tealwood Drive from Sedley Road, across Campion Avenue to Carew Road Construct 295 linear feet of 24-inch RCP along Sedley Road from Medwin Court to Teakwood Drive Construct 447 linear feet of 48-inch RCP across Addison Street, east of Snowdrift Circle that outfalls to the Lakes Construct 1221 linear feet of 42-inch RCP along Dryden Street from Campion Avenue to Addison Street Construct 192 linear feet of 24-inch RCP along Campion Avenue from Campion Court to Dryden Street Comments Alternative 3 Alternative Pipe Addition Construct 270 linear feet of 24-inch RCP along Campion Avenue from Crashaw Street to Dryden Street 112 Culvert Addition Construct 160 linear feet of 24-inch RCP south of Holland Windsor Shopping Center across The Lakes This pipe has a downstream invert of -21 feet NAVD and acts as a bleeder pipe to lower the pond upstream 113 Pipe Addition Construct 750 linear feet of 60-inch by 38-inch RECP along Windsor Oaks Boulevard west of Brittlebank Drive to Towne Square Drive 114 Pond Addition Construct a pond south of Lynnhaven Parkway, west of London Bridge Creek The pond has a 23-acre footprint at 6 feet NAVD 115 Pump station and gated control structure Addition Construct a new 600 cfs pump station and gated control structure The gated control structure closes when the water surface elevations upstream of the Creek along Lynnhaven Parkway are higher than the downstream tailwater elevation The pump station is proposed to operate at three levels: cfs pump that is triggered on at elevation 05 feet NAVD and shuts off at 0 feet NAVD cfs pump that is triggered on at elevation 3 feet NAVD and shuts off at 1 feet NAVD cfs pump that is triggered on at elevation 5 feet NAVD and shuts off at 2 feet NAVD No 116 Culvert Addition Construct 145 linear feet of three barrels 9 feet by 6 feet RCB connecting The Lakes to London Bridge Creek Once the water surface elevations upstream of Lynnhaven Parkway are less than the downstream tailwater elevation in London Bridge Creek, the control structure will fully open E-7

229 Appendix E Summary of Alternatives Improvements Item Feature Type Description Alternative Alternative Comments (Refer to Figure 5-2 or Figure Culvert Addition Construct 150 linear feet of 60-inch RCP across Holland Road, north of Warrick Drive connecting Green Run Lakes This pipe has a downstream invert of -5 feet NAVD and acts as a bleeder pipe to lower the lake upstream 118 Culvert Addition Construct 135 linear feet of 48-inch RCP across Lynnhaven Parkway, north of Crimson Holly Lane connecting Green Run Lakes This pipe has a downstream invert of -4 feet NAVD and acts as a bleeder pipe to lower the lake upstream 119 Culvert Addition Construct 132 linear feet of 24-inch RCP across Lynnhaven Parkway, west of Ski Lodge Road connecting Green Run Lakes This pipe has a downstream invert of -19 feet NAVD and acts as a bleeder pipe to lower the lake upstream 120 Ditch Addition Extend 724 linear feet of Green Run Lake 1, to include additional storage Extend the trapezoidal channel to have a 3845-feet wide bottom at -2 feet NAVD and 3:1 side slopes to meet existing west of the existing ditch south of Lynnhaven Parkway grade The center channel Manning s roughness value = Culvert Addition Construct 140 linear feet of 36-inch RCP across S Independence Boulevard, north of Riverside Drive connecting Green Run Lakes This pipe has a downstream invert of -19 feet NAVD and acts as a bleeder pipe to lower the lake upstream 122 Pond Addition Construct a pond west of Cinnamon Ridge Lake The pond has a 62-acre footprint at 12 feet NAVD 123 Control structure Addition Construct a control structure upstream of the confluence of London Bridge Creek and Canal No 2 The control structure prevents incoming flow from West Neck Creek to London Bridge Creek No E-8

230 Appendix E Summary of Alternatives Improvements Table E-2 Summary of Alternatives Model Results Alternative System Improvements Total New Pond Footprint (Acre Total New Pond Volume (Acre-Feet Simulated Number of Flooded Structures in 100-Year Design 1 Performance Criteria Met Level-of-service 2 Mitigate Adverse OffSite Impacts 3 Base Existing System Configuration N/A N/A 600 No N/A Performance criteria not met Northern pump station/gate structure (1,000 cfs Southern pump station/gate structure (600 cfs Set initial depth in London Bridge Creek to 0 feet NAVD Various ditch/canal improvements: Northgate Ditch, Bow Creek, Green Run Canals, East of Bethune Drive along South Lynnhaven Road Study area pipe improvements Same as Alternative 1, plus: Widen existing ditches within Bow Creek Golf Course Convert 16 acres (140 acre-feet of Plaza Northgate Park to a pond Green Run Canals: add 10 acres (total between two new ponds on vacant lots (70 acre-feet Refine study area pipe improvements Same as Alternative 2, plus Convert 61 acres (400 acre-feet of Bow Creek Golf Course to a lake Remove existing weir between The Lakes and Green Run Canal systems confluence on London Bridge Creek Add 2 acres (13 acre-feet of storage at the southern pump station location behind Title Max property on Lynnhaven Parkway Same as Alternative 3, except: Relocate northern pump station and gate structure to Potters Road (5,000 cfs London Bridge Creek dredging from the confluence with Canal 2 to the new pump station (1,500 linear feet Remove southern pump station and gate structure at Lynnhaven Parkway Install southern gate structure at Dam Neck Road (boundary between WS 6 and WS 10 Alternative Considered but Not Evaluated 5 Dredging and/or Reducing Channel Roughness of London Bridge Creek (to allow higher flows from the northern pump station Comment No Performance criteria not met Not evaluated Partially; met for 10-year, but not for 100-Year design, with exceptions for 11 structures, with exceptions for 19 structures Note: 1 Number of simulated flooded structures is approximate As noted in Section 41, obtaining additional surveys in the study area would provide a more accurate count of simulated flooding 2 Level-of-service criteria must be met for both 10-year and 100-year design storms for the study area (as defined in Section 41 of this report 3 Mitigate adverse offsite impacts is defined as modeled peak flood stages remaining the same (or lower than existing conditions at locations the study area (for all design storms Performance criteria not met 98 percent reduction in simulated number of flooded structures from Existing Conditions Base model 97 percent reduction in simulated number of flooded structures from Existing Conditions Base model Not considered due to uncertainty associated with potential environmental permitting requirements, land acquisition, liability and legal exposure E-9

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232 Legend 37 PS PumpStation Gated Control Structure C\ StorageImprovement Stormw aterpipeimprovement DitchImprovement Bow Creek GolfCours e 'Φ 'x 60' 'Φ 'Φ C\ 37 PS London Bridge Creek 3 Ü 113 Planned Dred ging Box Culvert PSTributary Area Red -Labeled Improvements Cores pond totablee-1 PlazaNorthgate Park Canal 2 42'Φ 'Φ 'Φ PS C\ 115 GreenRun Canals West Neck Creek Miles Figure E-1 Princess Anne Plaza and The Lakes Alternative 3: Proposed Stormwater Improvements - Itemized Projects

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234 Thalia Creek C\ 37 PS 2 1 Ü Bow Creek GolfCours e 'x 60' London Bridge Creek PlazaNorthgate Park Canal 'Φ 'Φ 'Φ GreenRun Canals C\ 123 Legend 37 PS PumpStation C\ Gated Control Structure StorageImprovement Canal 2 Stormw aterpipeimprovement DitchImprovement Planned Dred ging West Neck Creek 113 Box Culvert PSTributary Area Red -Labeled Improvements Cores pond totablee Miles Figure E-2 Princess Anne Plaza and The Lakes Alternative 4: Proposed Stormwater Improvements - Itemized Projects

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