Ashville Park Stormwater Management System Flood Mitigation Plan

Transcription

1 CITY OF VIRGINIA BEACH Ashville Park Stormwater Management System Flood Mitigation Plan FINAL DRAFT FEBRUARY 21, 2017

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3 Table of Contents Executive Summary... ES-1 Section 1 Introduction Section 2 Hydrologic and Hydraulic Model Development Basin Description Hydrologic and Hydraulic Model Setup Hydrologic Modeling Existing Conditions Hydrology Soils Land Use Imperviousness Build-out Conditions Hydrology Soils Land Use Imperviousness Hydraulic Modeling Existing Condition Stormwater System Proposed Build-out Condition Stormwater System Section 3 Model Calibration Calibration Storm Tidal Boundary Conditions Calibration Section 4 Existing and Build-out Model Comparison Target Level of Service (LOS Existing Model Analysis year Design Storm year Design Storm Build-out Model Analysis year Design Storm year Design Storm Section 5 Alternative Solution Sets Alternative Descriptions Alternative A Alternative B Alternative C Alternative D Alternative Results Results i

4 Table of Contents Section 6 Feasibility and Probable Cost of Alternative Solution Sets Feasibility Alternative Solution Set A Alternative Solution Set B Alternative Solution Set C Alternative Solution Set D Opinion of Probable Construction Cost Sea Level Rise Modifications to Alternative Solution Sets for Sea Level Rise Section 7 Summary and Conclusions Summary of Alternative Solution Sets Recommended Solution and Design Considerations List of Figures Figure ES-1 (Recommended Solution Set for Ashville Park: Alternative D... ES-5 Figure 1-1 Ashville Park Overview Figure 1-2 Ashville Park Subdivision Figure 2-1 Ashville Park Existing Scenario Figure 2-2 Ashville Park Build-out Scenario Figure 2-3 Ashville Park Soil Conductivity Distribution Figure 2-4 Breakdown of Ashville Park Existing Conditions Land Use Figure 2-5 Breakdown of Ashville Park Existing Conditions Imperviousness Figure 2-6 Breakdown of Ashville Park Build-out Conditions Land Use Figure 2-7 Breakdown of Ashville Park Build-out Conditions Imperviousness Figure 2-8 Ashville Park Existing Model Schematic Figure 2-9 Ashville Park Build-out Model Schematics Figure 3-1 September 19 22, 2016 Rainfall Hyetograph, Sandbridge Gauge Figure 3-2 Water Level at Ashville Bridge Creek Outfall, September 19 22, Figure 3-3 Ashville Park Model Calibration Locations Figure 5-1 Ashville Park Proposed Solution Set: Alternative A Figure 5-2 Ashville Park Proposed Solution Set: Alternative B Figure 5-3 Ashville Park Proposed Solution Set: Alternative C Figure 5-4 Ashville Park Proposed Solution Set: Alternative D Figure 6-1 Ashville Park Sea Level Rise: Elevations Figure 7-1 Recommended Alternative with Reconfiguration Refinements ii

5 Table of Contents List of Tables Table 2-1 Ashville Park Existing Condition Model Elements Table 2-2 Ashville Park Proposed Model Elements Table 3-1 September 21, 2016 Flood Elevations Table 4-1 Summary of Results for the Existing Conditions Table 4-2 Summary of Results for the Build-out Conditions Table 5-1 Summary of Elements and Features for Alternatives Table 5-2 Summary of Alternatives Simulation Results Table 6-1 Planning-level Opinion of Probable Construction Cost for Alternatives A through D Table 6-2 Modifications to Alternative Solution Sets for 1.5 Feet of SLR Table 6-3 Modifications to Alternative Solution Sets for 3.0 Feet of SLR Appendices Appendix A Field Investigation for Calibration Event Appendix B Stormwater Model Development Methodology Appendix C Model Results Appendix D Planning-level Opinion of Probable Construction Costs Appendix E Alternatives Analysis iii

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7 Executive Summary Ashville Park is a subdivision in southeastern Virginia Beach, located south of Sandbridge Road and East of Princess Anne Road in the City s transition area south of the Green Line. Building on the Ashville Park development began in 2006, along with the creation of a stormwater management system (SMS of linear lakes that outfall to a roadside ditch along Flanagan s Lane, which conveys flows to the channel along Sandbridge Road. The system ultimately discharges to Ashville Bridge Creek. Since its construction, Ashville Park has experienced flooding during heavy rain events. The conditions are most significant within the Ranier Village, Wilshire Village, and Ashville Park Boulevard areas. Rainfall events that produced the worst conditions include September 8 9, 2014, a more recent rainfall event from September 19 21, 2016 (remnants of Tropical Storm Julia, and Hurricane Matthew, October 8 9, The City of Virginia Beach retained CDM Smith to develop, and calibrate hydrologic and hydraulic (H&H model of Ashville Park to identify alternatives to alleviate flooding in Ashville Park and provide a level of service (LOS that contains the 10-year storm event and limits water in the roads to a maximum of 3-inches above roadway crowns for the 100-year storm event with no structural flooding. The model includes the Ashville Park SMS and external drainage features such as nearby ditches, pipes and channels - that interact with and influence the system. CDM Smith calibrated the H&H model to the September 19 21, 2016 storm event (the remnants of Tropical Storm Julia. For calibration, model results were compared with estimated flood depths derived from photos of observed water levels. The calibrated H&H model was used to identify areas of probable flooding in Ashville Park for the 10-year and 100-year storm events under existing and build-out conditions. The model identified two key requirements for the Ashville Park Subdivision SMS to meet the desired LOS: Significantly increase the size of the SMS to improve conveyance off site, from the existing Ranier and Wilshire Villages through to Ashville Bridge Creek Resize the stormwater collection systems in Ranier and Wilshire Villages with the addition of parallel systems, or supplemental piping and replacement of undersized pipes to accommodate the increased conveyance from Ashville Park CDM Smith developed more than two dozen initial alternatives, which were shortlisted to four potential solution sets (Alternatives A, B, C, and D through modeling and analysis. Each of the four flood mitigation alternatives consists of dozens of individual elements, including gated control structures, pumps stations, additional storage areas (ponds, ditches, and pipes varying in size from 18 to 60 inches in diameter. ES-1

8 Executive Summary Alternatives A, B and C use nearly identical SMS mitigation options in Ranier and Wilshire Villages, while the proposed SMS to convey flow downstream is considerably different in each alternative. Alternative D is similar to Alternative A, but with additional parallel Ranier Village SMS improvements located behind the lots in the southeastern portion of the Village. All the alternatives depend upon an additional outfall (or additional outfall capacity to convey water away from Ashville Park. From a feasibility and construction cost perspective, the four solution sets are comparable. Each alternative would impact wetlands and require wetland permitting. All of the alternatives will likely require individual permits from USACE and Virginia DEQ based on the anticipated magnitude and location of wetland and/or tidal impacts. The probable construction cost of each solution set is significant. The range in probable cost among the solution sets is limited, reducing the significance of project cost as a selection criterion. Ease of construction, impacts during construction, permitting complexity, and resiliency to sea level rise (SLR are relevant aspects that offer more differentiation among the alternative solution sets. Alternative D is the recommended solution set to advance to engineering design. This solution adds a ditch and outfall behind proposed Villages D and E, uses control structures to maintain normal water surface elevations and to prevent backflow, and uses a 20 cubic feet per second (cfs pump station to drawdown the subdivision s ponds, from their normal water level of 2 feet NAVD 88 to 1 foot NAVD 88, prior to the onset of a projected storm. The stormwater pump station and gate on the proposed outfall ditch are at a location that is resilient to SLR where the surrounding ground elevation is high, approximately 9 feet North American Vertical Datum of 1988 (NAVD 88. The proposed parallel system along the eastern roads of Ranier Village is located behind the properties, adjacent to Cayman Lane, to minimize impacts to existing development. Alternative D offers the following advantages: Lowest permitting complexity Least amount of modifications needed to accommodate SLR Least construction impact to existing development in Ranier Village Probable construction cost is one of the least at $21.2M 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. In addition to standard tasks associated with completing a Preliminary Engineering Report (PER and engineering design, CDM Smith proposes several recommendations for advancing Alternative D: Perform a wetland delineation (Preliminary Jurisdictional Determination (PJD and Wetland Confirmation from US Army Corps of Engineers (USACE to identify jurisdictional areas and probable project impacts prior to consultation with the regulatory agencies. ES-2

9 Executive Summary Evaluate refinements to the configuration of the recommended alternative to minimize wetland impacts, striving to reduce impacts below 1 acre to comply with USACE State Programmatic General Permit (12-SPGP-01 and Virginia Department of Environmental Quality (DEQ Virginia Water Protection Compliance Program (VWP permitting. Wetland impacts could be minimized by locating the proposed ditch, intake pond, and pump station outside wetland areas. Undertake early pre-application consultation with the USACE, the Virginia DEQ, and the Virginia Marine Resources Commission (VMRC to introduce the project and its objectives and address agency concerns. Consult with United States Fish and Wildlife Service (USFWS on potential impacts within the Back Bay National Wildlife Refuge along Sandbridge and New Bridge Roads, even if all impacts are within the right-of-way or city-owned easements. Acquisition of right-of-way, easements and property are essential for implementing the recommended solution (or any of the other final alternatives considered, and should be pursued as needed to support engineering design and project implementation. Conduct additional evaluations, such as a seepage analysis, to confirm interaction between proposed ditches and adjacent wetland areas, and determine if proposed channel will require improvements to prevent groundwater intrusion. Confirm the content of future development, impervious area, and size and location of additional Best Management Practices (BMPs. The analysis conducted for this study assumed future construction of BMPs 10A, 16 and 17 to store runoff from the future Villages C, D and E. The analysis also assumed culvert connections from BMPs 10A, 16 and 17 to the existing and proposed SMS per the recommendations provided by the developer s engineer. For the purposes of this report, the infrastructure items previously recommended by the developer s engineer are referred to as developer improvements or infrastructure, and CDM Smith s recommendations are referred to as proposed improvements. The combination of developer improvements and CDM Smith proposed improvements comprise the SMS improvements needed to meet the established LOS. ES-3

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11 Ü Legend Existing Stormwater Pipe BE AU XL RE N TY Planned Storage W AY!!! BU T Planned Ditch!!! Parcel TE R MI LK C T CH AM PIO NC %, L IR Proposed Improvements BMP 6 Expansion: approx. footprint at normal pool elv. PS 3 7 Pump Station \ C Control Structure 7 REIN LN Alternative Item No. Box Culvert Stormwater Pipe 18 Force Main DR CHANNING LN 8 1 RD RD PEPPERLIN DR NNE SA 6 0 CAY MA NL N AS HV Village D 3 6 CES BL VD ft NAVD Gated Control Structure 10A ILL EP AR K \ C TERRAMAR LN 36 N PRI 0 3 LN QUINCY WAY PRESTON DR MONTECITO DR GRANDON LOOP 5 1 DR 8 1 AG AN S R BRIGHTWOOD FLA N 7 X1 4 Rainier Village GE RD EMELITA DR ASH VILL E PA RK B L Village C 2 4 CAMARILLO LN 3 6 LUB AO BMP Addition or Expansion Pump Intake Pool Ditch 4 2 LN 24 Wilshire Village!!! SD !!! K LN BLY T HE Dredge Existing Ditch LO TU 3 0!!! KEO KIR 18!!! SANDB RID R 1 8 GE D 24 RID 24 KIT T 8 D BMP 2A: approx. footprint at normal pool elv. Ash vi SEAB OARD R lle B Village E ri d g US Fish and Wildlife Property e Cr eek 17 PS 3 7 \ C 1 0 X6 20 cfs Pump Station and Gated Control Structure Pump Intake Pool: approx 0.5 acres 10 X6 Elbow Farms Property W NE 0 1 in = 300 ft 900 Feet B DG RI E RD FIGURE ES-1 City of Virginia Beach Ashville Park Solution Set: Alternative D December 2016

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13 Section 1 Introduction The Ashville Park subdivision is in the southeastern portion of the City of Virginia Beach (City, located south of Sandbridge Road and east of Princess Anne Road. As presented in Figure 1-1, Ashville Park is in the City s transition area south of the Green Line ; construction of the current development began in The drainage system that serves Ashville Park consists of a system of linear lakes that outfall to a roadside ditch along Flanagan s Lane, which conveys flows to the channel along Sandbridge Road, ultimately discharging to Ashville Bridge Creek. Figure 1-2 depicts the existing development and plans for future growth. Since construction of the stormwater management system (SMS, localized flooding has been observed within the development. Flooding occurs during heavy rain events, and is most significant within the areas of Ranier Village, Wilshire Village, and Ashville Park Boulevard. Specific rainfall events that resulted in flooding reports include: September 8 9, 2014, a more recent rainfall event from September 19 21, 2016 (remnants of Tropical Storm Julia, and October 8 10, 2016 (Hurricane Matthew. To address these recurrent flooding events, the City retained CDM Smith to develop a hydrologic and hydraulic (H&H model of the study area for existing and build-out conditions. The H&H model used for this study includes the drainage system within Ashville Park and external drainage features that interact with and influence it. For example, the Heritage Park subdivision area that drains to the roadside ditch along Flanagan s Lane is included in the model, as well as the drainage system that conveys stormwater flows to Ashville Bridge Creek. Storage and overland flows have been added to more closely represent flood routing for conditions when stormwater runoff flows exceed system capacity. CDM Smith calibrated the model to the September 19 21, 2016 storm event (the remnants of Tropical Storm Julia. For calibration, model results were compared with estimated flood depths derived from photos of observed water levels. The site visit descriptions, photos, and methodologies are presented in Appendix A. To achieve calibration, model parameters were adjusted until a reasonable agreement between estimated flood depths and model 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 storm events under existing and build-out land use conditions. CDM Smith then modeled potential system improvements to evaluate the benefits of each. More than two dozen alternatives were evaluated to alleviate flooding for the 10-year and 100-year design storms and 1-1

14 Section 1 Introduction subsequently culled to nine overall draft alternatives for thorough modeling and evaluation of improvement alternatives. Alternatives were then evaluated on their ability to provide the following minimum level of service (LOS: No road flooding for the 10-year storm event Crowns of roads not to be exceeded by 3-inches for the 100-year storm event with no structural flooding occurring Ultimately, four of the nine draft alternatives were shortlisted as most feasible based on implementation and probable construction cost considerations. This report presents the net benefits of each alternative, implementation considerations, and opinions of probable construction cost to support the City s selection of a preferred solution. A brief description of all alternatives is provided in Appendix E. 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 for existing and build-out conditions with respect to 10-year and 100-year LOS. Section 5 provides alternative solution sets to achieve the established LOS. Section 6 describes anticipated environmental permitting required for each alternative solution set, a summary of the construction cost opinion and considerations to accommodate future sea level rise (SLR conditions. Section 7 presents a summary of the final alternative solution sets and the recommended solution. Appendix A contains field investigation documentation for observed flooding conditions 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. Appendix D contains detailed information about the opinion of probable construction costs presented in Section 5. Appendix E contains a brief description of all analyzed alternatives. 1-2

15 PRINCESS ANNE RD UPTON DR NIMMO PKWY SANDBRIDGE RD PRINCESS ANNE RD INDIAN RIVER RD N MUDDY CREEK RD FIGURE 1.1 City of Virginia Beach Ashville Park Overview Miles Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community Legend Ashville Park Subdivision Streets_Centerlines Transition Area Green Line February 2017

16 BMP7 BMP8 Wilshire Village Village C BMP1 BMP3 BMP2 BMP4 BMP5 BMP6 Ranier Village BMP15 BMP9 BMP12 BMP13 BMP14 BMP10 BMP11 Village D Village E FIGURE 1.2 City of Virginia Beach Ashville Park Subdivision Miles Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community Legend Ashville Park Subdivision February 2017

17 Section 2 Hydrologic and Hydraulic Model Development 2.1 Basin Description The Ashville Park Subdivision is spread across an area of 460 acres bounded by Princess Anne Road to the west and Sandbridge Road to the east. Prior to development, the land was used for agricultural purposes. In 2006, development began with the construction of Ashville Park Boulevard and Wilshire Village in the western portion of the subdivision (see Figure 1-2. The SMS was constructed prior to construction of the residential units. The SMS includes stormwater pipes that drain the streets within each village, stormwater pipes that drain Ashville Park Boulevard, and 15 linear BMP ponds that receive stormwater runoff conveyed by the stormwater pipes. The SMS discharges to three outfalls: two minor outfalls to the north (which discharge little flow, and one primary outfall to Ashville Bridge Creek to the east. Discharge to the outfall at Ashville Bridge Creek is conveyed from Ashville Park through ditches along Flanagans Lane and Sandbridge Road. 2.2 Hydrologic and Hydraulic Model Setup The primary objective for developing H&H models for the SMS is to provide a tool suitable for evaluating SMS performance. The US EPA Stormwater Management Model (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. Stormwater models were developed using PCSWMM Version 6.3 by Computational Hydraulics International (CHI. PCSWMM uses the US EPA SWMM Version computational engine, and includes a custom graphical user interface (GUI that offers GIS functionality, model building, calibration, and post processing tools. A number of data sources supported the development of the H&H model. Appendix B includes a detailed discussion of specific data sources and methodology. The best available data for topography is generally the citywide LiDAR coverage. For Ashville Park, the available LiDAR data was acquired prior to completion of grading and road construction in Ranier Village; as a result, it was not useful in southern Ranier Village, south of the southern branch of Grandon Loop Road. In areas where the City s spatial data and LiDAR coverage do not reflect the current extent of development, hydrologic parameters (soils, impervious area coverage, and land use were estimated based on available engineering plans and aerial imagery. Model parameters to represent the hydraulics of the SMS were derived from City GIS and as-built plans. 2-1

18 Section 2 Hydrologic and Hydraulic Model Development 2.3 Hydrologic Modeling The hydrologic portion of the H&H model represents the process for transforming rainfall to stormwater runoff that enters the SMS. For this study two hydrologic conditions were modeled: Existing Conditions: Present site condition, utilizing the original SMS, but with residential units in Wilshire and Ranier villages constructed (Figure 2-1. This condition was used for the calibration storm and for estimating existing LOS. Build-Out Conditions: Site conditions after completion of Villages C, D, and E, and improvements as previously recommended by the developer s engineer, including the following modifications in the eastern portion of the subdivision (Figure 2-2: Connecting BMP 10 and 15 by a channel that is approximately 900 linear feet with 3H:1V side slopes BMP 10A proposed to the South of Ashville Park Boulevard and connected to BMP 10 and 11 BMP 13 extended along Ashville Park Boulevard BMP 16 proposed in future Village C BMP 17 proposed in future Village E This condition was used to estimate the baseline future LOS, without additional improvements. As discussed in Section 5, select improvements previously recommended by the developer s engineer were used and modified as necessary to meet the LOS. Evaluating existing hydrology produces data on the performance of the existing SMS with current development conditions. Build-out condition analyses evaluate performance of the existing SMS under build-out conditions and with potential SMS improvements. The H&H model representation of Villages C, D, and E on the eastern side of the subdivision includes SMS infrastructure previously recommended by the developer s engineer consisting of three additional BMP ponds 10A, 16, and 17 and connections between BMP Ponds 15 and

19 BMP7 BMP8 BMP1 BMP3 BMP2 BMP4 BMP5 BMP6 BMP15 BMP9 BMP12 BMP13 BMP14 BMP10 BMP11 FIGURE 2.1 City of Virginia Beach Built Parcels: Existing Scenario Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community Legend Built Parcels Ashville Park Subdivision Miles February 2017

20 BMP7 BMP8 BMP1 BMP3 BMP2 BMP4 BMP5 BMP6 BMP16 BMP15 BMP9 BMP10 BMP11 BMP12 BMP13 BMP10A BMP17 FIGURE 2-2 City of Virginia Beach Built Parcels: Build Out Scenario Miles Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community Legend Developer Planned Channel Developer Planned Storage Built Parcels Ashville Park Subdivision December 2016

21 Section 2 Hydrologic and Hydraulic Model Development Existing Conditions Hydrology 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 Soils In general, local soils boring data and the Natural Resource Conservation Services (NRCS SSURGO soils database were used to assign soil parameters for pervious areas in the H&H model. For the remaining areas, the soils data was refined based on the soil borings from the developer. They were assigned parameters reflecting the high clay content of existing soils. Figure 2-3 shows the distribution in soil conductivity across the study area. For the purpose of sizing SMS improvements, it was conservative to use soil parameters that produce low infiltration rates. However, future refinement or application of the model to evaluate future development should use local boring data or SSURGO soils data for conducting pre- and post-development analyses to size detention volumes as opposed to relying on the final models for this study Land Use Appendix B provides a detailed description of the source and application of land use data to support model development. The City maintains a citywide GIS dataset of land use, and for existing development outside of Ashville Park, that data was used to define land use for existing hydrology in the H&H model. As discussed in Appendix B, the City s available GIS data does not reflect land use from recent development in Ashville Park. Therefore, the City s land use coverage was not used to estimate impervious cover (see Section It was applied to estimate additional hydrologic parameters, such as roughness and depression storage. These estimates are based on global databases developed over dozens of projects. It should be noted that the model is more sensitive to impervious cover than to these land use-derived hydrologic parameters. 2-5

22 FIGURE 2-3 City of Virginia Beach Ashville Park Soils Conductivity Miles Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and Legend the GIS User Community Ashville Park Subdivision Conductivity(in/hr January 2017

23 Section 2 Hydrologic and Hydraulic Model Development The majority of land use in the Ashville Park subdivision was estimated to be agricultural (56 percent. Medium density residential area accounts for 21 percent of the coverage. Waterbodies account for nearly 10 percent of the area, while the remaining land uses account for 13 percent of the total area. Figure 2-4 presents a breakdown of the land use in the Ashville Park subdivision. Agriculture 10% Medium Density Residential 7% Commercial, Light Industrial, and Institutional Heavy Industrial and Transportation 6% 21% 56% Waterbodies Figure 2-4 Breakdown of Ashville Park Existing Conditions Land Use Imperviousness Since the City s spatial data for impervious cover does not reflect the majority of the developed subdivision, CDM Smith estimated impervious values based on aerial coverage and lot plans as described in Appendix B. Based on the aerial coverage of developed parcels, the average lot impervious area was assumed to be 38% which is consistent with pervious area calculations. The aggregate impervious cover distribution of undeveloped lots is 5 percent, based on the area covered by sidewalks in these subcatchments. Figure 2-5 presents a breakdown of the impervious cover within Ashville Park for existing conditions, with Wilshire and Ranier Villages being fully developed. Based on the imperviousness assumptions stated above, 72 percent of the watershed is considered pervious, 18 percent is considered impervious, and 10 percent is considered impervious but routed to pervious areas, as discussed in Appendix B, Section B It should be noted that not all subcatchments are covered by lots. Some areas within the existing development are open space and some areas remain undeveloped, such as the proposed locations of Villages C, D, and E. 2-7

24 Section 2 Hydrologic and Hydraulic Model Development Impervious Pervious Figure 2-5 Breakdown of Ashville Park Existing Conditions Imperviousness Build-out Conditions Hydrology Soils The soil distribution for pervious areas within the study area was assumed to remain unchanged from existing to build-out conditions. As noted below, the land use type and impervious area are updated for build-out conditions Land Use The land use within Ranier and Wilshire Villages is the same for both existing and build-out conditions. Land use definitions were updated for areas where future development has been proposed, and the land use types for future development was assigned as medium density residential based on their assumed impervious cover, as explained in Appendix B. Figure 2-6 presents a breakdown of the land use for build-out conditions in the Ashville Park subdivision. 2-8

25 Section 2 Hydrologic and Hydraulic Model Development Agriculture Medium Density Residential 4% 5% 3% 9% Commercial, Light Industrial, and Institutional Heavy Industrial and Transportation Waterbodies 79% Figure 2-6 Breakdown of Ashville Park Build-out Conditions Land Use Imperviousness The impervious cover in the proposed Village C, D, and E subdivisions were assumed to be consistent with the density of impervious cover in Wilshire Village. The assumed impervious cover was based on Wilshire Village since it has similar lot sizes to the proposed lots contained in previously submitted development plans. Figure 2-7 presents a breakdown of the proposed build-out impervious cover within Ashville Park. Impervious Pervious Figure 2-7 Breakdown of Ashville Park Build-out Conditions Imperviousness 2-9

26 Section 2 Hydrologic and Hydraulic Model Development 2.4 Hydraulic Modeling Existing Condition Stormwater System A schematic of the SMS constructed to serve Wilshire and Ranier Villages is presented in Figure 2-8. Surface runoff is captured by street inlets and conveyed to 15 linear lakes (BMP Ponds 1 through 15, which in turn direct runoff to one outfall - OF1 to the east, which conveys flow from BMP Pond 14 to the ditch along Flanagan s Lane, extending along Sandbridge Road to Ashville Bridge Creek, eventually flowing to Back Bay. A small portion of the subdivision in Wilshire Village drains north to the historic ditch though an 18-inch diameter outfall pipe (OF2. OF4 conveys flow from a small portion of the development to BMP Pond 8, which discharges to Hell s Point Creek and Ashville Bridge Creek, north of Sandbridge Road. The system outfalls are shown in Figure 2-8. All inlets and pipes in the SMS were included in the model, with the model subbasin delineation refined to a level such that every inlet receives local runoff. Pipes were provided Manning s roughness (n values consistent with new, clean pipe within the subdivision. Local hydraulic losses were provided based on the Virginia Department of Transportation (VDOT guidelines. Appendix B provides detailed information on roughness values and local losses. This SMS includes 5.8 miles of modeled stormwater pipes. The SMS also includes approximately 5 miles of modeled canals and ditches. The Manning s roughness values for the center channel and flood banks for these canals and ditches were estimated from site visits, aerial photography and engineering judgment. A table of typical roughness values is provided in Appendix B, Table B-6. Within Rainier Village and Wilshire Village, most of the roads are fairly flat, with grading to each inlet. Therefore, when the roads flood, water pools above each inlet. If flooding is deep enough, water overtops the crown of the road and connects to the other side. In some cases, flooding can be deep enough to form a pool that connects to the next inlet either upstream or downstream in the SMS. This functionality is modeled with a series of storage units to represent ponding in the road and weir-type overflows to allow flow exchange between adjacent ponding areas. The storage units are provided depth area curves which allow for accurate estimates of flood depths above each inlet. The overflows are similar to weirs, in that they are short and wide (representing the crest of the road; however, irregular sections are used instead of weirs for model stability. More detailed information of overland model elements can be found in Appendix B. BMP Ponds 1 through 4 and BMP Pond 11 were modeled with a series of irregular links to represent the changing cross-sections of the linear ponds. The remaining ponds were modeled as storage nodes with stage storage area curves. Table 2-1 below presents a summary of the model elements for the Ashville Park model existing condition. Figure 2-8 shows the stormwater inventory for the model. 2-10

27 Section 2 Hydrologic and Hydraulic Model Development Table 2-1 Ashville Park Existing Condition Model Elements 1D Model Elements Subbasins 277 Junctions 143 Storage Nodes 226 Outfalls 5 Conduits Circular 309 Ellipse 0 Rectangular Open 0 Rectangular Closed 0 Irregular/Trapezoidal

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29 Ü os OF J1 J D D WIL_ BMP2_ B03A BMP4_2 BMP4_ BMP3_ E BMP3_ BMP4_ BMP E A A03A A A D03A D02B C C05A C C08A A14B A15B C09A F01A A A A A A11A A07D A06A A A08A A A BMP E A12B E A18B A18C E E E E A WS9_RAN_187_S E A13B BMP E E BMP11_ BMP A BMP ASH_ # * OF1 ASH_280 ASH_51 Legend # * OF # * BMP BMP11_ A B BMP11_ BMP BMP11_ BMP J E C10A BMP3_ E E C03B A B02A BMP2_ BMP2_ C02A BMP3_1 BMP3_2 BMP1_ BMP1_ BMP1_2 BMP1_ J D J BMP J OF # * BMP7 WS9_ASH Ashville Park Subdivision Outfalls Storages Junctions Pipes Ditch Lake Overland Flow Control Structure # * ,000 1,500 2,000 Feet # * OF6 10 X 6 OF5 FIGURE 2-8 City of Virginia Beach Ashville Park Existing Condition Model Elements Sources: Esri, HERE, DeLorme, USGS, Intermap, increment P Corp., NRCAN, Esri Japan, METI, Esri China (Hong Kong, Esri (Thailand, MapmyIndia, OpenStreetMap contributors, and the GIS User Community December 2016

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31 Section 2 Hydrologic and Hydraulic Model Development Proposed Build-out Condition Stormwater System Based on the layouts previously submitted by the developer s engineer for Villages C, D and E, additional BMP Ponds 10A, 16, and 17 will be constructed to augment the existing stormwater system. A schematic of the SMS constructed to serve build-out conditions is presented in Figure 2-9. For the model, CDM Smith assumed the SMS associated with the new development would be sufficient to meet the 100-year storm LOS (i.e., all new development runoff would be directed to the new stormwater ponds. Therefore, the SMS, within future villages, were not analyzed with the model. For the build-out condition, BMP Ponds 10A, 16, and 17 were added to the model, including pipes and ditches connecting the additional BMPs to the existing system. Changes have been made to the subbasin delineation to account for the additional ponds and changes to runoff load points. Changes to the model hydrology were discussed above. Table 2-2 below presents a summary of the model elements for the Ashville Park model build-out condition. Figure 2-9 shows the stormwater inventory for the model. Table 2-2 Ashville Park Proposed Model Elements 1D Model Elements Subbasins 279 Junctions 147 Storage Nodes 229 Outfalls 5 Circular 309 Ellipse 0 Conduits Rectangular Open 0 Rectangular Closed 0 Irregular/Trapezoidal

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33 Ü os OF J1 J D3 D BMP1_ BMP1_2 WIL_ C03B C BMP4_ BMP3_ A A03A E VC_6-5 VC_6-2 BMP A11A C10A A A A A A18B A18C A A36 BMP E E BMP E BMP10A BMP11_ BMP BMP10A BMP11_ # * BMP10A # * OF4 OF1 BMP17 ASH_280 ASH_51 Legend BMP # * BMP A B BMP11_ BMP BMP11_ A BMP BMP15_J E E E E21 J E A12B WS9_RAN_187_S E A13B A14B A15B C09A F01A A09 VC_ E A06A A08A A47 VC_6-6 BMP E BMP3_ BMP4_ A A D03A D02B BMP4_1 BMP2_ B03A BMP3_ C08A C C11A E E BMP3_1 BMP3_ A B02A BMP2_ C02A BMP2_ BMP1_4 BMP1_ D J J BMP J OF # * BMP # * Ashville Park Subdivision Outfalls Storages Junctions Pipes Ditch Lake Overland Flow Control Structure # * ,000 1,500 2,000 Feet OF6 10 X 6 OF5 FIGURE 2-9 City of Virginia Beach Ashville Park Build Out Condition Model Elements Sources: Esri, HERE, DeLorme, USGS, Intermap, increment P Corp., NRCAN, Esri Japan, METI, Esri China (Hong Kong, Esri (Thailand, MapmyIndia, OpenStreetMap contributors, and the GIS User Community December 2016

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35 Section 3 Model Calibration Model calibration is the adjustment of model parameters to match observed conditions. The Ashville Park model was calibrated based on photographs of flooding during the September 2016 (remnants of Tropical Storm Julia storm. The September 2016 event was used for model calibration because it occurred with existing hydrologic conditions and detailed observed flood elevation data was available. Additionally, since the tide data for this event was recorded by the City installed temporary gauge at Ashville Bridge Creek, a true representative boundary condition could be applied within the model. CDM Smith reviewed photos documenting the event and estimated peak flood depths at multiple locations. 3.1 Calibration Storm The storm selected as model calibration event is characterized below. September 2016 (Remnants of Tropical Storm Julia: Approximately inches of precipitation over approximately a 60-hour period in the City. Maximum intensity was 0.26 inches in a 5-minute period (3.1 inches per hour. Maximum hourly total was 1.5 inches. Total storm volume was much larger than the 100-year storm. Peak intensity was closer to the 2-year storm (see Table B-2 in Appendix B for design storm volumes and intensities. Peak stage in Ashville Bridge Creek reached 3.2 feet NAVD 88 near 11 p.m. on September 21, 2016 (approximately 50 hours after the beginning of the storm. Two rainfall gauges were examined to characterize the rainfall spatial and temporal variation. Rainfall was provided in 5-minute intervals from the City s Building 2 and Sandbridge gauges (Figure 3-1. The Building 2 gauge recorded a total rainfall of 14.9 inches and the Sandbridge gauge recorded 14.4 inches. Since the total rainfall depths at the two locations are comparable and follow similar distributions, the Sandbridge gauge was assigned to all the subcatchments. 3-1

36 Section 3 Model Calibration Figure 3-1 September 19 22, 2016 Rainfall Hyetograph, Sandbridge Gauge 3.2 Tidal Boundary Conditions Tide data was recorded by the City-installed temporary gauge at Ashville Bridge Creek for the September 2016 remnants of Tropical Storm Julia. Data extracted from this gauge in feet NAVD 88 was used directly as a time series boundary condition for the September 2016 storm in the model and assigned to outfalls OF1, OF4, and OF6 (Figure 3-2. The outfall stage value at OF2 (5.3 feet NAVD 88 was derived from LiDAR and surrounding stage elevations. Outfall OF5 discharges to agricultural land south of the study area and was modeled using a normal depth boundary condition. Figure 3-2 Water Level at Ashville Bridge Creek Outfall, September 19 22,

37 Section 3 Model Calibration 3.3 Calibration The existing conditions model was run for the remnants of Tropical Storm Julia from September 19, 2016, at 8 a.m. to September 24, 2016, at 8:00 a.m. The start time was set at a relatively low tide condition, prior to the onset of rainfall, to match the initial depths in the model setup. The end time was set to allow for estimates of recovery time. Since the calibration observations occurred at approximately 10:00 a.m. on September 21, 2016, and 2:00 a.m. on September 22, 2016, the model elevations at the calibration locations were extracted at those times. The calibration model results for the locations shown in Figure 3-3 are shown in Table 3-1 at approximately 10:00 a.m. on September 21, These flood elevations were observed during the field reconnaissance described in Appendix A, which includes photographing the flooding. Appendix A describes the methodology used to estimate flood elevations and associated uncertainty. The uncertainty does not take into account the vertical accuracy of the LiDAR Digital Elevation Model (DEM (see Appendix B. Because the model was built from the DEM, the absolute accuracy of the LiDAR is not as important as the relative differences. One of the elements contributing to model calibration was selection of soil infiltration parameters. Soil borings were available for portions of Ashville Park Boulevard and within the Ashville Park subdivision. The borings showed clay content, and were used to define the Green- Ampt Soils parameters throughout the model. During calibration the soils parameters in the Wilshire neighborhood were defined using SSURGO soils data where soil borings were not available and to improve the match to observed flood conditions. As shown in Figure 2-3, soil infiltration parameters reflecting low infiltration rates were also applied to portions of the study area outside of the Ashville Park subdivision. For the purpose of sizing SMS improvements, it was conservative to use soil parameters that produce low infiltration rates. However, future refinement or application of the model to evaluate future development should use local boring data or SSURGO soils data for conducting pre- and post-development analyses to size detention volumes as opposed to relying on the final models for this study. 3-3

38 Section 3 Model Calibration Table 3-1 September 21, 2016 Flood Elevations ID Location Observed Flood Elevations (feet NAVD 88 Model Delta (feet Emelita Dr and Wilshire Dr Blythe Dr and Wilshire Dr Lubao Ln and Aldea Cir Aldea Cir and Blythe Dr Lubao Ln between Emelita Dr and Ashville Park Blvd Lubao Ln and Ashville Park Blvd C04 Near 1908 Channing Ln C05A Channing Ln and Grandon Loop Rd A17A Grandon Loop Rd and Pepperlin Dr A05A Grandon Loop Rd and Brightwood Dr A02 Channing Ln and Brightwood Dr Camarillo Ln and Wilshire Dr A Grandon Loop Rd (9/22, 2 am

39 REX LN REIN LN PRINCESS ANNE RD KITTRIDGE DR LUBAO LN ^_ ALDEA CIR ^_ BLYTHE DR ^_ ^_ WILSHIRE DR ^_ KEOKIRK LN CAMARILLO LN BENECIA DR EMELITA DR ^_ ^_ ^_ C04 ^_ MONTECITO DR BRIGHTWOOD DR ^_ A C05A ^_ CHANNING LN GRANDON LOOP RD A05A ASHVILLE PARK BLVD ^_ A38 ^_ A17A CAYMAN LN FIGURE 3-3 City of Virginia Beach Ashville Park Model Calibration Locations September 2016 Source: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS User Community ^_ Legend Calibration Locations Ashville Park Subdivision Miles January 2017

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41 Section 4 Existing and Build-out Model Comparison The calibrated model was run for the 10-year, and 100-year NOAA Type C 24-hour design storm events under existing and build-out conditions. The methodologies used to develop the design storm hyetographs are presented in Appendix B, Section B.4. The 10-year design storm volume is 5.6 inches with a 6-minute peak intensity of 6.4 inches per hour. The 100-year design storm volume is 9.5 inches with a peak 6-minute intensity of 10.7 inches per hour (the hyetographs are discretized in 6-minute intervals. The existing condition model is similar to the calibrated model. The existing hydrologic conditions (such as the amount of impervious cover and primary hydraulic conditions (e.g., pipes, culverts, and ditches are the same in both. There is one main difference: The tailwater condition in Ashville Bridge Creek is set to a fixed stage determined by storm. Use of fixed stage boundary conditions is a conservative estimate, and in the case of Ashville Bridge Creek, appropriate due to possible wind-driven tailwater effects. Fixed tailwater elevations in Ashville Bridge Creek were set at 2.3 feet NAVD 88 for the 10-year storm and 3.1 feet NAVD 88 for the 100-year storm. Due to the unknown levels in the historic ditch north of Wilshire Village, the stage for Outfall OF2, representing the ditch level, remained fixed at 5.3 (same as for the validation for both storms. Initial water depths in the model for both the 10- year and 100-year storm simulations are identical within the Ashville Park subdivision because they are set by the outfall control structure. Outside of the subdivision, initial water depths are set to equilibrium with the fixed outfall stages. The build-out model represents a proposed condition where Ashville Park Villages C, D, and E are developed, with the associated stormwater ponds and additional conveyances. As discussed above, the most significant changes from the existing model to the build-out model include: Increased impervious cover Re-delineation of subbasins to encompass the new development and stormwater ponds Additional stormwater BMP Ponds 10A, 16, and 17 Stormwater pipes from BMP Ponds 16 and 17 to the existing system Additional ditches along Ashville Park Blvd connecting the outfall of the pipe from BMP Pond 17 to BMP Pond 10 and BMP Pond 10A 4-1

42 Section 4 Existing and Build-out Model Comparison 4.1 Target Level of Service (LOS The target LOS for the 10-year and 100-year, 24-hour design storms are similar to the City s Public Works Standards. The guidance specifies no flooding above stormwater inlet rim elevations (or 6 inches above a drop inlet rim elevation for the 10-year storm and visible road crowns or top of curb for the 100-year storm. To model the LOS for this project, indicator elevations were estimated from the DEM, where recent LiDAR was available and from design plans where it was not (south Ranier Village. Some depth of flooding above a road inlet as runoff flows into the stormwater system is expected. The system is considered to not meet LOS at the inlets and road crowns if the indicator elevations are exceeded by more than 3 inches (0.25 feet. Indicator elevations were provided to all roads and inlets in the Ranier and Wilshire Villages, to Ashville Park Boulevard inlets, and to critical locations in Flanagan s Lane and Sandbridge Road downstream of the subdivision. 4.2 Existing Model Analysis year Design Storm The results of the existing condition simulations indicate that 126 of 227 inlets do not meet the LOS described above for the 10-year storm. The complete list of indicator elevations and flood depths is presented in Appendix C. Table 4-1 is a summary of the expected flooding at some of the critical road and/or intersection low points. The maximum flood depth is 1.5 feet above the inlet indicator elevation for the 10-year storm at Lubao Drive near Aldea Circle. This summary table represents locations in the subdivision that experience the deepest flooding in the neighborhood or locations where it has proven difficult to identify alternatives to relieve flooding (see below year Design Storm The results of the existing condition simulations indicate that 171 of 227 locations do not meet the LOS described above for the 100-year storm. The complete list of 100-year storm flood depths for the existing condition are presented in Appendix C. Table 4-1 summarizes the expected flooding for the 100-year storm at critical locations. The maximum flood depth is 2.1 feet above the road crown indicator elevation for the 100-year storm at Lubao Drive, west of BMP Pond 1 between Kittridge Drive and Ashville Park Boulevard. For locations such as this one, where the road is separated with a median, the road crown is the represented by the highest side of the single lane, not the height of the median. Additional insight into the existing conditions flood depths was provided by additional model simulations that were completed with BMP Ponds 1 through 4 maintained at their normal water level of 2 feet NAVD 88 throughout the simulation. Even in this optimum and physically impossible condition the existing SMS still did not meet the desired LOS in southern portion of Ranier Village and the northern portion of Wilshire Village, indicating that the stormwater pipes in both villages are undersized. Unlike the model calibration, which was completed to match observed flooding, the design storm simulation with static boundary conditions at BMP Ponds 1 through 4 was completed to evaluate the LOS provided by the existing stormwater pipes. 4-2

43 Section 4 Existing and Build-out Model Comparison Table 4-1 Summary of Results for the Existing Conditions Location Model Node Critical Elevations (feet NAVD 88 Flowline (10-yr Crown (100-yr 10-yr Storm Peak Stages (feet NAVD 88 Delta from Flowline (feet 100-yr Storm Peak Stages (feet NAVD 88 Lubao Dr west of BMP Wilshire Dr & Blythe Dr Channing Ln & Grandon Loop Rd Brightwood Dr & Ashville Park Blvd Delta from Crown (feet C11A A West Grandon Loop Rd C09A West Channing Ln C03B West Quincy Wy & Grandon Loop Rd East Quincy Wy & Grandon Loop Rd A Montecito Dr A14B Brightwood Dr & Grandon Loop Rd Ashville Park Blvd & Lubao Dr A4A A Lubao Dr & Aldea Cir Quincy Way & Pepperlin Dr Ashville Park Blvd south of BMP 1 Grandon Loop Rd & Pepperlin Dr A18B Blythe Dr & Aldea Cir Lubao Dr north of Kittridge Dr Camarillo Ln Northeast Loop Camarillo Ln east of Keokirk Ln Terramar Ln & Pepperlin Dr Kittridge Dr Loop Bencia Dr east of Keokirk Ln Terramar Ln east of Pepperlin Camarillo Ln Northwest Loop

44 Section 4 Existing and Build-out Model Comparison 4.3 Build-out Model Analysis year Design Storm The results of the build-out condition simulations indicate that 130 of 227 inlets do not meet the LOS described above for the 10-year storm. The complete list of flood depths for the build-out conditions and 10-year storm are also presented in Appendix C. Table 4-2 summarizes the expected flooding at the same critical locations as Table 4-1. For the 10-year storm, the maximum flood depth is again at Lubao Drive near Aldea Circle, 1.5 feet above the inlet indicator elevation. The build-out condition is expected to perform slightly worse than the existing conditions, with a maximum increase of 0.2 feet and an average increase of 0.02 feet over the 227 indicator elevations. Additionally, durations of flooding are expected to increase. The primary cause of both the increase in peak stages and the duration of flooding are the additional volumes of floodwaters added to the primary drainage system. Though the newer subdivisions are expected to be contained in the new BMPs, additional conveyance to the existing system should cause the entire system to drain more slowly year Design Storm The results of the build-out condition simulations indicate that 180 of 227 locations do not meet the LOS described above for the 100-year storm. The complete list of flood depths for the 100-year storm under build-out conditions are presented in Appendix C. Table 4-2 summarizes the expected flooding for the 100-year storm at critical locations. The maximum flood depth is again 2.1 feet above the road crown indicator elevation for the 100-year storm at Lubao Drive, west of BMP Pond 1 between Kittridge Drive and Ashville Park Boulevard. The build-out condition is expected to perform worse than the existing condition for the 100-year storm as well. In this case, the average increase over the 227 indicator elevation is 0.03 feet with a range of change from feet to plus 0.3 feet. Again, the additional volume of flow through the existing system is likely to cause the increase in peak stage at most locations, as well as an increase in flooding duration. 4-4

45 Section 4 Existing and Build-out Model Comparison Table 4-2 Summary of Results for the Build-out Conditions Location Model Node Critical Elevations (feet NAVD 88 Flowline (10-yr Crown (100-yr 10-yr Storm Peak Stages (feet NAVD 88 Delta from Flowline (feet 100-yr Storm Peak Stages (feet NAVD 88 Lubao Dr west of BMP Wilshire Dr & Blythe Dr Channing Ln & Grandon Loop Rd Brightwood Dr & Ashville Park Blvd Delta from Crown (feet C11A A West Grandon Loop Rd C09A West Channing Ln C03B West Quincy Wy & Grandon Loop Rd East Quincy Wy & Grandon Loop Rd A Montecito Dr A14B Brightwood Dr & Grandon Loop Rd Ashville Park Blvd & Lubao Dr A4A A Lubao Dr & Aldea Cir Quincy Way & Pepperlin Dr Ashville Park Blvd south of BMP 1 Grandon Loop Rd & Pepperlin Dr A18B Blythe Dr & Aldea Cir Lubao Dr north of Kittridge Dr Camarillo Ln Northeast Loop Camarillo Ln east of Keokirk Ln Terramar Ln & Pepperlin Dr Kittridge Dr Loop Bencia Dr east of Keokirk Ln Terramar Ln east of Pepperlin Camarillo Ln Northwest Loop

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47 Section 5 Alternative Solution Sets Though all systems in the Ashville Park subdivision were originally designed to the 10-year storm, the existing condition model results indicate that this LOS is not being met. For the 100-year storm, and the recent extreme weather events, the SMS lacks adequate capacity to convey flows away from the subdivision. The flood depths are too deep for normal traffic to pass through and the duration of flooding is measured in days as opposed to minutes or hours. Two critical issues need to be resolved for the Ashville Park SMS to meet the desired LOS: Increasing Capacity in the Ashville Park SMS The SMS needs to be increased to improve conveyance offsite, from the existing Ranier and Wilshire Villages through to Ashville Bridge Creek. The conveyance of this system backbone needs to be enlarged significantly to reduce the slope of the hydraulic grade profile. Note that the crown of the road at some locations in Table 4-1 are as low as 5.5 feet NAVD 88, while the fixed outfall stage in Ashville Bridge Creek is 3.1 feet NAVD 88. LOS cannot be met until head losses in the SMS from Ranier and Wilshire Villages to Ashville Bridge Creek are reduced to approximately 2.5 feet. Expanding Conveyance Capacity Within the Ranier and Wilshire Villages Increasing the conveyance from the existing Ranier and Wilshire Villages to Ashville Bridge Creek alone will not fix the flooding problems in entire subdivision. Model simulations were completed with BMP Ponds 1 through 4 maintained at 2 feet NAVD 88 throughout the simulation and the existing SMS still did not meet the desired LOS for the 10-year storm in southern portion of Ranier Village and the northern portion of Wilshire Village. It should be noted that the modeled NOAA Type C 10-year storm is of a greater intensity than recent named storms such as Hurricane Matthew and the remnants of Tropical Storm Julia. Hence, these areas show flooding for the 10-year simulation while they may not have been flooded during recent observed rainfall events. The analysis results demonstrate that the existing stormwater pipes within the villages are not large enough to convey flows to the BMP ponds and meet the LOS. Four alternatives were developed from more than two dozen potential combinations of proposed flood mitigation options. These four solutions all significantly increase the conveyance and storage in the SMS from Ranier and Wilshire Villages to Ashville Bridge Creek, and provide parallel conveyance within the Ranier and Wilshire Villages. The first three alternatives use nearly identical options within the Ranier and Wilshire Villages, while the SMS improvements downstream are considerably different. The fourth alternative is similar to the first, but with the additional parallel SMS improvements for the Ranier Village system located behind the lots in the southeastern portion of the Village. Many of the flood mitigation options are identical in all four alternatives. 5-1

48 Section 5 Alternative Solution Sets The following alternatives are derived from the build-out conditions models, because the peak stages are slightly higher in this condition. Therefore, any potential solution to the build-out model is sufficient for meeting LOS for existing conditions as well. 5.1 Alternative Descriptions The following presents a summary of features associated with Alternatives A through D. Table 5-1 and Figure 5-1 through Figure 5-4 provide additional detail of the improvements that comprise each alternative. The alternative improvements identified in this study are conceptual. 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 or to account for future development of Villages C, D and E Alternative A Alternative A expands the SMS conveyance capacity by adding a ditch and outfall behind proposed Villages D and E; uses control structures to maintain lower normal water surface elevations and to prevent backflow; and includes a 20 cfs pump station to draw down the Subdivision s ponds to 1 foot NAVD 88 from their normal water level of 2 feet NAVD 88 prior to the onset of a storm. Depending on seasonal baseflow, this size pump station should be able to draw down the entire system in 24 to 48 hours. Figure 5-1 presents a schematic of the included options in this alternative, Table 5-1 describes each of the improvement, and Table D-1 in Appendix D details the estimated probable cost of each item. For Alternative A, the planning-level opinion of probable construction cost is $20,100,00. Generally, the elements listed for all alternatives are parallel to existing SMS facilities, though occasionally an increase in existing pipe size is required. In either case, these elements are conceptual, and new pipe, parallel pipe, or different shaped pipes could be used if the conveyance capacity is equivalent Alternative B Alternative B expands the SMS conveyance capacity along Ashville Park Boulevard, Flanagan s Lane, and Sandbridge Road; by using control structures to maintain lower normal water surface elevations and to prevent backflow; and by using a 20 cfs pump station to drawdown the Subdivision s ponds to 1 foot NAVD 88 from their normal water level of 2 feet NAVD 88 prior to the onset of a storm. As in Alternative A, this size pump station will be able to draw down the entire system in 24 to 48 hours. The proposed ditch from Alternative A, behind Villages D and E is not included in this alternative. Figure 5-2 presents a schematic of the included options in this alternative, Table 5-1 describes each of the improvement, and Table D-1 in Appendix D details the estimated probable cost of each item. For Alternative B, the planning-level opinion of probable construction cost is $23,500,

49 Section 5 Alternative Solution Sets Alternative C Alternative C expands the SMS conveyance capacity by adding a ditch and outfall behind proposed Villages D and E, by using control structures to maintain lower normal water surface elevations and to prevent backflow, and by using a 50 cfs pump station west of Lubao Drive, near BMP Pond 1 to both lower stages pre-storm from normal water level of 2 feet NAVD 88 to a foot NAVD 88, and help lower stages in BMP Ponds 1 through 4 during the storm. This pump station includes a force main under Princess Anne Road to an improved ditch to a western outfall. This alternative functions similarly to Alternative A, with additional pump capacity to further lower peak stages in BMP Ponds 1 through 4, thus allowing for smaller SMS mitigation systems in Wilshire and Ranier Villages. Figure 5-3 presents a schematic of the included options in this alternative, Table 5-1 describes each of the improvement, and Table D-1 in Appendix D details the estimated probable cost of each item. For Alternative C, the planning-level opinion of probable construction cost is $24,000, Alternative D Alternative D expands the SMS conveyance capacity by adding a ditch and outfall behind the proposed Villages D and E, by using control structures to maintain lower normal water surface elevations and to prevent backflow, and by using a 20 cfs pump station to drawdown the Subdivision s ponds to 1 foot NAVD 88 from their normal water level of 2 feet NAVD 88 prior to the onset of a storm. This size pump station should be able to draw down the entire system in 24 to 48 hours. Alternative D is similar to Alternative A, but with the pump station and gate on the proposed outfall ditch moved from New Bridge Road to a location where the surrounding ground elevation is high. The natural ground elevation at the location of the proposed control structure will serve as a barrier to SLR (on top of surge event. Discussion of improvements to accommodate SLR is presented in Section 6.3. Additionally, the proposed parallel system along the eastern roads of Ranier Village has been moved to behind the properties, adjacent to Cayman Lane. Figure 5-4 presents a schematic of the included options in this alternative, Table 5-1 describes each of the improvement, and Table D-1 in Appendix D details the estimated probable cost of each item. For Alternative D, the planning-level opinion of probable construction cost is $21,200,

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51 Section 5 Alternative Solution Sets Table 5-1 Summary of Elements and Features for Alternatives Table 5-1 Summary of Elements and Features for Alternatives Alternatives Item Feature Type A B C D Comments Refer to Figure 5-1 Refer to Figure 5-2 Refer to Figure 5-3 Refer to Figure 5-4 A.1 Culvert Replace Replace existing culvert under New Bridge Road, ~1,900 feet south of Sandbridge Road, with a 6-feet X 10-feet box culvert, 40 feet in length. N/A Same as Alternate A Same as Alternate A Regrade the New Bridge Road elevations around the culvert. A.2 Pump Station Addition A.3 Ditch Addition A.4 Ditch Improve A.5 Control Structure Addition A.6 Ditch Addition A.7 Ditch Addition Construct a new 20-cfs pump station (PS and gated control structure (CS. This PS must be designed to allow for future upgrades. Construct a 4,800 long ditch behind Villages D and E from BMP Pond 10 southeast to the proposed New Bridge Road CS. Improve 1,050 feet of ditch along Flanagan s Lane and Sandbridge Road. Construct a 6-feet tall by 10-feet wide CS between BMP Pond 14 and the ditch along Flanagan s Lane. Construct 800 feet long ditch between BMP Ponds 15 and 10. Construct a 1,100 linear feet long ditch adjacent to Ashville Park Boulevard between BMP Ponds 4 and 15. N/A N/A Same as Alternate A N/A Same as Alternate A N/A N/A Same as Alternate A Same as Alternate A N/A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A A.8 Culvert Addition Construct 80 feet long 7-feet by 14-feet box culvert. Same as Alternate A Same as Alternate A Same as Alternate A PS operates when the CS is closed and draws down the upstream ponds from the normal water surface elevation of 2 feet to 1 foot NAVD 88 prior to a storm. The PS maintains the elevation until pumping no longer required. Once the effects of a storm have subsided, and the upstream pond elevations are higher than the downstram tailwater elevation in the Ashville Bridge Creek, the CS will fully open and the PS shut off. May require a platting adjustment for Village D. The ditch has a 4-feet bottom and 3:1 side slopes, running from a depth of near -1 foot NAVD 88 to -2 feet NAVD 88. The depth expected to be between 7 to 9 feet, with an ~50-feet wide footprint at grade. The Manning s roughness value of the center channel is 0.025, for a clean, earthen/gravel channel or some version of a lined channel. Table B-6 in Appendix B presents Manning s n values for a range of channel type (from Chow, VT, Open Channel Hydraulics, McGraw-Hill, A seepage barrier may be necessary along a portion of the ditch that abuts wetlands. A seepage analysis is recommended during design to estimate the extent of the barrier. The cost analysis in Appendix D uses a conservative estimate of this barrier The upstream portion of this ditch is lowered from approximately 2 feet NAVD 88 to 0 foot NAVD 88. Improvements end ~ 570 feet east of the intersection of Flanagan s Lane and Sandbridge Road and matching the existing grade at that point. The CS operates to pass baseflow and maintain a normal water surface elevation of ~ 2.0 feet NAVD 88 (current normal water surface elevation is ~3.1 feet NAVD 88. The CS closes to prevent backflow when Ashville Bridge Creek is higher than 2.0 feet. Ditch has a 10-feet wide bottom near -1.5 feet NAVD 88, 3:1 side slopes and a proposed center channel roughness of 0.03 (clean straight channel with short grass. The channel footprint approaches 75 feet in width at grade. Ditch has a 10-feet wide bottom near -1.5 feet NAVD 88, 3:1 side slopes and a proposed center channel roughness of 0.03 (clean straight channel with short grass. The channel footprint approaches 75 feet in width at grade. Culvert required because the property narrows between the subdivision boundary and Ashville Park Boulevard. Provide wing walls on both sides of culvert to minimize head loss. Culvert invert matches the ditch at ~ -1.3 feet NAVD

52 Section 5 Alternative Solution Sets Table 5-1 Summary of Elements and Features for Alternatives Alternatives Item Feature Type A B C D Comments A.9 Pipe Parallel/Addition A.10 Pipe Addition A.11 Culvert Addition A.12 Pipe Parallel/Addition A.13 Pond Addition Refer to Figure 5-1 Refer to Figure 5-2 Refer to Figure 5-3 Refer to Figure 5-4 Construct a parallel system to the existing stormwater pipes from Terramar Lane to a proposed 48-inch outfall to BMP Pond 4, along the eastern roads of Ranier Village. Construct 260 feet long, twin 54-inch diameter outfalls from Channing Lane to BMP Pond 4. Construct a 55 feet long 54-inch diameter culvert across Brightwood Drive. Construct a 2,000 feet long parallel system to the existing stormwater pipes from Terramar Lane to a proposed 54-inch outfall to BMP Pond 4. System ties into the proposed 54-inch connection to BMP Pond 2 (described in A.11. Construct BMP Pond 2A between Princess Anne Road and Rainier Village. Same as Alternate A Same as Alternate A N/A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A, except 60- inch outfall to BMP Pond 4 Same as Alternate A, except 60-inch outfall to BMP Pond 4 Same as Alternate A Same as Alternate A Same as Alternate A N/A The system is ~ 1,600 feet long and from Terramar Lane approximately 550 east of Pepperlin Drive and Quincy Way, approximately 280 feet east of Pepperlin Drive to the intersection of Terramar Lane and Quincy Way, along Quincy Way to Grandon Loop Road, along Grandon Loop Road to Channing Lane and then along Channing Lane to an outfall that parallels the exist. outfall. Pipe diameter range from 24- to 48-inches. One pipe equalizes flow from BMP Pond 2 to BMP Pond 4, while the second is part of the upgrades in Ranier Village described in A.12 below. Construct from the exist 48-inch outfall from Channing Lane to BMP Pond 2, to the proposed outfall from Channing Lane to BMP Pond 4 described in A.12. The system extends from ~ 100 feet west and 200 feet east of Pepperlin Drive along Terramar Lane, along Pepperlin Drive from Terramar Lane to southern Grandon Loop Road, around the square along Montecito Drive and then from northern Grandon Loop Road to Channing Lane along Brightwood Drive. Pipe diameter range from 30- to 60-inches. This pond does not connect to existing BMP Pond 2. The pond bottom is ~ -3.5 feet NAVD 88, with the normal water surface covering ~ 1.4 acres. The total footprint is expected to be 2.8 acres at grade. A.14 Pipe Addition Install a 200 feet long, 36-inch pipe to convey flow from Grandon Loop Road to proposed Pond 2A. Same as Alternate A Same as Alternate A Same as Alternate A A.15 Pipe Addition Construct a new system with 800 feet of pipe along Grandon Loop Road to the intersection of Grandon Loop Road and Quincy Way, and along Quincy Way to the same intersection, tying into the proposed 36- inch pipe (Item A.14 that outfalls to proposed BMP Pond 2A. Same as Alternate A Same as Alternate A Same as Alternate A except pipe diameter range from 18-inch in diameter to a 19-inch by 30-inch elliptical pipe. This system connects to the existing systems on both streets and is parallel to some of the smaller pipe on Quincy Way and a short section of Terramar Lane. A.16 Culvert Parallel/Addition Construct 230 feet of parallel 18-inch diameter culvert to connect BMP Pond 4 to Pond 3. Same as Alternate A Same as Alternate A Same as Alternate A A.17 Culvert Parallel/Addition Construct 240 feet of parallel 60-inch diameter culvert to connect BMP Pond 1 to Pond 3. Same as Alternate A Same as Alternate A Same as Alternate A A.18 Pipe Addition Construct 200 feet of 34-by 53-inch elliptical pipe to connect the improved historic ditch (Item A.19 to BMP Pond 3. Same as Alternate A Same as Alternate A, except install 29- by 45- inch elliptical pipe Same as Alternate A, except install 32- by 49-inch elliptical pipe. 5-6

53 Section 5 Alternative Solution Sets Table 5-1 Summary of Elements and Features for Alternatives Alternatives Item Feature Type A B C D Comments A.19 Ditch Improve A.20 Pipe Abandon Refer to Figure 5-1 Refer to Figure 5-2 Refer to Figure 5-3 Refer to Figure 5-4 Improve the historic ditch located between Wilshire Drive and Keokirk Lane that extends from Emelita Drive to Camarillo Lane. Plug and abandon the 18-inch diameter pipe under Camarillo Lane. Same as Alternate A Same as Alternate A Same as Alternate A Improve to a trapezoidal channel, with an 8-feet wide bottom at 2 feet NAVD 88 and 3:1 side slopes to meet existing grade. The center channel Manning s roughness value = The existing break in the ditch (likely a remnant of an old farm road to be removed (the removal of the farm road is planned by the City, regardless of alternative and thus not included in the cost estimate; however, the channel dredging is included. Same as Alternate A Same as Alternate A Same as Alternate A Required to prevent flow north of the subdivision entering the system. A.21 Pipe Parallel/Addition Construct a parallel system to the existing pipes along Camarillo Lane from the northwest corner loop to the northeast corner loop, tying into the proposed 42-inch outfall to the expanded BMP Pond 6 (Item A.23. The system is ~ 1,500 feet long with pipe diameter range from 18- to 36-inches. Same as Alternate A Same as Alternate A, except pipe diameter range from 18- to 30- inches. Same as Alternate A, except pipe diameter range from 18- to 30-inches. This system connects to the historic ditch with open outfalls/intakes. A.22 Pond Improve A.23 Pipe Addition Expand the northern portion of BMP Pond 6 to the west towards Camarillo Lane. Install two 42-inch diameter pipes to convey flows from Camarillo Lane to the expanded BMP Pond 6. Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A The bottom of the expanded pond will be similar to the existing section, with the normal water surface covering an additional 3 acres. The total additional footprint is expected to be close to 4.5 acres at grade. The northern pipe is 375 feet long, from the northeast corner of Camarillo Lane to BMP Pond 6. The southern pipe is 400 feet long from the intersection of Benecia Drive and Camarillo Lane to BMP Pond 6. A.24 Pipe Parallel/Addition Install 700 feet of additional 24-inch diameter pipe parallel to the existing stormwater lines along Bencia Drive from Keokirk Lane to Camarillo Lane, tying into the proposed 42-inch outfall to the expanded BMP Pond 6 (Item A.23. Same as Alternate A Same as Alternate A Same as Alternate A This parallel system connects into existing systems on both ends. A.25 Pipe Parallel/Addition Install 1,150 feet of pipe parallel to the existing stormwater lines along Wilshire Drive from 200 feet north of Blythe Drive to an outfall to BMP Pond 1. Same as Alternate A Same as Alternate A Same as Alternate A Pipe diameter range from 18- to 36-inches. This system ties into the outfall to the historic ditch and eventually the Benecia Drive system, and a proposed Blythe Drive system (Item A.26. A.26 Pipe Parallel/Addition Install 680 feet of pipe parallel to the existing stormwater pipes along western Camarillo Lane and Blythe Drive from 220 feet north of Blythe Drive to the intersection of Blythe Drive and Wilshire Drive. Same as Alternate A Same as Alternate A Same as Alternate A Pipe diameter range from 24- to 36-inches. This parallel system ties into the proposed Wilshire system (Item A

54 Section 5 Alternative Solution Sets Table 5-1 Summary of Elements and Features for Alternatives Alternatives Item Feature Type A B C D Comments Refer to Figure 5-1 Refer to Figure 5-2 Refer to Figure 5-3 Refer to Figure 5-4 A.27 Pipe Addition Install 500 feet of pipe from Wilshire Drive to the intersection of Keokirk Lane and Bencia Drive, tying into the proposed system along Wilshire Drive and a new parallel system along Bencia Drive (Item A.24. Includes 34- by 22-inch elliptical and 18-inch diameter circular pipe. Same as Alternate A Same as Alternate A, except includes 38- by 24-inch elliptical and 24- inch diameter circular pipe. Same as Alternate A The system connects to the historic ditch with open outfalls/intakes. A.28 Culvert Parallel/Addition A.29 Pipe Parallel/Addition Install 230 feet of parallel 36-inch diameter culvert to connect BMP Pond 2 to Pond 1. Install 1,300 feet of pipe parallel to the existing stormwater pipes along northern Kittridge Drive, Aldea Circle, and Lubao Drive from 400 feet west of Aldea Circle to an outfall to BMP Pond 1. Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Same as Alternate A Pipe diameter range from 18- to 48-inches. A.30 Pipe Improve Replace 145 feet of exist. 15-inch diameter pipe that crosses Lubao Lane and discharges to BMP Pond 1 with 30- and 36-inch diameter pipe. Same as Alternate A, except all pipe is 30-inch diameter N/A Same as Alternate A, except all pipe is 30-inch diameter A.31 Pipe Parallel/Addition A.32 Pipe Addition Install 1,350 feet of pipe parallel to the existing stormwater pipes along Kittridge Drive to Lubao Lane tying into an existing 36-inch diameter culvert under Lubao Lane. Install two additional 15-inch pipe outfalls from Emelita Drive to BMP Pond 1. Same as Alternate A Same as Alternate A Same as Alternate A Pipe diameter range from 18- to 24-inches. Same as Alternate A Same as Alternate A Same as Alternate A A.33 Pipe Addition Install three 18-inch pipe outfalls from Ashville Park Boulevard to the proposed ditch between BMP Ponds 4 and 15. Same as Alternate A Same as Alternate A Same as Alternate A A.34 Ditch Addition The ditch previously recommended by the developer s engineer and BMP Pond 10A along Ashville Park Boulevard from proposed Village E to just east of existing Pond 10, tying into the existing SMS at the 60-inch pipe where it connects BMP Ponds 10 and 11. See B.8 Below Same as Alternative A Same as Alternative A The ditch has a 6-feet wide bottom and 4:1 side slopes for the bottom 2 feet, a 6:1 side slope for the next foot, then 3:1 side slopes until the ditch meets grade. The ditch bottom is proposed at 1.0 foot NAVD 88 with a Manning s roughness in the center channel (the bottom 2 feet and a 0.05 roughness value in the remainder of the section. BMP Pond 10A has an ~ 1.5 acre footprint. A.35 Pond Addition BMP Ponds 16 and 17 shown previously recommended by the developer s engineer and culvert connections to the existing and proposed SMS Same as Alternative A Same as Alternative A Same as Alternative A A.36 Pipe Addition Minor additional connectors from the existing system to the parallel proposed system, to equalize across streets (Not shown on figures Same as Alternate A Same as Alternate A Same as Alternate A 5-8

55 Section 5 Alternative Solution Sets Table 5-1 Summary of Elements and Features for Alternatives Alternatives Item Feature Type A B C D Comments B.1 Culvert Replace N/A B.2 Culvert Replace N/A B.3 B.4 Control Structure Pump Station Addition Addition N/A N/A B.5 Ditch Addition N/A B.6 Ditch Addition N/A B.7 Culvert Addition N/A Refer to Figure 5-1 Refer to Figure 5-2 Refer to Figure 5-3 Refer to Figure 5-4 Install a 1,225 feet long 8-feet by 8-feet box culvert, along Sandbridge Road, from Flanagan s N/A N/A Lane to Ashville Bridge Creek, replacing the existing ditch. Install A 385 feet long 8-feet by 8- feet box culvert, along Flanagan s Lane from the CS and PS to N/A N/A Sandbridge Road, replacing the existing ditch. Install an 8-feet tall by 10-feet wide CS between BMP Pond 14 and the ditch along Flanagan s Lane. Construct a 20 cfs PS adjacent to the CS (between BMP Pond 14 and Flanagan s Lane Construct a 350 feet long ditch between BMP Ponds 13 and 14. Construct a 760 feet long ditch along Ashville Park Boulevard between the proposed entrance road to Village E and BMP Pond 13. Install a 220 feet long culvert or bridge under the proposed entrance to Village E, to convey flow to the proposed downstream ditch (Item B.6. N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A The box culvert will also replace the existing culverts under New Bridge Road. The bottom has a slight (< 0.1% grade with inverts ranging from -4.5 feet NAVD 88 to -5.0 feet NAVD 88. New Bridge Road will be re-graded, after construction of the box culvert, to higher elevations than the existing condition. The bottom has a slight (< 0.1% grade with inverts near -4.5 feet NAVD 88. This CS will close when Ashville Bridge Creek is higher than the upstream pond levels within the subdivision. The CS opens and operates as the primary outfall when upstream pond elevations are greater than the tailwater elevations in Ashville Bridge Creek. Under normal (non-storm conditions, this CS should be operated to maintain normal water surface elevations in the Ashville Park Subdivision at 2.0 feet NAVD 88. The PS operates when the CS is closed and draws down the upstream ponds from the normal water surface elevation of 2 feet to 1 foot NAVD 88 prior to a storm. The PS will maintain this elevation until pumping is no longer required. Once the effects of a storm have subsided and the upstream pond elevations are higher than the downstram tailwater elevation in the Ashville Bridge Creek, the CS will fully open and the PS will shut off. Construct with a 10-feet wide bottom near -2 feet NAVD 88, 3:1 side slopes and a proposed center channel roughness of 0.03 (clean straight channel with short grass. The channel footprint approaches 75 feet in width at grade. This ditch replaces the existing 48-inch culverts between these ponds, though as with the existing pipes, it needs to bend around and to the south of the existing offsite property on Flanagan s Lane. Construct with a 10-feet wide bottom near -2 feet NAVD 88, 3:1 side slopes and a proposed center channel roughness of 0.03 (clean straight channel with short grass. The channel footprint approaches 75 feet in width at grade. This ditch replaces a shallow roadside swale that is part of the subdivision development plan. This element is modeled an open trapezoidal section, with a 10 feet wide bottom near -2.0 feet NAVD 88, 3:1 side slopes, and a roughness of Note that this matches the center section of the proposed Ashville Park Boulevard Ditch and therefore is modeled as a bridge with a lower chord above peak water surface. This 220-feet long section may also be designed as a concrete box culvert, though the culvert would necessarily be quite large to provide equivalent conveyance capacity. 5-9

56 Section 5 Alternative Solution Sets Table 5-1 Summary of Elements and Features for Alternatives Alternatives Item Feature Type A B C D Comments B.8 Ditch Addition N/A B.9 Culvert Addition N/A B.10 Pipe Improve N/A C.1 Ditch Addition N/A N/A C.2 Culvert Parallel/Addition N/A N/A C.3 Ditch Improve N/A N/A C.4 Pipe Addition N/A N/A C.5 Pump Station Refer to Figure 5-1 Refer to Figure 5-2 Refer to Figure 5-3 Refer to Figure 5-4 Addition N/A N/A Install a proposed ditch and BMP Pond 10A along Ashville Park Boulevard from proposed Village E to just east of existing Pond 10. Install a 200 feet long culvert or bridge under the proposed entrance to Village D, conveying flow to the proposed Ashville Park Boulevard ditches on either side of the entrance. Install 18-inch diameter pipe to Pond 10 near the entrance to Village D. N/A N/A N/A N/A N/A N/A Install 2,300 feet long ditch from Princess Anne Lane to an existing stream north of Seaboard Road. Install a 50-feet long, 36-inch diameter pipe Improve the farm ditch west of Princess Anne Road. Install 500 feet of 36- inch diameter force main from the proposed PS to the ditch described in Item C.3. Construct a 50 cfs PS west of Lubao Lane and ~ 200 feet south of Kittridge Drive. N/A N/A N/A N/A N/A The ditch ties into the proposed bridges/culverts under the entrance roads to Villages D and E (see Items B.7 and B.9. The ditch has a 10- feet wide bottom and 3:1 side slopes. The invert ranges from -1.6 feet to -1.8 feet NAVD 88 with a 0.03 Manning s roughness in the center channel. The ditch is deeper and wider than the ditch in the subdivision s build-out development plan (which was used as originally planned in Alternative A. BMP Pond 10A, with an ~ 1.5 acre footprint is left unchanged from the subdivision s development plan and therefore not included in the cost estimate. This element is modeled as an open trapezoidal section, with a 10 feet wide bottom near -1.6 feet NAVD 88, 3:1 side slopes, and a roughness of Note this matches the center section of the proposed Ashville Park Boulevard Ditch and therefore is modeled as a bridge with a lower chord above peak water surface. This 200-feet long section may also be designed as a concrete box culvert, though the culvert would necessarily be quite large to provide equivalent conveyance capacity. The ditch is trapezoidal, with a bottom width of 1-4 feet and 3:1 side slopes. The bottom of the ditch falls from 10.5 feet NAVD 88 near Princess Anne Lane to 6 feet NAVD 88 at the outfall, while the depth ranges from 4 to 5 feet, with resulting top widths from 25 to 30 feet. The outfall is modeled as a normal outfall which means flow in the ditch is not significantly affected by the tailwater condition. A tailwater condition as high as 10 feet NAVD 88 is unlikely to affect the model results. However, if the discharge stream is flooding out-ofbank (above 10 feet NAVD 88, then the bottom reach of this ditch should be expected to flood out-of-bank as well. Install parallel to the existing 36-inch culvert under Princess Anne Lane. Construct with a trapezoidal center section, a bottom width of 2.5 feet at approximately 10.5 feet NAVD 88, 3:1 side slopes and a center roughness of 0.03 (clean straight channel with short grass. The ditch will be 35 feet wide at grade for a length of 175 feet. The existing culvert under Princess Anne Road at this location (two barrels of 36-inch RCP will be plugged to remove the potential to circulate flow back to BMP Pond 1. The PS will drawdown the subdivision ponds from the normal water surface elevation of 2 feet NAVD 88 to 1 feet NAVD 88 prior to a storm. However, for this alternative, the PS will also be used to reduce peak stages in BMP Ponds 1-4. The PS is modeled with a 20 cfs pump to drawdown the subdivision ponds within 48 hours and an additional 30-cfs pump to provide a total capacity of 50 cfs. The final pump 5-10

57 Section 5 Alternative Solution Sets Table 5-1 Summary of Elements and Features for Alternatives Alternatives Item Feature Type A B C D Comments Refer to Figure 5-1 Refer to Figure 5-2 Refer to Figure 5-3 Refer to Figure 5-4 station design may use a different combination of pump sizes to provide the same functionality. C.6 Pipe Addition N/A N/A C.7 Pipe Addition N/A N/A D.1 Ditch Addition N/A N/A N/A Install 200 feet of twin 36-inch diameter intake pipe from BMP 1 to the proposed PS. Install a new 140 feet long, 36-inch diameter pipe from southbound Lubao Lane to the proposed PS. N/A N/A Install a 1,800 foot long ditch behind Village E to convey flow from the CS (Item A.2 to the proposed box culvert at New Bridge Road. D.2 Pond Addition Construct a Pump Intake Pond D.3 Ditch Addition N/A N/A N/A D.4 Pipe Parallel/Addition N/A N/A N/A Construct a 3,000 feet long ditch behind Villages D and E to convey flow from BMP Pond 10 southeast to the proposed CS. Install a 3,400 feet long system parallel to the existing stormwater pipes on Terramar Lane to a proposed 60-inch outfall to the ditch downstream of BMP Pond 4, behind the eastern edge of Ranier Village. The ditch has a 4-feet bottom and 3:1 side slopes, running from a depth of near -1.1 feet NAVD 88 to -2 feet NAVD 88. The depth depends on natural grade, but is expected to be 7 to 9 feet, resulting in an ~ 50-feet wide footprint. The Manning s roughness value of the center channel is 0.025, which approximates a clean, earthen/gravel channel or some version of a lined channel. Due to an existing wetland adjacent to the proposed ditch, a seepage barrier (e.g. sheeting may be necessary. A seepage analysis should be performed during design to estimate the length and depth of the barrier. May require an adjustment in the proposed platting of Village D. The ditch will have a 4-feet bottom and 3:1 side slopes, running from a depth of near -0.8 feet NAVD 88 to -1.1 feet NAVD 88. The depth depends on natural grade, but is expected to be 7 to 9 feet, resulting in an ~ 50-feet wide footprint. The Manning s roughness value of the center channel is set to 0.025, which ~s a clean, earthen/gravel channel or some version of a lined channel. Due to a wetland adjacent to the proposed ditch, a seepage barrier (e.g. sheeting may be necessary along a portion of the ditch. A seepage analysis should be performed during design to estimate the length and depth of the barrier. The system contains many tributary sections. The upstream end of the system starts parallel to the existing system on Terramar Lane (100 feet west of and 200 feet east of Pepperlin Drive, then cuts south between properties to Cayman Lane. The system runs adjacent to Cayman Lane, approximately under the existing sidewalk. It follows Cayman Lane near the end of the cul-de-sac, where it cuts north to the proposed ditch between BMP Ponds 4 and 15 (~ 350 feet east of BMP Pond 4. The system has 4 additional tributary pipes connecting the existing pipes to the new system near Cayman Lane. The 5-11

58 Section 5 Alternative Solution Sets Table 5-1 Summary of Elements and Features for Alternatives Alternatives Item Feature Type A B C D Comments Refer to Figure 5-1 Refer to Figure 5-2 Refer to Figure 5-3 Refer to Figure 5-4 connections are at Terramar Lane, Quincy Way, and at the southern and northern curves of Grandon Loop Road. D.5 Pipe Parallel/Addition N/A N/A N/A D.6 Pipe Parallel/Addition N/A N/A N/A D.7 Pipe Addition N/A N/A N/A Install 650 feet of pipe parallel to the existing stormwater pipes from northern Grandon Loop Road to Channing Lane along Brightwood Drive. Install 300 feet of 18-inch pipe parallel to existing system along Grandon Loop Road, 300 feet west of Quincy Way. Install 600 feet of pipe along Quincy Way, from Pepperlin Drive to Terramar Lane, becoming parallel to the existing system approximately halfway. Pipe diameter range from 15-to 36-inches. This parallel system through the interior of Ranier Village is significantly shorter and smaller than Alternatives A, B, and C. Pipe diameter range from 30- to 36-inches. 5-12

59 FURYWAY Ü REXLN BUTTERMILKCT BEAUTYWAY Legend ExistingS torm wa ter Pipe Pla nneds tora ge!!!!!! Pla nnedditch,l % Pa rcel Alterna tiv eitem No. LOS CONAES WAY BMP6Expa nsion:a pprox. footprinta tnorm a lpoolelv. REINLN 7 CHAMPIONCIR Proposed Improvements 37 PS Pum p S ta tion C\ ControlS tructure BoxCulv ert KITTRIDGEDR A.26 BLYTHEDR A.21 24!!!!!!!!! A.20 A KEOKIRKLN 30 A A.23 A.22 8 S torm wa ter Pipe ForceMa in!!!!!! DredgeExistingDitch!!!!!! Ditch BMPAddition or Expa nsion Pum p Inta kepool A A LUBAO LN 48 1 A Wilshire Village A.32 A A.25 A A A.10!!!!!!!!!!!!!!! A.18 A.16 4 A.19 CHANNING LN 18 3 AS HVILLEPARKBL!!! A.24 EMELITADR!!!!!!!!! CAMARILLO LN A.33 5!!!!!!!!!!!!!!!!!!! A X14!!!!!! A.8 6 Village C 16 A.35 FLANAGANS LN 2.0ftNAVDGa ted ControlS tructure S ANDBRIDGERD PRINCES S ANNERD 48 A A MONTECITO DR 48 BRIGHTWOODDR Rainier Village PRES TONDR A QUINCYWAY GRANDONLOOPRD A.9 CAYMANLN 15!!!!!!!!!!!!!!!!!!!!! A.6!!!!!!!!! 9!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Village D!!!!!!!!!!! AS HVILLEPARKBLVD 12!!!!!!!!!!!!!!!! C\!!!!!!!!!!!! A.5!!!!!!!!!!!!!!!!!! A.4 LOTUS DR!!! A PEPPERLINDR 30 TERRAMARLN A.34!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 10A!!!!!!!!!!!!!!!!!! 17 A.35 S EABOARDRD BMP2A:a pprox. footprinta tnorm a lpoolelv.!!!!!!!!! Village E US Fish and Wildlife Property Ashville Bridge Creek A.3 20cfsPum p S ta tion a ndga tedcontrol S tructure 2.0ftNAVDGa ted ControlS tructure!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Elbow Farms Property A.2 37 PS!!!!!! C\ 10X6 A.1 NEW BRIDGERD 1in =300ft 0 900Feet FIGURE 5-1 City of Virginia Beach Ashville Park Solution Set: Alternative A Decem ber 2016

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61 Ü BMP6Expa nsion:a pprox. footprinta tnorm a lpoolelv. REXLN REINLN BUTTERMILKCT 7 CHAMPIONCIR BEAUTYWAY Legend ExistingS torm wa ter Pipe Pla nneds tora ge!!!!!! Pla nnedditch,l % Pa rcel Alterna tiv eitem No. (See Table 5 1 Proposed Improvements 37 PS Pum p S ta tion C\ ControlS tructure BoxCulv ert A.31 KITTRIDGEDR A LUBAO LN 1 A Wilshire Village A A.25 A.26 BLYTHEDR A.17 A.28 18!!!!!!!!!!!!!!! A.10 A.11 A.21 A.19 A.18 A.16!!!!!!!!! 3 AS HVILLEPARKBL CHANNING LN A.20 A KEOKIRKLN!!! A.24 EMELITADR!!!!!!!!! CAMARILLO LN A !!!!!!!!!!!!!!!!!!! A.7 A.23 7X A.8!!!!!! A.22 6 Village C 16 A cfsPum p S ta tion a nd2.0ftnavd Ga tedcontrols tructure FLANAGANS LN S ANDBRIDGERD S torm wa ter Pipe ForceMa in!!!!!! DredgeExistingDitch!!!!!! Ditch BMPAddition or Expa nsion Pum p Inta kepool PRINCES S ANNERD A.13 A A MONTECITO DR BRIGHTWOODDR Rainier Village PEPPERLINDR PRES TONDR QUINCYWAY GRANDONLOOPRD TERRAMARLN A.9 A CAYMANLN 15!!!!!!!!!!!!!!!!!!!!! A.6!!!!!!!!! 9 B X10 B.9!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Village D B.8 AS HVILLEPARKBLVD 10A 11!!!!! 8X10 B.7 12!!!!!!!!!!!!!!!!!! 17 B.6!!!!!! A.35 13!!!!!!!!! B.5!!! 14 B.4 37 PS B.3 B.2 B.1 LOTUS DR S EABOARDRD BMP2A:a pprox. footprinta tnorm a lpoolelv. Village E US Fish and Wildlife Property Ashville Bridge Creek Elbow Farms Property NEW BRIDGERD 1in =300ft 0 900Feet FIGURE 5-2 City of Virginia Beach Ashville Park Solution Set: Alternative B Decem ber 2016

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63 15 15!!!!!! Ü A.30 BMP6Expa nsion:a pprox. footprinta tnorm a lpoolelv. REXLN REINLN BUTTERMILKCT 7 BEAUTYWAY FURYWAY Legend ExistingS torm wa ter Pipe Pla nneds tora ge!!!!!! Pla nnedditch,l % Pa rcel Alterna tiv eitem No. (See Table 5 1 Proposed Improvements 37 PS Pum p S ta tion C\ ControlS tructure BoxCulv ert!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! C.1 PRINCES S ANNELN 50cfsPum p S ta tion!!!!!!!!!!!! C !!!!!!!!! C.2 A C.4 KITTRIDGEDR 37 PS LUBAO LN AS HVILLEPARKBLVD A.14 A ALDEACIR CAMARILLO LN Wilshire A.25 Village A A.32 C.5 C.6 C A BLYTHEDR 60 MONTECITO DR WILS HIREDR CHANNING LN!!!!!!!!!!!!!!!!!! Rainier Village A.11 A A.28 A PEPPERLIN DR A PRES TONDR A.12 A QUINCYWAY A.19 A.16!!! A KEOKIRKLN AS HVILLEPARKBL GRANDONLOOPRD A.27 TERRAMARLN CAMARILLO LN !!! 42 A A.24 A.18 BENECIADR 24 EMELITADR A.33 CAMARILLO LN!!!!!!!!!!!!!!!!!! A.7 A A.23!!!!!! X14!!!!!! A.8 15 A.22 6!!!!!!!!!!!!!!!!!!!!!!!!! A.6 Village C 9 16 A.35!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Village D!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! 8 FLANAGANS LN AS HVILLEPARKBLVD A.34 10A!!!!!! 2.0ftNAVD Ga tedcontrols tructure!!!!!!!!!!!!!!!!!! 12!!!!!!!!! C\ S ANDBRIDGERD!!!!!!!!!!!!!!!!!!!!!!!!!! A.5 A.4 S torm wa ter Pipe LOTUS DR!!!!!! DredgeExistingDitch!!!!!! Ditch ForceMa in BMPAddition or Expa nsion Pum p Inta kepool LOTUS DR COUNTYPL S EABOARDRD BMP2A:a pprox. footprinta tnorm a lpoolelv. PRINCES S ANNERD CAYMANLN A.3 17 Village E A.35 US Fish and Wildlife Property!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Ashville Bridge Creek 10X6 A.1 Elbow Farms Property!!!!!! NEW BRIDGERD A.2 1in =350ft 0 1,050Feet FIGURE 5-3 City of Virginia Beach Ashville Park Solution Set: Alternative C Decem ber 2016

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65 Ü Legend BE AU XL RE N TE R MI LK C T CH 7 A.23 3 CHANNING LN D.7 A.6 D.4 10 AS HV Village D TERRAMAR LN R ILL EP AR K BL VD 17 US Fish and Wildlife Property Village E A.12 BMP 2A: approx. footprint at normal pool elv. lle B D.3 Ash vi D A ft NAVD Gated Control Structure A.35 SEAB OARD R A A A.34 \ C e Cr eek 15 RD PEPPERLIN DR NNE SA 8 1 CES A.13 LN 6 3 D AG AN S ri d g N PRI QUINCY WAY Rainier Village 5 1 RD FLA N 7 X1 4 SD GRANDON LOOP PRESTON DR D.5 GE RD A.7 LO TU A.8 A DR MONTECITO DR A.33 A BRIGHTWOOD 6 3 ASH VILL A.16 E PA RK B L EMELITA DR 6 0 CAY MA NL N A.15 CAMARILLO LN A.18 A Ditch A.24 A.11 A.14!!! Village C A.19 A.17 A.30!!! K LN KEO KIR Dredge Existing Ditch BMP Addition or Expansion A.32!!! Pump Intake Pool A.25 A.29!!! SANDB RID Wilshire Village Control Structure Force Main A LUB AO LN 24 A.31 A DR Pump Station Box Culvert 8 A.26 BLY T HE 3 0 Proposed Improvements IR \ C A Alternative Item No. Stormwater Pipe R AM PIO NC %, L A GE D Planned Ditch!!! PS 3 7 REIN LN RID Planned Storage W AY Parcel BMP 6 Expansion: approx. footprint at normal pool elv. KIT T TY!!! BU T 18 Existing Stormwater Pipe PS 3 7 \ C 1 0 X6 D.2 Pump Intake Pool: approx 0.5 acres 20 cfs Pump Station and Gated Control Structure A.2 Elbow Farms Property 10 X6 D.1 W NE 1 in = 300 ft Feet B DG RI E A.1 RD FIGURE 5-4 City of Virginia Beach Ashville Park Solution Set: Alternative D December 2016

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67 Section 5 Alternative Solution Sets 5.2 Alternative Results Results The simulation results for each of the alternatives are shown in Table 5-2. A full table of results is presented in Appendix C. As discussed above, the LOS for the 100-year storm is visible crown, which for this project was set to 3 inches (0.25 feet above the indicator elevation; therefore, all values shown in Table 5-1 meet the desired LOS. Note that in meeting LOS for the 100-year storm, the peak stages for the 10-year storm are well below inlet grade. The results for Alternative B are nearly identical to Alternative A, as were most of the flood mitigation elements within the Wilshire and Ranier Villages. This was by design, as both alternatives were designed to first mitigate the head losses in the primary outfall system. The resultant flattening of the system HGL lowers the peak stages in BMP Ponds 1 through 4. The SMS in the Wilshire and Ranier Villages outfall to similar peak stages in these ponds and therefore are similar. Minor differences do occur due to very small differences in the timing and peaks in Ponds 1 through 4. The results for Alternative C are nearly identical to Alternatives A and B. The peak stages in BMP Ponds 1 through 4 have been reduced by approximately 3 to 6 inches for the 100-year storm versus Alternative A. Therefore, the SMS systems in the Wilshire and Ranier Villages were reduced in size, where possible. A larger pump station may be useful to further reduce the peak stages in Ponds 1 through 4; however, additional flows on peak to the flows under Seaboard Road may cause offsite problems. The results of on-going analyses by the City (Sherwood Lakes will be needed to determine the amount of additional on-peak flows that could be added to the western outfall and/or what mitigation is needed for that system to handle these flows. Alternative C is a more conservative option. The results for Alternative D are nearly identical to Alternative A. The peak stages in BMP Ponds 1 through 4 are nearly the same for both alternatives. The primary difference in the Ranier Village SMS is the removal of much of the parallel system running through the center of the village and the removal of the parallel system proposed on the eastern village roads. The flood mitigation elements for this alternative have moved from these roads to behind the village properties. In December 2016, finished garage elevations (FGEs for houses in Ranier Village were surveyed by the City of Virginia Beach. Additional analyses were performed to compare the performance of the SMS for the existing (October, 2016 Ashville Park subdivision with the hydraulic elements recommended in Alternative D for both the 100-year design storm and Hurricane Matthew. Peak model flood elevations were compared to the surveyed FGEs for each storm. Model results indicate that only 3 of the FGEs were exceeded for Hurricane Matthew under for the current condition with the Alternative D improvements. In contrast, over 90 homes in Ranier Village experienced water in garages varying from 6 to 9 inches deep during Hurricane Matthew. No surveyed FGEs were exceeded for the 100-year storm with the Alternative D improvements. For the build out condition model in Alternative D, the roads experience no more than 0.2 ft of flooding above crown for the 100-year design storm; therefore, none of the garage elevations will be exceeded. Additionally, since all four alternatives have similar flood stages in the neighborhood, the SMS performance with respect to surveyed FGEs is expected to be similar to Alternative D. 5-21

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69 Section 5 Alternative Solution Sets Table 5-2 Summary of Alternatives Simulation Results Alternatives A B C D 10-yr Storm 100-yr Storm 10-yr Storm 100-yr Storm 10-yr Storm 100-yr Storm 10-yr Storm 100-yr Storm Critical Elevations Peak Peak Peak Peak Peak Peak Peak Peak (feet NAVD 88 Stages Delta Stages Delta Stages Delta Stages Delta Stages Delta Stages Delta Stages Delta Stages Delta Crown Flowline (feet from (feet from (feet from (feet from (feet from (feet from (feet from (feet from Model (100- (10-yr NAVD Flowline NAVD Crown NAVD Flowline NAVD Crown NAVD Flowline NAVD Crown NAVD Flowline NAVD Crown Location Node yr 88 (feet 88 (feet 88 (feet 88 (feet 88 (feet 88 (feet 88 (feet 88 (feet East Quincy Wy & Grandon Loop Quincy Way & Pepperlin Dr Terramar Ln & Pepperlin Dr Terramar Ln east of Pepperlin Brightwood Dr & Ashville Park Blvd A Montecito Dr A14B Grandon Loop & Pepperlin Dr A18B West Quincy Wy & Grandon Loop A Ashville Park Blvd & Lubao Dr A Brightwood Dr & Grandon Loop A4A West Channing Ln C03B West Grandon Loop Rd C09A Channing Ln & Grandon Loop C11A Ashville Park Blvd south of BMP Kittridge Dr Loop Lubao Dr & Aldea Cir Lubao Dr north of Kittridge Dr Camarillo Lane Northwest Loop Wilshire Dr & Blythe Dr Blythe Dr & Aldea Cir Camarillo Ln east of Keokirk Ln Camarillo Lane Northeast Loop Bencia Dr east of Keokirk Ln Lubao Dr west of BMP

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71 Section 6 Feasibility and Probable Cost of Alternative Solution Sets 6.1 Feasibility Each of the alternatives for Ashville Park SMS improvements 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 USACE, the Virginia DEQ, and the VMRC to introduce the project and its objectives and identify agency concerns. A wetland delineation (PJD and Wetland Confirmation from USACE should be performed prior to this meeting to identify jurisdictional areas. Kimley-Horn and Associates, Inc., performed a wetland delineation in 2014 that may be used if it was certified by USACE and a PJD or Approved Jurisdictional Determination (JD was performed. All portions of the project area not covered by this delineation must be delineated. For this assessment, the National Wetlands Inventory (NWI maintained by the US Fish and Wildlife Service (USFWS was consulted in addition to the 2014 Kimley-Horn delineation. 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. The existing stormwater management ponds are shown on NWI maps as wetlands (palustrine unconsolidated bottom. These areas might not be considered jurisdictional since they were constructed wholly in upland areas. This must be confirmed with the permitting agencies. The existing drainage ditches connecting wetland areas would, however, be considered jurisdictional. Permits from the USACE, the Virginia DEQ, and the VMRC would be applied for through a single Joint Permit Application, available at the USACE Norfolk District Regulatory Branch website ( If impacts result in the loss of ½ acre or less of wetlands, stream impacts are 300 linear feet or less, and all impacts are nontidal, USACE and Virginia DEQ permitting may be possible using Nationwide Permit (NWP 43 for stormwater management. If wetland losses equal one acre or less and stream impacts 2,000 linear feet or less, and all impacts are non-tidal, the USACE State Programmatic General Permit (12-SPGP-01 may be used along with Virginia DEQ VWP Compliance Program permitting. If impacts exceed 1 acre, exceed 2,000 linear feet of stream, or are tidal, individual permits from USACE and Virginia DEQ would be required. Additionally, 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. 6-1

72 Section 6 Feasibility and Probable Cost of Alternative Solution Sets Additional pre-application consultation is recommended with United States Fish and Wildlife Service (USFWS due to potential impacts within the Back Bay National Wildlife Refuge along Sandbridge Road and New Bridge Road, even if all impacts are within the right-of-way or cityowned easements. Any impacts to lands within the Back Bay National Wildlife Refuge must undergo a Compatibility Determination, conducted by the USFWS. The permitting agencies (USACE, Virginia 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 demonstrate to the agencies that the project s sole objective is additional stormwater management for the developed areas that are experiencing flooding. It is important to communicate that the project s size and scope is not intended to accommodate additional flows from unpermitted developments. While connections to the SMS for the undeveloped villages will be permitted in the future, these villages will require additional stormwater management controls that meet the City s stormwater permit requirements. 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. In addition, any ditches within wetlands will need to be designed to prevent draining the wetlands. The proposed ditches in each of the alternatives will be constructed from clay, which is expected to minimize draining wetland groundwater. Additionally, an adjacent berm would minimize wetland drainage from overland flow. The surface water in the stormwater ponds and drainage ditches is expected to be at 2 feet (NAVD during most times, and would represent equilibrium with groundwater. Drainage of groundwater that does infiltrate the ditch is expected to be minimal due to the relatively flat topography. Prior to large expected storm events, the water level would be lowered by approximately 1 foot to accommodate additional storm flow. The only time that groundwater would be drained would be just prior to these storm events. A modelling study should be submitted with the permit application to illustrate this design. 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 1.5:1 for scrub-shrub wetland impacts 1:1 for emergent wetland impacts 6-2

73 Section 6 Feasibility and Probable Cost of Alternative Solution Sets An IPaC Trust Resources Report generated for preliminary research of the project area indicates the only Threatened and Endangered Species present is the northern long-eared bat (Myotis septentrionalis. Consultation with USFWS in accordance with Section 7 of the Endangered Species Act of 1973 (ESA will be required to identify any mitigation measures that must be undertaken to prevent a take of this species. Such measures may include time of year restrictions for tree clearing. The IPaC Trust report also indicates a variety of migratory birds (including bald eagle (Haliaeetus leucocephalus that could potentially be present in the project area. Migratory birds are protected by the Migratory Bird Treaty Act of 1918 and bald eagles are protected by the Bald and Golden Eagle Protection Act. Any activity resulting in a take of migratory birds or bald eagles is prohibited unless authorized by USFWS. During the design of the project, the potential for impacts should be identified and mitigation measures taken, if needed, to prevent a take of these species. The permitting process will further assess impacts to protected species through consultation with the Virginia Department of Conservation and Recreation (DCR 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 Alternative Solution Set A Alternative A would impact palustrine forested wetlands and palustrine emergent wetlands along portions of the proposed 6,625 linear feet of ditch that would run along the southeast property line for Ashville Park and Elbow Farms, as well as the 1,687 linear feet of ditch improvements along Sandbridge Road and Flanagan Lane. Additional smaller impacts would occur in existing ditches within the agricultural fields. Impacts to tidal wetlands are possible at the outfall structure along New Bridge Road. The proposed new BMP 2A and expansion of BMP 6 are not identified by NWI mapping as wetlands; this needs to be confirmed in the field. Wetland impacts could be minimized by adjusting the locations of the proposed ditch and pump station to move them outside wetland areas. Such an adjustment may be required by regulatory agencies, unless the City can demonstrate it is not practical to do so. This alternative will likely require individual permits from USACE and Virginia DEQ as impacts are expected to exceed 1 acre and/or tidal impacts may be included. The proposed work along Sandbridge Road and on the north side of the Elbow Farms Property has the potential to impact the Back Bay National Wildlife, which would necessitate individual permits. If tidal wetlands are impacted at the gated outfall structure, a permit from VMRC would also be required. Impacts would also occur where the SMS connects to existing stormwater ponds, though regulatory agencies may consider these ponds non-jurisdictional since they were constructed wholly in uplands. 6-3

74 Section 6 Feasibility and Probable Cost of Alternative Solution Sets Compensatory mitigation may be required due to the loss of vegetated wetlands from the berm adjacent to the proposed ditch, as well as conversion of vegetated wetlands to open water (within the ditches, 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 will activities within or adjacent to the Back Bay National Wildlife Refuge Alternative Solution Set B Alternative B would impact wetland areas along the proposed box culvert on Sandbridge Road and potentially at the location of the proposed pump station. The wetlands along Sandbridge Road are mapped by NWI as palustrine forested. Although the ditch along the roadway is marked as riverine on NWI maps, the City could contend that it is simply a wetland ditch, not a stream. Since this area is within or adjacent to the Back Bay National Wildlife Refuge, the City must coordinate with USFWS as described above. This alternative would have minimal wetland impacts along the 2,600 linear feet of stormwater pipe connecting the BMPs and along Ashville Park Boulevard. In addition, the proposed locations of the new BMP 2A and expansion of BMP 6 are not identified by NWI mapping as wetlands; this must be confirmed in the field. Impacts would also occur where the stormwater system connects to existing stormwater ponds, though regulatory agencies may consider these ponds non-jurisdictional since they were constructed wholly in uplands. The wetland impacts from this alternative may be small enough to allow the use of NWP 43 or 12-SPGP-01; however, the work along Sandbridge Road has the likelihood to impact the Back Bay National Wildlife Refuge and may necessitate individual permits. Compensatory mitigation may be required due to the loss of vegetated wetlands from the box culvert and pump station, as well as 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 will activities within or adjacent to the Back Bay National Wildlife Refuge Alternative Solution Set C Alternative C would impact palustrine forested wetlands and palustrine emergent wetlands along portions of the proposed 6,625 linear feet of ditch that would run along the southeast property line for Ashville Park and Elbow Farms, as well as the 1,687 linear feet of ditch improvements along Sandbridge Road and Flanagan Lane. Additional smaller impacts would occur in existing ditches within the agricultural fields. Impacts to tidal wetlands might occur at the outfall structure along New Bridge Road. The proposed new BMP 2A and expansion of BMP 6 are not identified by NWI mapping as wetlands, nor is the area west of Wilshire Village in which the force main, 50 cfs pump station, and ditch would be located; this needs to be confirmed in the field. 6-4

75 Section 6 Feasibility and Probable Cost of Alternative Solution Sets Wetland impacts could be minimized by adjusting the locations of the proposed ditch and pump station to move them outside wetland areas. Such an adjustment may be required by regulatory agencies, unless the City can demonstrate that it is not practical to do so. This alternative will likely require individual permits from USACE and Virginia DEQ as impacts are expected to exceed 1 acre and/or tidal impacts may be included. The proposed work along Sandbridge Road and on the north side of the Elbow Farms Property has the likelihood to impact the Back Bay National Wildlife Refuge and as such may require individual permits. If tidal wetlands are impacted at the gated outfall structure, a permit from VMRC would also be required. Impacts would also occur where the stormwater system connects to existing stormwater ponds; though the regulatory agencies may consider these ponds non-jurisdictional since they were constructed wholly in uplands. Compensatory mitigation may be required due to the loss of vegetated wetlands from the berm adjacent to the proposed ditch, as well as conversion of vegetated wetlands to open water (within the ditches, 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 will activities within or adjacent to the Back Bay National Wildlife Refuge Alternative Solution Set D Alternative D would impact palustrine forested wetlands and palustrine emergent wetlands along portions of the proposed 6,405 linear feet of ditch that would run along the southeast property line of Ashville Park and Elbow Farms, the 0.5-acre intake pool and pump station along the 1,687 linear feet of ditch improvements along Sandbridge Road and Flanagan Lane, and at the rectangular box culvert under New Bridge Road. Additional smaller impacts would occur in existing ditches within the agricultural fields. The proposed new BMP 2A and expansion of BMP 6 are not identified by NWI mapping as wetlands; this needs to be confirmed in the field. Wetland impacts could be minimized by adjusting the locations of the proposed ditch, intake pond, and pump station to move them outside wetland areas. Such an adjustment may be required by regulatory agencies, unless the City can demonstrate that it is not practical. This alternative will likely require individual permits from USACE and Virginia DEQ as impacts are expected to exceed 1 acre and/or tidal impacts may be included. The proposed work along Sandbridge Road and on the north side of the Elbow Farms Property has the likelihood to impact the Back Bay National Wildlife Refuge, which may necessitate individual permits. Impacts would also occur where the SMS connects to existing stormwater ponds, though regulatory agencies may deem these ponds non-jurisdictional since they were constructed wholly in uplands. 6-5

76 Section 6 Feasibility and Probable Cost of Alternative Solution Sets Compensatory mitigation may be required due to the loss of vegetated wetlands from the berm adjacent to the proposed ditch, as well as conversion of vegetated wetlands to open water (within the ditches, 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 will activities within or adjacent to the Back Bay National Wildlife Refuge. 6.2 Opinion of Probable Construction Cost The planning-level opinion of probable construction cost (OPCC developed for this study is based on a conceptual level of detail. The costs presented are planning-level costs and represent the project team s best engineering judgment. However, additional information will be developed during project design, and actual costs at the time of construction are largely dictated by market conditions at the time of bidding. CDM Smith cannot guarantee that bids and actual construction costs will not vary from the OPCC presented in this section. In addition, there are no 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 the contractor s scope. Table 6-1 Planning-level Opinion of Probable Construction Cost for Alternatives A through D Category Cost Alternative A Alternative B Alternative C Alternative D Channels $2,894,444 $959,978 $2,703,256 $3,049,728 Culverts $224,000 $4,377,400 $562,000 $562,000 Demolition $492,215 $503,415 $492,215 $492,215 E&SC $276,255 $267,130 $287,830 $275,855 Miscellaneous $54,600 $51,600 $64,200 $51,600 Pavement Replacement $2,035,034 $1,985,034 $2,037,362 $2,015,870 Piping $2,734,869 $2,727,927 $3,066,041 $2,906,556 Ponds $1,064,027 $1,094,027 $1,064,027 $1,153,099 Pump Station $3,000,000 $3,000,000 $5,000,000 $3,000,000 Subtotal $12,775,444 $14,966,511 $15,276,931 $13,506,923 Mobilization $668,772 $778,326 $793,847 $705,346 Contingency (30% $4,033,265 $4,723,451 $4,821,233 $4,263,681 Engineering, Survey, & Permitting (15% $2,621,622 $3,070,243 $3,133,802 $2,771,393 Total $20,099,103 $23,538,531 $24,025,813 $21,247,343 USE $20,100,000 $23,500,000 $24,000,000 $21,200,

77 Section 6 Feasibility and Probable Cost of Alternative Solution Sets 6.3 Sea Level Rise Virginia Beach and the Hampton Roads area have been experiencing sea level rise (SLR at a slow, gradual pace since tide levels have been recorded. Future increases in sea level will increase water levels in Ashville Bridge Creek, which will impact to performance of the Ashville Park SMS. For planning purposes, two SLR conditions were evaluated: 1.5 feet and 3.0 feet of increase in water levels. Section 4 discussed the assignment of boundary conditions in the H&H model to represent tailwater conditions at Ashville Bridge Creek. The existing conditions water elevation at Ashville Bridge Creek was defined at 3.1 feet NAVD 88 for the 100-year rainfall event. The two SLR scenarios produce water elevations of 4.6 feet NAVD 88 and 6.1 feet NAVD 88, respectively Modifications to Alternative Solution Sets for Sea Level Rise Each solution set will require modifications to maintain hydraulic performance with sea level rise conditions. Table 6-2 and Table 6-3 list the improvements that will be required to maintain the design LOS with 1.5 feet and 3.0 feet of SLR, respectively. The improvements required with SLR are generally located at the downstream portion of the SMS. As shown in Figure 6-1, the existing land elevation in Ashville Park is above elevation 6.1 feet NAVD 88. However, with SLR the SMS improvements east of Ashville Park will be need to be modified to prevent backflow from Ashville Bridge Creek and maintain drainage when water levels are elevated. As SLR increases over time the ability to discharge by gravity through the gated control structure to Ashville Bridge Creek will diminish and the pump station will need to pump more frequently, longer durations, and at an increased rate in order to maintain the LOS. The bullets below provide highlights for each alternative solution set and Table 6-2 and Table 6-3 provide additional detail for maintaining system performance with SLR. Alternative Solution Set A the pump station, control structures and downstream portion of the ditch improvement will need to be modified to function with an increased tailwater condition. With the pump station location near Ashville Bridge Creek in Alternative A, the ditch will need to be confined with berms or levees to maintain conveyance to the pump station, and the pump station and control structure will need to be elevated. Alternative Solution Set B a gravity conveyance system to Ashville Bridge Creek is included in Alternative B with existing sea level conditions. However, with SLR a pump station and control structure similar to Alternative D will be needed. As with Alternative D, the ditch to Ashville Bridge Creek will need to be confined with berms or levees to maintain conveyance where the top of bank is below elevated water levels from SLR. Alternative Solution Set C the pump station and control structure are located at BMP 14, a location where existing topography is at or above both SLR scenarios. With existing sea level, the pump station discharges to the proposed culvert adjacent to Sandbridge Road and flows by gravity to Ashville Park Creek. With SLR gravity flow will not be possible and a forcemain will be required to convey flows from the pump station to the creek. Alternative Solution Set D the pump station location is located within Ashville Park at a higher ground elevation. 6-7

78 Section 6 Feasibility and Probable Cost of Alternative Solution Sets Table 6-2 Modifications to Alternative Solution Sets for 1.5 Feet of SLR Item Item Type Modifications to Alternative Solution Sets for 1.5 feet of SLR A B C D A.2 A.5 Pump Station Control Structure A.37 Berm A.38 Ditch B.3 B.4 Control Structure Pump Station Elevate pump station and control structure by 1.5 feet Increase pump station capacity Raise control structure by 1.5 feet Construct berm to contain SMS flow within channel to New Bridge Road and protect Ashville Park Expand ditch from New Bridge Road to Ashville Bridge Creek, and add energy dissipation structure N/A N/A B.11 Roadway N/A B.12 Culvert N/A C.8 C.9 Pump Station Control Structure N/A N/A N/A N/A Construct berm to protect Ashville Park Construct pump station and control structure Raise control structure by 1.5 feet Construct berm to protect Ashville Park Increase pump station capacity N/A N/A N/A N/A Construct pump station above SLR elevation Increase pump station capacity Raise control structure by 1.5 feet Raise Sandbridge Road Construct force main to convey flow to Ashville Bridge Creek, and add energy dissipation structure N/A N/A N/A N/A N/A N/A Construct pump station to discharge to Ashville Bridge Creek Construct control structure to prevent backflow from Ashville Bridge Creek Construct berm to protect Ashville Park N/A N/A N/A N/A N/A N/A 6-8

79 Section 6 Feasibility and Probable Cost of Alternative Solution Sets Table 6-3 Modifications to Alternative Solution Sets for 3.0 Feet of SLR Item Item Type Modifications to Alternative Solution Sets for 3.0 feet of SLR A B C D A.2 A.5 Pump Station Control Structure A.37 Berm A.38 Ditch B.3 B.4 Control Structure Pump Station Elevate pump station and control structure by 3.0 feet Increase pump station capacity Raise control structure by 3.0 feet Construct levee to contain SMS flow within channel to New Bridge Road Expand ditch from New Bridge Road to Ashville Bridge Creek, and add energy dissipation structure N/A N/A B.11 Roadway N/A B.12 Culvert N/A C.8 C.9 Pump Station Control Structure N/A N/A N/A N/A N/A N/A Construct pump station above SLR elevation Increase pump station capacity Raise control structure by 1.5 feet Raise Sandbridge Road Construct force main to convey flow to Ashville Bridge Creek, and add energy dissipation structure N/A N/A Construct pump station and control structure Raise control structure by 1.5 feet Construct berm to contain SMS flow within channel to New Bridge Road Expand ditch from New Bridge Road to Ashville Bridge Creek, and add energy dissipation structure N/A N/A N/A N/A Construct pump station to discharge to Ashville Bridge Creek Construct control structure to prevent backflow from Ashville Bridge Creek Increase pump station capacity Construct berm to contain SMS flow within channel to New Bridge Road Expand ditch from New Bridge Road to Ashville Bridge Creek, and add energy dissipation structure N/A N/A N/A N/A N/A N/A 6-9

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81 Legend Study_area 4.6 contour: 1.5 SLR contour: 3.0 SLR Parcels 8 High : Low : IN PR 2 4 FL A 5 NA GA 6 NS LN CE SS AN NE 9 RD HV IL LE PA R KB SAN 14 DBR 11 IDG ER D LV D ± FIGURE 6-1 City of Virginia Beach Ashville Park Sea Level Rise: Elevations Miles W NE US Fish and Wildlife Property BR IDG ER D 10 AS 13

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83 Section 7 Summary and Conclusions 7.1 Summary of Alternative Solution Sets CDM Smith applied the calibrated H&H model to identify areas of probable flooding in Ashville Park for the 10-year and 100-year storm events under existing and build-out conditions. Section 5 presented the components and hydraulic performance of each solution set. Section 6 presented the feasibility and opinion of probable construction cost of each solution set. CDM Smith identified more than two dozen initial alternatives to alleviate flooding in Ashville Park by increasing the capacity of the Ashville Park Subdivision SMS to improve off site conveyance and resizing the SMS in Ranier and Wilshire Villages, generally by adding parallel systems to the existing system. Through modeling and analysis, the alternatives were shortlisted to four potential solution sets (Alternatives A, B, C, and D to provide a LOS consistent with the City s design criteria (no road flooding for the 10-year storm event and visible road crowns for the 100-year storm event. Each of the four flood mitigation alternatives consists of dozens of individual elements, including gated control structures, pumps stations, additional storage areas (ponds, ditches, and pipes varying in size from 18 to 60 inches in diameter. Many components of the flood mitigation options are identical throughout the four alternatives. Alternatives A, B and C contain similar improvements within the Ranier and Wilshire Villages. The primary difference in improvements in each alternative are located downstream of the Ranier and Wilshire Villages. Alternative D is similar to Alternative A, but with the additional parallel Ranier Village SMS improvements are located behind the lots in the southeastern portion of the Village. From a feasibility and construction cost perspective, the four solution sets are comparable. Each alternative would impact wetlands and require wetland permitting. All of the alternatives will likely require individual permits from USACE and Virginia DEQ based on the anticipated magnitude and location of wetland and/or tidal impacts. The probable construction cost of each solution set is significant. The range in probable cost among the solution sets is limited, reducing the significance of project cost as a selection criterion. Risk during construction, permitting complexity, project impacts and sustainability are relevant aspects that offer more differentiation among the alternative solution sets. The primary differentiators among the four solution sets are as follows: Alternative B poses the most significant risk and uncertainty during construction, and has nearly the highest cost of the four solution sets. Alternative C requires construction of improvements to discharge to two outfalls. The feasibility of Alternative C depends on expansion of available downstream capacity to receive flows from the west outfall, which carries significant permitting complexity. 7-1

84 Section 7 Summary and Conclusions Alternatives B and D locate the stormwater pump station in a more sustainable location than Alternative A. Alternative D has less impacts within the existing Ranier Village development than Alternatives A, B, and C. 7.2 Recommended Solution and Design Considerations Alternative D is the recommended solution set to advance to engineering design. Alternative D adds an additional ditch and outfall behind proposed Villages D and E, uses control structures to maintain normal water surface elevations and to prevent backflow, and uses a 20 cfs pump station to drawdown the subdivision s ponds prior to the onset of a storm. The stormwater pump station and gate on the proposed outfall ditch is at a sustainable location where the surrounding ground elevation is high. Additionally, the proposed parallel system along the eastern roads of Ranier Village are located behind the properties, adjacent to Cayman Lane, to minimize impacts to existing development. Alternative D is recommended based on the following advantages over the solution sets: Most constructible alternative Lowest permitting complexity Most resilient configuration with respect to SLR Least construction impact to existing development in Ranier Village Probable construction cost is nearly the least cost option at $21.2M While D is the recommended alternative based on the advantages listed above, as engineering design proceeds additional information will become available. If aspects of Alternative D prove to be infeasible or difficult to implement the City may wish to re-evaluate the alternatives. Based on categories listed above, the order of preference from most preferred to least preferred is: D, A, B, and C. As design engineering proceeds, it is also possible that additional information and design detail will indicate that select components of Alternative D are infeasible, will require additional cost during construction or are difficult to implement. It is possible that components from one of the other final alternatives (A, B or C may be more viable than select portions of the improvements identified for Alternative D. 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. In addition to standard tasks associated with completing a Preliminary Engineering Report (PER and engineering design, the following recommendations are provided for advancing the recommended alternative: 7-2

85 Section 7 Summary and Conclusions A wetland delineation (PJD and Wetland Confirmation from USACE should be performed to identify jurisdictional areas and probable project impacts prior to consultation with the regulatory agencies. As presented in Figure 7-1, refinements to the configuration should be considered to minimize wetland impacts, striving to achieve less than 1 acre to comply with USACE State Programmatic General Permit (12-SPGP-01 and the Virginia DEQ VWP Compliance Program permitting. Wetland impacts could be minimized by moving the proposed ditch, intake pond, and pump station outside wetland areas. Undertake early pre-application consultation with the USACE, the Virginia DEQ, and the VMRC to introduce the project and its objectives and identify agency concerns. Confirm that the regulatory agencies consider the existing stormwater management ponds non-jurisdictional since they were constructed wholly in upland areas. Consult with USFWS on potential impacts within the Back Bay National Wildlife Refuge along Sandbridge Road and New Bridge Road, even if all impacts are within the right-ofway or City-owned easements. Agency consultation should also include permitting requirements for new outfalls as required. Acquisition of right-of-way, easements and property are essential for implementing the recommended solution (or any of the other final alternatives considered, and should be pursued as needed to support engineering design and project implementation. Conduct additional evaluations, such as a seepage analysis, to confirm interaction between proposed ditches and adjacent wetland areas, and determine if proposed channel improvements will require lining to prevent groundwater intrusion. Confirm the content of future development, impervious area, and size and location of additional Best Management Practices (BMPs. The analysis conducted for this study assumed future construction of BMPs 10A, 16 and 17 to store runoff from the future Villages C, D and E. The analysis also assumed culvert connections from BMPs 10A, 16 and 17 to the existing and proposed SMS per the previous recommendations provided by the developer s engineer. 7-3

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87 Ü Legend BE AU XL RE N Existing Stormwater Pipe TY Planned Storage W AY!!! BU T TE R Planned Ditch!!! Parcel MI LK C T CH BMP 6 Expansion: approx. footprint at normal pool elv. AM PIO NC %, L Alternative Item No. Proposed Improvements IR PS REIN LN \ C Stormwater Pipe 3 0 SANDB RID 9 10 Village D TERRAMAR LN AS HV ILL EP AR K BL VD ft NAVD Gated Control Structure 10A 17 US Fish and Wildlife Property Re-align ditch to minimize wetland impacts Ash vi D BMP 2A: approx. footprint at normal pool elv. lle B Village E SEAB OARD R \ C e Cr eek 15 RD PEPPERLIN DR NNE SA 6 3 Evalute reconfiguration of ditch as box culvert to minimize wetland impacts CES QUINCY WAY LN ri d g RD AG AN S R 1 8 FLA N SD GRANDON LOOP PRESTON DR N PRI DR X1 4 Rainier Village LO TU ASH VILL E PA RK B L CHANNING LN MONTECITO DR GE RD CAMARILLO LN 4 BRIGHTWOOD 6 3 Ditch Village C 2 4 EMELITA DR !!! K LN !!! BMP Addition or Expansion Dredge Existing Ditch Pump Intake Pool 6 0 CAY MA NL N LN LUB AO DR!!! KEO KIR Wilshire Village BLY T HE !!! R 24 GE D Force Main RID Control Structure Box Culvert 18 KIT T Pump Station PS 3 7 \ C 1 0 X6 20 cfs Pump Station and Gated Control Structure Pump Intake Pool: approx 0.5 acres 10 X6 Elbow Farms Property Re-align ditch to minimize wetland impacts W NE 1 in = 300 ft Feet B DG RI E RD FIGURE 7-1 City of Virginia Beach Ashville Park Solution Set: Alternative D December 2016

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89 Appendix A Field Investigation for Calibration Event A-1

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91 Memorandum To: File From: James Duvall Date: November 28, 2016 Subject: WO3A - Ashville Park Stormwater Management System Technical Analysis Model Validation Supporting Documentation September 21, 2016 Event On September 21, 2016 the Virginia Beach Department of Public Works conduced a field investigation in the Wilshire and Ranier Villages of the Ashville Park neighborhood. Observed flooding depths were recorded along the flow line between 9:15 and 10:30 AM. Flooding depths were added to the rim elevation of the corresponding stormwater structure in order to determine the water surface elevation. The uncertainty value reflects the degree to which the rim elevation at the chosen node and the true ground surface elevation at the point of measurement could differ based on the ground slope within a 10-foot radius of the structure. Therefore, the uncertainty provides an upper and lower bound on the reported flood elevation that can reasonably be expected to contain the actual flood elevation. Where possible, photos of the validation points are included on the following pages. The table and below summarize the findings.

92 Memorandum November 28, 2016 Page 2 Table 1 Summary of Observed flooding in Ashville Park on September 21, 2016 Node ID Structure Type Observed Depth of Flooding (ft Rim Elev. (ft, NAVD 88 Flood Elev. (ft, NAVD 88 Uncertainty (ft MH ± Inlet ± MH ± Inlet ± Inlet ± Inlet ± C04 MH ± C05A Inlet ± A17A Inlet ± A05A MH ± A02 Inlet ± MH ± A38 Inlet * ± 0.3 *Depth recorded by resident of 2025 Grandon Loop Road at 2:00 am on September 22, 2016.

93 Memorandum November 28, 2016 Page 3 Figure 1 Map of the Ashville Park Validation Points for Tropical Storm Julia

94 Memorandum November 28, 2016 Page 4 Figure 2: Approximate Location: Intersection of Emelita Drive and Wilshire Drive Water Surface Elevation (NAVD 88: 7.5 ft

95 Memorandum November 28, 2016 Page 5 Figure 3: Approximate Location: Intersection of Lubao Lane and Aldea Circle Water Surface Elevation (NAVD 88: 7.6 ft

96 Memorandum November 28, 2016 Page 6 Figure 4: Approximate Location: Intersection of Lubao Lane and Emelita Drive, looking south toward Ashville Park Boulevard Water Surface Elevation (NAVD 88: 7.3 ft

97 Memorandum November 28, 2016 Page 7 Figure 5: Approximate Location: Lubao Lane between Emelita Drive and Ashville Park Boulevard Water Surface Elevation (NAVD 88: 7.3 ft

98 Memorandum November 28, 2016 Page 8 Figure 6: Approximate Location: Intersection of Lubao Lane and Ashville Park Boulevard Water Surface Elevation (NAVD 88: 7.6 ft

99 Memorandum November 28, 2016 Page 9 Figure 7: Approximate Location: Intersection of Lubao Lane and Ashville Park Boulevard Water Surface Elevation (NAVD 88: 7.6 ft

100 Memorandum November 28, 2016 Page 10 Figure 8: Approximate Location: Intersection of Lubao Lane and Ashville Park Boulevard, looking east toward BMP 2. Water Surface Elevation (NAVD 88: 7.6 ft

101 Memorandum November 28, 2016 Page 11 Figure 9: CO5A Approximate Location: Intersection of Channing Lane and Grandon Loop Road Water Surface Elevation (NAVD 88: 7.3 ft

102 Memorandum November 28, 2016 Page 12 Figure 10: A05A Approximate Location: Intersection of Grandon Loop Road and Brightwood Drive Water Surface Elevation (NAVD 88: 7.1 ft

103 Memorandum November 28, 2016 Page 13 Figure 11: A05A Approximate Location: Intersection of Grandon Loop Road and Brightwood Drive Water Surface Elevation (NAVD 88: 7.1 ft

104 Memorandum November 28, 2016 Page 14 Figure 12: A05A Approximate Location: Intersection of Grandon Loop Road and Brightwood Drive Water Surface Elevation (NAVD 88: 7.1 ft

105 Memorandum November 28, 2016 Page 15 Figure 13: A02 Approximate Location: Intersection of Channing Lane and Brightwood Drive Water Surface Elevation (NAVD 88: 7.0 ft

106 Memorandum November 28, 2016 Page 16 Figure 14: A02 Approximate Location: Intersection of Channing Lane and Brightwood Drive Water Surface Elevation (NAVD 88: 7.0 ft

107 Memorandum November 28, 2016 Page 17 Figure 15: A02 Approximate Location: Intersection of Channing Lane and Brightwood Drive Water Surface Elevation (NAVD 88: 7.0 ft

108 Memorandum November 28, 2016 Page 18 Figure 16: Approximate Location: Intersection of Carmarillo Lane and Wilshire Drive Water Surface Elevation (NAVD 88: 7.1 ft

109 Memorandum November 28, 2016 Page 19 Figure 17: A38 Approximate Location: Near 2025 Grandon Loop Road Water Surface Elevation (NAVD 88: 7.5 ft Comment: This water surface elevation was observed at 2:00 am on September 22, The measurement was submitted by a resident and not made by the Department of Public Works.

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111 Appendix B Stormwater Model Development Methodology B-1

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113 Appendix B Stormwater Model Development Methodology B.1 Introduction The primary objective for developing hydrologic and hydraulic (H/H models of the Ashville Park Subdivision was to provide a tool suitable for evaluating the performance of the stormwater management system and evaluate alternative improvements to meet the desired level of service for flood control. This Appendix describes the approach taken to develop and apply H/H model for this purpose. This section proceeds through the model development process, 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 (i.e. boundary condition. B.2 Model Development and Application B.2.1 Stormwater Model Software Stormwater models were developed using PCSWMM Version 6.3 by CHI (Computational Hydraulics International. PCSWMM uses the U.S. 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-2-d development tool that allows gridded overland flow modeling using fully compliant EPA SWMM input. B.2.2 Levels of Detail, Temporal Scales and Numerical Time Steps The levels of detail must be adequate to accurately define and characterize flooding and erosion problems and represent the local and sub watershed-wide effects of each alternative and/or series of alternatives in order to cost-effectively size projects to solve existing problems. To achieve the objectives of the evaluation of the Ashville Park Subdivision, it was considered necessary to include all pipes and inlets including yard drainage as well as ditches and ponds. To accurately represent watershed hydrology, the rainfall interval should be less than the travel times within the smallest subbasin. For this project, a 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. B.3 Model Development Process This section presents an overview of the modeling development process that was applied. B.3.1 Collect Data and Characterize Watershed Table B-1 provides a list of data sources that were used for model development. B-3

114 Appendix B Stormwater Model Development Methodology Table B-1 Virginia Beach Stormwater Master Plan Available GIS Data Data Type Name Description Topographic Data LiDAR_DSM_ foot DEM This digital surface model (DSM has been prepared from LiDAR data acquired in the spring of The fundamental vertical accuracy for bare earth elevations is 0.42 feet, with a consolidated vertical accuracy of 0.64 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. Impervious Area Land Use Structures and Physical Features (2014 LandTable.mdb (2015 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. Stormwater Infrastructure Stormwater (2014 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. Other sources of information used for this study included: Watershed 9, Watershed Management Plan, URS 2003 Stormwater Management Study, Ashville Park, MSA, P.C., September 2016 Construction Plans and As Built Drawings of the SMS City GIS Data 1990 CDM Master Plan and EPA SWMM 4 Model 2003 URS EPA SWMM 4 Model September 2016 MSA, P.C. Autodesk Storm and Sanitary Analysis (SSA Model 2014 Homefed-Ashville Park Wetlands Delineation, Kimley-Horn and Associates Inc Plans for future villages and applicable development plans Soil borings from Grandview Village and Ashville Park Clubhouse Field survey of control structures for the existing detention basins and water levels Documentation of observed flooding conditions B-4

115 Appendix B Stormwater Model Development Methodology 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. Since much of the development was built after the land use, impervious, and LiDAR datasets were developed, assumptions were made for areas that had a significantly different coverage based on building permits and aerial pictures. 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 UNITID (Unit Identification in the junction shapefiles (labeled as: inlet, manholes, nodes, and misc. For the storm main (conduit shapefile, critical attributes include UNITID, pipe shape, pipe ht (height, pipe diam (diameter for circular pipes, width for elliptical, and upstream and downstream 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. B.3.2 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 upon the locations of inlets, manholes, and other miscellaneous nodes in the GIS layers. in addition to the GIS data, SMS data obtained from field survey and/or design plans have been added to the model. After the model network was defined, subbasins were delineated based on available topographic and aerial coverage data, and local stormwater system maps, and the hydrograph load points 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. Boundary Conditions for the stormwater system evaluation includes design-frequency tailwater elevation in combination with coincident rainfall applicable to tidal flood conditions. Section B.5.8 below provides a more detailed discussion of the development of model boundary conditions. B-5

116 Appendix B Stormwater Model Development Methodology B.3.3 Model Calibration Following initial model development, the simulation results were compared against known flooding conditions within the study area. 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 calibration storms, historical flood data, and results. B.4 Hydrologic Data and Parameters B.4.1 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 four 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. B-6

117 Appendix B Stormwater Model Development Methodology Normalized Rainfall (hr Figure B-1 Ashville Park Design Rainfall Data 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, 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. Table B-2 Design Storm Volumes Return Period/ Duration Rainfall Depth (inches Peak Intensity (inches/hr 2-year year year year year year B.4.2 Topography and Vertical Datum Time (min Topographic data are used to define hydrologic boundaries, runoff flowpaths 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 two dimensional surface with elevation values at discrete points B-7

118 Appendix B Stormwater Model Development Methodology on the surface. These discrete points are tiles each having a specific elevation value and a resolution of 2.5 feet in length and width. The fundamental vertical accuracy for bare earth elevations is 0.42 feet, with a consolidated vertical accuracy of 0.64 feet. This DEM was utilized in order to determine hydrologic and hydraulic overland flow paths, stagearea storage relationships, and subbasin boundaries. The elevation data used in the computer models and provided in this report are referenced to the North American Vertical Datum of 1988 (NAVD 88. B.4.3 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 on primarily on the DEM, land use, aerial photographs, 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 is an additional input parameter (see Section B below and was estimated by land use, building footprints from aerial images, and building permits issued at the time. Subbasin roughness and initial abstraction were also assigned according to the land use within the subbasin. The soils parameters are estimated from the soils coverage and soil boring logs. Section B.4.7 below describes how these data were reduced for use in the model. B.4.4 Land Use Parameters and Impervious Areas Land use data are used to estimate impervious to pervious routing, surface friction factors, and initial abstractions for each subbasin. Available land use data obtained from the City does not reflect the development of Ashville Park subsystem. Therefore, for purpose of determining the hydrologic characteristics for the existing site conditions, any area that had an impervious coverage greater than 5 percent was assumed to be medium density residential. Imperviousness was estimated based on building footprints from aerial images and building permits issued at the time corresponding to model simulation. An aggregate impervious percentage was calculated for these built lots based on available aerial and GIS information. For buildings with a lot coverage higher than the average, the GIS impervious coverage was used. The aggregate impervious coverage distribution of undeveloped lots is 5 percent, based on the area covered by sidewalks in these subcatchments. 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 B-8

119 Appendix B Stormwater Model Development Methodology 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 ten categories of relatively homogeneous geophysical parameters. Present land uses within the study area include: Golf Courses and Agriculture (Ag/GC Low Density Residential (LDR Medium Density Residential (MDR Commercial, Light Industrial, and Institutional (Comm Wetlands; and (Wetlnd Waterbodies (Water B 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: 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. 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 B-9

120 Appendix B Stormwater Model Development Methodology 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 set to 25 percent, unless adjusted for validation. 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 0.05 inches to 0.5 inches and vary by subbasin and land cover. A typical channel section from the model is presented in Figure B-2. Table B-3 Published Values of Manning n Roughness Coefficients for Overland Flow Source Ground Cover Manning n Range Crawford and Linsley ( Smooth asphalt Asphalt of concrete paving Packed clay Packed clay Light turf Dense turf Dense shrubbery and forest litter Engman ( Concrete or asphalt Bare sand Graveled Surface Bare clay-loam (eroded Range (natural Bluegrass sod Short grass prairie Bermuda grass Note: Global values of land use dependent variables are compiled in Table B-4. 1 Crawford, N.H. and Linsley, R.K., Digital Simulation in Hydrology: Stanford Watershed Model IV, Tech. Report No. 39, Civil Engineering Department, Stanford University, Palo Alto, CA, July Engman, E.T., Roughness Coefficients for Routing Surface Runoff, Journal of Irrigation and Drainage Engineering, ASCE, Vol. 112, No. 1, February 1986, pp B-10

121 Appendix B Stormwater Model Development Methodology 6.5 Transect DW9_SB_ Elevation (ft Station (ft Figure B-2 Typical Channel Section B-11

122 Appendix B Stormwater Model Development Methodology 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 above for heading land use definitions. The parameters in Table B-4 were incorporated 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. B Impervious Area Any rainfall that occurs on impervious area becomes surface runoff once its depression storage is filled. Since the City s impervious coverage data does not reflect the state of development of the subdivision, the number of built lots were estimated from aerial images and building permits issued. Based on the lot plans and available GIS information, an aggregate impervious percentage of 38% was calculated for these lots. Some buildings, particularly in Wilshire village, had a higher lot coverage than the 38% average. Since the impervious footprint of these buildings were present in the City s Impervious GIS layer, the actual coverage was assigned to these lots. B.4.5 Runoff Parameters For this project, kinematic wave flow routing techniques (EPA SWMM RUNOFF methodology have been used as opposed to more traditional unit hydrograph techniques for the following reasons: 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. SWMM runoff is a more rigorous, parameter based methodology which 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 storm water runoff hydrograph are defined as opposed to a unit hydrograph. Each model subbasin requires the following runoff parameters: Subbasin Area The total subbasin area calculated in GIS. B-12

123 Appendix B Stormwater Model Development Methodology 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. B Length and Slope The length (L parameter is the average area-weighted travel length to the hydraulic model load point. For ponded or detention storage areas, the hydraulic model load point is typically the centroid of ponding. 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 (i.e., 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 load point located along the modeled stormwater system. The subbasin average surface slope was determined as the area-weighted average slope for the representative flow paths. Area-weighted flow path lengths and slopes were initially determined utilizing available topographic data. The topography within the study area can be alternating steep and flat areas. Therefore, it is important to consider the effect of sudden drops in elevation with respect to the remainder of the flow path when defining a subbasin average surface slope. For example, a relatively shallow slope of a few feet over many thousands of feet in distance should not have an included 10 feet drop over 30 feet (or less at the bank of a stream. The slope B-13

124 Appendix B Stormwater Model Development Methodology representing the overall subbasin is the former slope, where adding the latter steep slope would skew the overall velocity calculation. B.4.6 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, to avoid the inadvertent loss of infiltration capacity that can occur under certain conditions with the original algorithm. The Green-Ampt equation was used 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 soils 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 soils porosity and its field capacity. Estimated values for all of these parameters can be found in Table B-5. 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-5 Soil Parameter Estimates Soil Texture Hydraulic Conductivity (inches/hr 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-14

125 Appendix B Stormwater Model Development Methodology B.4.7 Hydraulic 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 are calculated. Once the area-weighted parameters were set for each subbasins, the values were moved to PCSWMM using the PCSWMM import tools. B.5 Hydraulic Data and Parameters The H/H model uses a node/link (junction/conduit representation of the stormwater management system. For this study, the SMS links were primarily circular pipes. 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 All nodes in the modeling area were set at least 10 feet above ground elevation, to provide a consistent offset. In the case of manholes, adding 10 feet to the rim effectively seals the manhole, by not allowing water to flood out of the model at that location. B.5.1 Stage Area Relationships Storage in the model is accounted for explicitly 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 either the topography from LiDAR, GIS, or the data from the developer s plans. 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 B-15

126 Appendix B Stormwater Model Development Methodology storage junctions as some subbasins have no storage beyond that which is represented in the model links. Since BMP ponds 1, 2, 3, 4, and 11 are linear in nature, they were modeled as conduits with irregular cross sections. The model cross sections for these ponds were derived from LiDAR and design profiles from construction plans. 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, where applicable. B.5.2 Culverts and Pipes Survey and as built 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. 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 and corrugated metal pipe (CMP roughness values were set to For HDPE and PVC pipes, a value of 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 (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 coefficients (Ke were set to Exit loss coefficients (Ko were set to 0.25 for manholes, 0.5 for pipes and culverts discharging into moving water, and 1.0 for pipes and culverts discharging into still water. Under the additional minor loss in SWMM, bend losses were added based on VDOT Table 9-9, Losses in Junction Due to Change in Direction of Flow Lateral. For example, a K value 0.7 is used for 90-degree bends and T s, a value of approximately 0.5 is used for 45-degree bends and a value of approximately 0.25 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. Plunging losses occur at the upstream elements in a drainage system, where the inflows are larger than 20 percent of the pipe flow. A factor of 1.3 was applied to inlets that were expected to meet this criterion. 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. B.5.3 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 B-16

127 Appendix B Stormwater Model Development Methodology that the hydraulic properties defined for the transect in 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 0.03 to 0.06 based on vegetation and engineering judgment. Roughness in the overbanks ranged from 0.04 to 0.08 based on vegetation and engineering judgment. Additional guidance on channel roughness is provided in Table B-6 (from Chow, VT, Open Channel Hydraulics, McGraw-Hill, Table B-6 Open Channel Manning s Roughness Values* Channel Type Main Channels Minimum Normal Maximum 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 Floodplains a. Pasture, no brush 1. Short grass High grass b. Cultivated areas 1. No crop Mature row crops Mature field crops c. Brush B-17

128 Appendix B Stormwater Model Development Methodology Channel Type Floodplains Minimum Normal Maximum 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 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 B-18

129 Appendix B Stormwater Model Development Methodology a. Cement Channel Type Lined or Constructed Minimum Normal Maximum 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 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 B-19

130 Appendix B Stormwater Model Development Methodology f. Brick Channel Type Lined or Constructed Minimum Normal Maximum 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 * - Only categories relevant to Virginia Beach are presented in this table. B.5.4 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. B.5.5 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 pipes discharging into receiving waters with fixed and time varying boundary conditions. The system outfall draining to Ashville Bridge Creek is modeled with a time varying boundary condition based on data collected from the City operated gage located at the creek. For subbasins discharging to a water body without pipes, these may be simulated as sheet flow to the river from the subbasins along the shore. B.5.6 Overland Flow Conduits The depth of flooding and movement of flood waters is controlled by two methods: stage storage curves in the storage nodes (see Section 2.5.1, and overland flow conduits. 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 need to be included in the model. There are two basic types of overland flow conduits: a weir type and a channel type. B 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, B-20

131 Appendix B Stormwater Model Development Methodology 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 (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 to 0.05 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-3 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 (e.g., 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. B 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 to 0.05 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 of 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 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 cannot be as accurate as the stage-area calculation performed in GIS. Therefore, most of the overland links are weir-types, with channel and mixed 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-21

132 Appendix B Stormwater Model Development Methodology B.5.8 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 provides flexibility for defining boundary conditions, and can adopt fixed stage boundaries or time series boundaries as necessary. The model outfall elevations were set based on field survey data and LiDAR estimates, as well as gage readings recorded during the calibration event. See the main report for information on the boundary condition used for each model outfall. B-22

133 Appendix C Model Results C-1

134 Appendix C Model Results This page intentionally left blank. C-2

135 Appendix C Model Results Table C-1 Model Results for Ashville Park Existing and Build-out Conditions (10-year Storm Model Node Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Flowline (feet Existing Buildout Existing Buildout C11A A C09A C03B A A14B A4A A A18B C05A C10A A C02A C-3

136 Appendix C Model Results Model Node Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Flowline (feet Existing Buildout Existing Buildout E A15B A08A A07D C06A A A03A A A A E E D02B D03A A12B A13B E E C-4

137 Appendix C Model Results Model Node Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Flowline (feet Existing Buildout Existing Buildout A18C E E A17A A A A05A A11A A10A E B03A B02A E E C-5

138 Appendix C Model Results Model Node Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Flowline (feet Existing Buildout Existing Buildout E C-6

139 Appendix C Model Results Model Node Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Flowline (feet Existing Buildout Existing Buildout C-7

140 Appendix C Model Results Model Node Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Flowline (feet Existing Buildout Existing Buildout C-8

141 Appendix C Model Results Model Node Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Flowline (feet Existing Buildout Existing Buildout Table C-2 Model Results for Ashville Park Existing and Build-out Conditions (100-year Storm Name Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Crown (feet Existing Buildout Existing Buildout C11A A C09A C03B A A14B A4A C-9

142 Appendix C Model Results Name Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Crown (feet Existing Buildout Existing Buildout A A18B C05A C10A A C02A E A15B A08A A07D C06A A A03A A C-10

143 Appendix C Model Results Name Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Crown (feet Existing Buildout Existing Buildout A A E E D02B D03A A12B A13B E E A18C E E A17A A A C-11

144 Appendix C Model Results Name Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Crown (feet Existing Buildout Existing Buildout A05A A11A A10A E B03A B02A E E C-12

145 Appendix C Model Results Name Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Crown (feet Existing Buildout Existing Buildout E C-13

146 Appendix C Model Results Name Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Crown (feet Existing Buildout Existing Buildout C-14

147 Appendix C Model Results Name Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Crown (feet Existing Buildout Existing Buildout C-15

148 Appendix C Model Results Name Critical Elevations (feet NAVD 88 Flowline (10-year Crown (100-year Peak Stages (feet NAVD year Storm Delta from Crown (feet Existing Buildout Existing Buildout Table C-3 10-year Storm Model Results for Alternatives A, B, C, and D Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D E E C11A A08A A18C A18B C05A A13B C-16

149 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D A C10A C09A A12B C03B A07D C06A A11A A A15B A14B C02A A4A A A10A A A A E E A C-17

150 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D A17A D03A B03A E A03A A05A A D02B E A C-18

151 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D E A E E B02A C-19

152 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D E E C-20

153 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D C-21

154 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D Table C year Storm Model Results for Alternatives A, B, C, and D Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D C11A A08A A13B A12B A07D A C-22

155 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D A11A E E C05A D03A A10A B03A C10A C-23

156 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D A A C06A A18C A18B A E C-24

157 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D A15B C03B D02B A14B A17A B02A C02A C09A A C-25

158 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D A4A A A03A A E A A A E A05A E E C-26

159 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D E E E E C-27

160 Appendix C Model Results Name Critical Elevations (feet NAVD year Storm Peak Stages (feet NAVD year Storm Delta from Crown (feet Flowline Crown Alt A Alt B Alt C Alt D Alt A Alt B Alt C Alt D C-28

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

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163 Table D-1 Ashville Park Stormwater Management System Flood Mitigation Plan Planning-Level Opinion of Probable Construction Cost Item Alternative A Alternative B Alternative C Alternative D Item Description Category Unit Unit Cost No. Qty Total Cost Qty Total Cost Qty Total Cost Qty Total Cost 1 15 RCP, Class III Piping LF $ $ 13, $ 13, $ 13, $ 28, RCP, Class III Piping LF $ 72 2,135 $ 153,720 2,335 $ 168,120 2,005 $ 144,360 2,403 $ 173, RCP, Class III Piping LF $ 89 3,653 $ 325,117 3,781 $ 336,509 4,111 $ 365,879 3,845 $ 342, RCP, Class III Piping LF $ $ 82, $ 56,592 1,000 $ 108,000 1,394 $ 150, RCP, Class III Piping LF $ 140 2,547 $ 356,580 2,617 $ 366,380 3,162 $ 442,680 3,589 $ 502, RCP, Class III Piping LF $ 170 1,900 $ 323,000 1,682 $ 285,940 1,197 $ 203, $ 153, RCP, Class III Piping LF $ 185 1,704 $ 315,240 1,762 $ 325,970 1,827 $ 337, $ 171, RCP, Class III Piping LF $ $ 184, $ 112, $ 15, $ 71, RCP, Class III Piping LF $ $ 84, $ 175,000 1,000 $ 350,000 1,250 $ 437, X 14 Box Culvert Culverts LF $ 2, $ 160, $ 498, $ 498, $ 498, x 30 RECP Piping LF $ $ - - $ - - $ $ 50, x 34 RECP Piping LF $ $ 36, $ 36,160 - $ $ 36, x 38 RECP Piping LF $ $ 61, $ 61, $ 104,500 - $ x 45 RECP Piping LF $ $ - - $ $ 44,000 - $ x 49 RECP Piping LF $ $ $ 50,000 - $ $ 50, x 53 RECP Piping LF $ $ 60, Cap/Plug Existing 18 RCP Piping EA $ 1,200 1 $ 1,200 1 $ 1,200 1 $ 1,200 1 $ 1, Drop Inlets Piping EA $ 5, $ 645, $ 645, $ 645, $ 645, Demolition of Pavement Demolition SY $ 15 26,345 $ 395,175 26,345 $ 395,175 26,345 $ 395,175 26,345 $ 395, Remove Existing Curb & Gutter Demolition LF $ 10 9,704 $ 97,040 9,704 $ 97,040 9,704 $ 97,040 9,704 $ 97, Select Material Pavement Replacement CY $ 35 8,782 $ 307,370 8,782 $ 307,370 8,782 $ 307,370 8,782 $ 307, Aggregate Base Material No. 21A Pavement Replacement TON $ 50 9,484 $ 474,200 9,484 $ 474,200 9,484 $ 474,200 9,484 $ 474, Asphalt Concrete Base Course (Type BM-25 Pavement Replacement TON $ 110 4,347 $ 478,170 4,347 $ 478,170 4,347 $ 478,170 4,347 $ 478, Asphalt Concrete (Type SM-9.5 Pavement Replacement TON $ 110 2,173 $ 239,030 2,173 $ 239,030 2,173 $ 239,030 2,173 $ 239, Curb and Gutter (CG-6 Pavement Replacement LF $ 30 9,704 $ 291,120 9,704 $ 291,120 9,704 $ 291,120 9,704 $ 291, Inlet Protection E&SC EA $ $ 38, $ 38, $ 38, $ 38, End Section (ES-1 Piping EA $ 1, $ 26, $ 26, $ 26, $ 26, ES-1A for elliptical pipes Piping EA $ 1,600 3 $ 4,800 3 $ 4,800 3 $ 4,800 3 $ 4, Culvert Outlet Protection (EC-1, Class I Riprap Piping TON $ $ 61, $ 61, $ 61, $ 61, Detour Signs Miscellaneous EA/DAY $ $ 9, $ 9, $ 9, $ 9, Flaggers Miscellaneous HR $ $ 9, $ 9, $ 19, $ 9, Traffic signs Miscellaneous EA/DAY $ $ 6, $ 3, $ 6, $ 3, Temporary Silt Fence E&SC LF $ 5 11,000 $ 55,000 11,000 $ 55,000 11,000 $ 55,000 11,000 $ 55, Coffer Dams Ponds EA $ 30,000 4 $ 120,000 5 $ 150,000 4 $ 120,000 4 $ 120, Pond Excavation Ponds CY $ 16 47,237 $ 755,792 47,237 $ 755,792 47,237 $ 755,792 47,237 $ 755, Turbidity Curtain Ponds LF $ $ 10, $ 10, $ 10, $ 10, Temporary Safety Fence Ponds LF $ $ 2, $ 2, $ 2, $ 2, Bank Stabilization E&SC AC $ 15,000 7 $ 97,950 7 $ 97,950 7 $ 97,950 7 $ 97, Clearing and Grubbing Miscellaneous LS $ 30,000 1 $ 30,000 1 $ 30,000 1 $ 30,000 1 $ 30, Ditch Dredging Channels CY $ 22 5,820 $ 128,040 5,820 $ 128,040 5,820 $ 128,040 5,820 $ 128,040 Page 1 of 2

164 Table D-1 Ashville Park Stormwater Management System Flood Mitigation Plan Planning-Level Opinion of Probable Construction Cost Item Alternative A Alternative B Alternative C Alternative D Item Description Category Unit Unit Cost No. Qty Total Cost Qty Total Cost Qty Total Cost Qty Total Cost 41 Ditch Construction Channels CY $ 22 47,727 $ 1,049,994 24,179 $ 531,938 52,673 $ 1,158,806 45,649 $ 1,004, Rock Check Dam E&SC EA $ $ 4,900 7 $ 4,900 8 $ 5,600 8 $ 5, Temporary Silt Fence E&SC LF $ 5 15,341 $ 76,705 13,516 $ 67,580 17,516 $ 87,580 15,121 $ 75, Gated Outfall Channels EA $ 300,000 2 $ 600,000 1 $ 300,000 1 $ 300,000 2 $ 600, Construction Entrance E&SC EA $ 1,500 2 $ 3,000 2 $ 3,000 2 $ 3,000 2 $ 3, cfs Stormwater Pump Station Pump Station EA $ 3,000,000 1 $ 3,000,000 1 $ 3,000,000 - $ - 1 $ 3,000, cfs Stormwater Pump Station Pump Station EA $ 5,000,000 - $ - - $ - 1 $ 5,000,000 - $ X 10 Box Culverts - Ashville Park Blvd. Culverts LF $ 1,800 - $ $ 756,000 - $ - - $ - 49 Remove existing twin 36 pipes Demolition LF $ $ - 80 $ 11,200 - $ - - $ X 8 Box Culvert - Sandbridge Road Culverts LF $ 1,300 - $ - 1,610 $ 2,093,000 - $ - - $ - 51 Sheet pile Culverts SF $ 40 - $ - 25,760 $ 1,030,400 - $ - - $ - 52 Bored Road Crossing - Princess Anne Road Piping LF $ $ - - $ - 80 $ 64,000 - $ - 53 Jacking Pits Piping EA $ 4,000 - $ - - $ - 2 $ 8,000 - $ Force Main Piping LF $ $ - - $ $ 125,000 - $ x 6 Box Culvert - New Bridge Road Culverts LF $ 1, $ 64,000 - $ - 40 $ 64, $ 64, Mill & Overlay Pavement Replacement SY $ 12 16,262 $ 195,144 16,262 $ 195,144 16,456 $ 197,472 14,665 $ 175, Seepage Barrier - Sheeting (10 Deep Channels SF $ 15 51,650 $ 774,750 - $ - 51,650 $ 774,750 65,050 $ 975, Regrade New Bridge Culvert Pavement Replacement LS $ 50,000 1 $ 50,000 - $ - 1 $ 50,000 1 $ 50, Intake Pond for PS (Alternative D Ponds CY $ 16 - $ - - $ - - $ - 5,567 $ 89, Developer Ponds 10A, 16, and 17 Ponds CY $ 16 10,925 $ 174,800 10,925 $ 174,800 10,925 $ 174,800 10,925 $ 174, Developer Ditch from Pond 10 to Pond 13 Channels CY $ 22 15,530 $ 341,660 - $ - 15,530 $ 341,660 15,530 $ 341,660 Subtotal $ 12,775,444 $ 14,966,511 $ 15,276,931 $ 13,506,923 Mobilization $ 668,772 $ 778,326 $ 793,847 $ 705,346 Contingency (30% $ 4,033,265 $ 4,723,451 $ 4,821,233 $ 4,263,681 Engineering, Survey, & Permitting (15% $ 2,621,622 $ 3,070,243 $ 3,133,802 $ 2,771,393 Total $ 20,099,103 $ 23,538,531 $ 24,025,813 $ 21,247,343 USE SAY $ 20,100,000 $ 23,500,000 $ 24,000,000 $ 21,200,000 Page 2 of 2

165 Appendix E Alternatives Analysis E-1

166 Appendix E Alternatives Analysis E-2

167 Appendix E Alternatives Analysis Table E-1 Alternative Improvements Evaluated* Alternative Description Evaluation Performance Comments 1 Clean pipes, make outfall ditches bigger Rejected Does not meet LOS Improvements shown on the developer s conceptual plan (BMPs 16, 17, 10A and stormwater system/ditches to move water off proposed developments to BMPs to meet City LOS, BMP 2A, ditch from BMP 4 through BMP 10A, clean pipes, make outfall ditches bigger Improvements shown on the developer s conceptual plan from Alternative 2 plus local storage in pocket parks, additional pipe network to pocket parks, BMP 6 Alternative 3 plus lower normal pool to 2 feet Improvements shown on the developer s conceptual plan from Alternative 2, expand BMP 6, significant additional piping to BMPs 1 through 4 and 2A, lower normal pool to 2 feet NAVD 88 Improvements shown on the developer s conceptual plan from Alternative 2, significant additional piping to BMPs 1 through 4 and 2A, lower normal pool to 2 feet NAVD 88, set aside large parcel north of Wilshire for additional storage - with improvements necessary to move water north Lowered BMPs 1 through 4 to a fixed elevation of 2 feet NAVD 88 - MSA Improvements, but no other changes to primary stormwater management system (PSMS Improvements shown on the developer s conceptual plan from Alternative 2, ditch along Sandbridge Rd that would encroach on the Fish and Wildlife property, large box culverts between BMPs 1 through 14 Improvements shown on the developer s conceptual plan from Alternative 2, covered open box culvert along Sandbridge Rd, large box culverts between BMPs 1 through 14, large box culverts inside of Wilshire and Ranier to move water to BMPs 1 through 4, 2A and expanded BMP 6 Rejected Rejected Rejected Rejected Rejected Rejected Rejected Rejected Does not meet LOS Does not meet LOS Concerns about public acceptance of loss of pocket parks and would not likely meet 100-yr storm LOS Meets 10-yr LOS, does not meet 100-yr LOS Meets 10-yr LOS, does not meet 100- yr LOS, concerns about available capacity of existing drainage system to the north Conducted as a test to see if LOS could be met within Wilshire and Ranier if the peak stages in BMPs 1 through 4 could be maintained at 2 feet NAVD 88. Neither the 10-yr nor 100-yr LOS could be met with the existing stormwater system, thus additional piping is needed in all alternatives Concerns about the land or easement acquisition that would be required from USFWS HGL between BMPs 1 through 4 and BMP 14 can be kept flatter with open ditches than it can with large boxes, at significantly less cost E-3

168 Appendix E Alternatives Analysis Alternative Description Evaluation Performance Comments Improvements shown on the developer s conceptual plan from Alternative 2, additional piping to BMPs 1 through 4 and 2A, 8 by 15 feet box culverts between BMP 14 and Lotus Creek, revised BMP 14 structure, ditches from BMP 4 to BMP 14 (using/expanding on MSA options, weirs in BMPs 3 and 4 and small pump to lower normal pools to 2 feet NAVD 88, deepen historical ditch in Wilshire, add northern storage area (see test 6 Alternative 10 plus box culverts from BMP 17 to an outfall ditch behind Village E (prior to city rejecting northern storage - described in further detail for more accepted version, see below Alternative 10 plus use of outfall ditch behind Villages D and E (prior to city rejecting northern storage - described in further detail for more accepted version, see below Alternative 10 plus pumping west across Princess Ann Rd (prior to city rejecting northern storage - described in further detail for more accepted version, see below Improvements shown on the developer s conceptual plan from Alternative 2, expand BMP 6, significant additional piping to BMPs 1 through 4, to the expanded BMP 6, and to 2A, lower normal pool to 2 feet NAVD 88 using weirs and a pump between BMPs 6 and 9 (lowers normal pool in BMPs 6 through 9 versus above, deepen historical ditch in Wilshire and connect to BMP 6 pipes. Add 8 feet by 15 feet box culverts between BMP 14 and Lotus Creek, revised BMP 14 structure, ditches from BMP 4 to BMP 14 (using/expanding on MSA options with a short section of box culvert between BMP 4 and BMP 15 where the available land is narrow Alternative 14 plus an additional box culvert connection between BMPs 6 and 10 Old Alternative 1 (see below with ditches along Ashville Park Blvd having concrete bottom Rejected Rejected Rejected Rejected Rejected Rejected Rejected Prefer to expand BMP 6 and moving floodwater east instead of north. Location and land acquisition for the north parcel is a concern Prefer outfall ditch behind Village E (also - northern storage was rejected in favor expanding BMP 6 and moving floodwater east instead of north was preferred by the City Prefer to expand BMP 6 and moving floodwater east instead of north. Location and land acquisition for the north parcel is a concern Prefer to expand BMP 6 and moving floodwater east instead of north. Location and land acquisition for the north parcel is a concern Meets LOS, options below are preferred and less costly Meets LOS, options below are preferred and less costly. The additional culverts did not significantly lower stages, but did help drain the system post-storm Meets LOS, options below are preferred and less costly. The alternatives below work with center channel Mannings n = i.e., can be well maintained grass bottom E-4

169 Appendix E Alternatives Analysis Alternative Description Evaluation Performance Comments Alternative 1 - Ditch behind Villages D and E: MSA Improvements (BMPs 16, 17, 10A and stormwater system/ditches to move water off proposed developments to BMPs to meet City LOS, new BMP 2A, expand BMP 6, ditches (10-feet bottom near -1.5 feet NAVD 88, 3:1 ss, n=0.03 along Ashville Park Blvd from BMP 4 through BMP 10, ditches (10 feet bottom from -1 to feet NAVD 88, 3:1 side slopes (ss, center n = from BMP 10 to New Bridge Rd, operable gate and 6 feet by 10 feet box under New Bridge Rd, 20 cfs PS in same location - to be used to lower normal pool pre-storm to 1.0 foot NAVD 88, gated 10 feet wide control structure between BMP 14 and Flanagans Ln at 1.7 feet NAVD 88 to allow Ashville Park BMPs to pool at 2 feet NAVD 88 by gravity and prevent backflow to Ashville Park if Lotus Creek is high. Improved ditch along Flanagans Ln and Sandbridge Rd and 570 feet along Sandbridge Rd towards Lotus Creek. Significant additional parallel PSMS within Wilshire and Ranier Alternative 1 with 20 cfs pump off after pool lowered, but prior to storm peak Alternative 2 - Alternative 1 with expanded BMP 1/removal of a section of Lubao Ln Alternative 3 - Improve Sandbridge Rd: MSA Improvements (BMPs 16, 17, 10A and stormwater system/ditches to move water off proposed developments to BMPs to meet City LOS, new BMP 2A, expand BMP 6, ditches (10 feet bottom sloping from -1 to -2 feet NAVD 88, 3:1 ss, n = 0.03 along Ashville Park Blvd from BMP 4 through BMP 14, very large culverts under entrance roads to both D and E (may be bridges, operable gate and 8 feet by 10 feet box at BMP 14 to control Ashville Park flows by gravity for a 2 feet NAVD 88 normal pool and prevent backflow, 20 cfs PS in same location - to lower normal pool pre-storm to 1.0 foot NAVD 88, 8 ft by 8 ft box culverts along Flanagans Ln and Sandbridge Rd to Lotus Creek, significant additional parallel PSMS within Wilshire and Ranier Alternative A Rejected Rejected Alternative B Meets LOS Does not meet LOS. The pump station needs to operate and maintaining the pool at 1 foot NAVD 88. It may be turned off at the peak of the storm, however Meets LOS, options below are preferred and less costly. Change in the additional PSMS in Wilshire and Ranier is minimal Meets LOS E-5

170 Appendix E Alternatives Analysis Alternative Description Evaluation Performance Comments Alternative 4 - This is similar to Alternative 1, but without the ditch behind Villages D and E, though the gravity control structure and improved ditch at and downstream of BMP 14 remain. This alternative includes a 150 cfs pump station near BMP 1 and 800 feet of 4 feet forcemain to and under Princess Ann Rd. West of Princess Ann Rd, the ditch is improved to a 4 feet wide bottom from 10 feet to 5 feet NAVD 88 with 3:1 ss and n = The culvert under a farm rd is also improved to a 4 feet by 8 feet box. This alternative includes two 5 feet circular intake pipes from BMPs 1 and 2 to the PS on the west side of Lubao Ln. As before, it requires significant additional parallel PSMS within Wilshire and Ranier Alternative 5-50 cfs Pump West. This is similar to Alternative 1, with the ditch behind Villages D and E, the gated structure under New Bridge Rd, and the gravity control structure and improved ditch at BMP 14. This alternative includes a 50 cfs pump station near BMP 1 and 500 feet of 3 feet forcemain to and under Princess Ann Rd. West of Princess Ann Rd, the ditch is improved as above, but to lower depths (ditches are improved to 3:1 ss with n = The culvert under the farm road is improved to a 42 inch RCP. This alternative includes two barrels of 3 feet circular intake pipes from BMPs 1 to the PS on the west side of Lubao Ln. The number of and size of the additional parallel PSMS within Wilshire and Ranier is reduced from Alternative 1 Alternative 3 with no BMP 6 expansion - change additional PSMS Alternative 1 with expanded BMPs 10A and 17, plus a ditch connecting BMP 8 and BMP 11 Alternative 8 - Move PSMS behind Ranier Village. This alternative is the same as Alternative 1, but with a portion of the additional PSMS moved to a system that is proposed behind (south and east of Ranier. The additional proposed system running through Ranier are removed or significantly downsized Rejected Alternative C Rejected Rejected Rejected Receiving waters west of Princess Ann Rd cannot accommodate an additional 150 cfs on peak Meets LOS Additional storage is needed to attenuate flows Did not produce improved results Pump station and gate are located in a less resilient location than included in Alternative 26 E-6

171 Appendix E Alternatives Analysis Alternative Description Evaluation Performance Comments 26 Alternative 9 - Move gate and PS closer to Village E, away from New Bridge Rd. This alternative is Alternative 8, but with the 20 cfs PS and gate moved up the outfall ditch to a location adjacent to Village E. This location is higher and upstream of a natural ridge. The pump station and control gate are kept together to maintain the backup power and controls at one location. This alternative includes a non-gated 6 feet by 10 feet box culvert under New Bridge Rd * Refer to Figure 7-1 for conceptual infrastructure layouts Alternative D Meets LOS E-7

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