SouthEast Connector Flood Analysis Memorandum

Transcription

1 TECHNICAL MEMORANDUM SouthEast Connector Flood Analysis Memorandum PREPARED FOR: COPY TO: PREPARED BY: Garth Oksol/RTC Jeff Hale/RTC Cindy Potter/CH2M HILL Matthew Setty/CH2M HILL Mary Horvath/CH2M HILL DATE: October 2, 2013 PROJECT NUMBER: This flood analysis memorandum presents the hydraulic design process used to design the SouthEast Connector roadway and floodplain. The proposed design is in compliance with local agency flood management ordinances and appropriate design criteria. The design team used multiple techniques based on industry standards to determine the project design components; the team used the same techniques to determine the interaction of the proposed infrastructure on flood event performance and impacts to the designated flood storage pools and areas within Critical Flood Zone 1. This memorandum presents additional information to supplement the 50% Drainage Design Report; it is not intended to supplant the Drainage Design Report developed for the 50% design (which will be updated at the 90% design level). This memorandum details project compliance with the local flood management ordinances. Introduction The SouthEast Connector (SEC) is a 5.5 mile, six lane arterial transportation corridor along the eastern side of the Truckee Meadows. It extends Veterans Parkway from South Meadows Parkway to the intersection of Greg Street and Sparks Boulevard. Phase 1 extends from the intersection of Sparks Boulevard and Greg Street to just south of Clean Water Way. Phase 2, hereinafter referred to as the project, extends from the south end of Phase 1 to the intersection of Veterans Parkway and South Meadows Parkway. The proposed Veterans Parkway extension, or SEC, includes 3 new signalized intersections at Greg Street, Pembroke Drive, and Mira Loma (see Figure 1; all figures and tables are located at the end of the memorandum). The alignment bisects a regulatory floodplain and floodway, resulting in additional off site design considerations and flood pool volume mitigation. The purpose of the SEC is to provide improved north south regional linkages between the South Truckee Meadows and the City of Sparks for all modes of travel. The project will alleviate congestion on I 80, US 395/I 580, and McCarran Boulevard, as well as other well traveled streets. The SEC provides three travel lanes in each direction, a multi use path, and maintenance access where appropriate. Purpose and Objective of Flood Analysis This flood analysis memorandum presents the hydrologic, hydraulic, and civil design data required to demonstrate compliance with the City of Reno and Washoe County flood management ordinances. To achieve compliance with the flood management ordinances, the SEC design complies with the volumetric mitigation by removing a greater volume of material from the flood storage pool (Critical Flood Zone 1) than the fill requirements for construction of the roadway. In addition, this memorandum presents the analyses that demonstrate how the overall system will perform to reduce the flood risk in the design event throughout the project, including developed areas and areas for future development. RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH) 1

2 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM The objectives of the hydraulic design of the roadway and floodplain (and subsequent analysis of that design) are as follows: (1) to provide for compliance with applicable flood management ordinances, and (2) to demonstrate flood system performance that complies with standard design criteria, balances hydraulic forces throughout the roadway footprint, provides for appropriate hydraulic connectivity across the roadway prism, and achieves the intent of the flood management ordinances throughout the Steamboat Creek and Truckee River floodplain for the design storm event. This project has been designed to comply with Policy 3.1.b of the Washoe County Comprehensive Regional Water Management Plan (Flood Plain Storage within the Truckee River Watershed) and Ordinance No of the City of Reno Code of Development. The SEC is located within the Critical Flood Zone 1 as defined in the Washoe County Plan and the City of Reno. The SEC s footprint of impervious area is extremely small in relation to the Truckee River Watershed. Runoff will have an extremely short travel time to both the Steamboat Creek and the Truckee River compared to their overall watersheds. Because of this, any increase in peak runoff or runoff volume from the area of the roadway will be negligible and will not increase the peak flow rate or volume for the Truckee River or Steamboat Creek. Stormwater runoff from the roadway will drain far in advance of the much larger off site flows from the upper watershed that create the maximum peak flows and flood pool elevations. Therefore, the governing criteria for the off site portion of this project will be the flood volume mitigation that is required for development within the Critical Flood Zone 1, and demonstrating a no rise condition for the flood pool in the post project condition. Mandated flood volume mitigation, as defined by the Regional Water Management Plan Policy 3.1.b plan and within City of Reno Ordinance No. 6164, is the excavation of one part volume for every one part volume of flood storage volume displaced. The excavation must occur within the same vicinity and at the same elevation band as the flood volume displaced. The text of the applicable ordinances is provided below. To mitigate the impact of the proposed embankment to the flood pool and upstream floodplain of the Steamboat Creek, mitigation volume will be excavated adjacent to the Steamboat Creek and/or roadway embankment. To allow conveyance through the embankment, large equalization culverts will be constructed at multiple locations along the embankment. Additionally, five bridges are included in the design at the following locations: Huffaker Narrows, upstream of Mira Loma Drive, within the Rosewood Lake Golf Course, at Boynton Slough, and over Yori Drain. The goal of the regional flood analyses is to show that the mitigation volume and equalization culverts/bridges provide adequate flood storage and conveyance to allow for no rise of the maximum water surface elevation during the design flood event in the with project conditions. The no rise (no adverse impact) analysis for the Truckee Meadows Critical Flood Pool is being completed using the United Stated States Army Corp of Engineers (USACE) Truckee Meadows HEC RAS model (updated in 2012) for use in planning of the Truckee River Project. The SEC proposed conditions HEC RAS model is identical to the SEC existing conditions model, except that the cross sections, lateral weirs, storage areas, and storage area connections impacted by the SEC project are modified to reflect the proposed project geometry of Phase 1 and 2 of the SEC, including the proposed embankments, mitigation volume areas, equalization culverts, bridges, and floodplain grading modifications. The proposed conditions model is being run with the 117 year design event (which is considered to be close to the flood of record on the Truckee River that occurred in December 1997). Results from the proposed and existing conditions models are compared to ensure that the design does not cause a rise in peak water surface elevation within the Critical Flood Zone 1 or peak flood flows on the Steamboat Creek or Truckee River downstream of the SEC project area. City of Reno Ordinance No The City of Reno has adopted Ordinance No As detailed in the Reno Municipal Code , this ordinance dictates conditions required to meet the No Adverse Impact when displacement of available flood storage volume occurs. 2 RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH)

3 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM Section Critical Flood Pools Zone 1 (a) Any public or other development that changes existing grades, places fill, imports and or displaces any volume of water within Critical Flood Zone 1 for the Truckee River shall meet the following requirements: (1) Storm water discharges from the project shall be limited to predevelopment conditions relative to peak flows: and (2) Flood storage volume mitigation meeting No Adverse Impact when displacement of available flood storage volume occurs below 1997 water surface elevation of the Critical Flood Zone 1. (b) Subject to the concurrence of the administrator, the requirements of (a)(1) and (a)(2) above may be met by: (1) Excavation of one part volume for every one part volume of flood storage volume displaced. The excavation must occur within a. the same Flood Storage Area as the volume placed, and b. at the same elevation band as the volume displaced per the Elevation Map. (2) When mitigation within the same Flood Storage Area and within the same elevation band cannot be met because of utility conflicts, topography, high groundwater, or other conditions exist, mitigation outside of the same Flood Storage Area but within the Critical Flood Zone 1 must: i. provide for at least one part excavation volume to one part volume displaced and ii. demonstrate through application and output of the appropriate Truckee River Project Mitigation Model the actual mitigation required (3) The entire mitigation volume of mitigation areas must be available for flood storage during any flood event. Detention basins required by other ordinances are not eligible for mitigation of lost storage volumes. (4) Mitigation cannot occur in conflict with and/or at the same location of approved Truckee River Flood Project Features which are shown on the Truckee River Flood Project Features Map on file with the City of Reno Community Development Department and Public Works Department, as amended, and incorporated by reference. (5) Mitigation must occur concurrently with, or prior to, a reduction of the flood storage volume. Washoe County Development Code (as amended by Ordinance 1257, effective date ) Standards for All Development in Critical Flood Storage Zones. The standards for development in critical storage zones are set forth in this section. (a) Any activity that decreases the volume for flood storage Zone 1 shall be prohibited. (b) All development located in critical flood storage Zone 1 shall provide compensatory storage at one to one (1:1) ratio on the project site, or in a hydrologically connected basin, as determined by the Department of Public Works. (c) Compensatory storage may be, but is not required to be, hydrologically connected to on site drainage design required under Article 420 of the Development Code. (d) No critical facilities will be allowed in Critical Flood Storage Zone 1. (e) Compensatory storage on a one to one (1:1) ratio may be allowed in an existing hydrologically connected regional storage mitigation facility determined adequate by the Department of Public Works. RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH) 3

4 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM Truckee River Flood Project Mitigation Models The aforementioned ordinances are met using approved USACE and Truckee Meadows Flood Management Authority (TRFMA) HEC RAS models, with updates to reflect existing conditions of the SEC project area. The use of an approved existing and proposed project conditions model will be used to assess the difference in water surface elevation within the Critical Flood Zone 1, as determined by comparing the existing condition water surface elevation with the proposed project water surface elevations. Through discussions with City of Reno and TRFMA, it was determined that comparison of the existing conditions to the proposed project conditions is a more reliable means of assessing flood storage volume impacts than a comparison to the 1997 Map Book elevations as discussed in City of Reno Ordinance No Section (a)(2). Project Area The SEC project area included in the flood analysis extends from the southern limits of the roadway project at South Meadows Parkway to the confluence of Steamboat Creek with the Truckee River in the north. This overall project reach includes the main channel and floodplain of Steamboat Creek, tributary inflows from defined water courses entering from the west, and the Truckee River in the eastern portion of the Truckee Meadows. The extent of the hydraulic modeling extends over the entire flood pool storage area defined by the USACE Truckee Meadows Model. The SEC project area is discussed in four sections for clarity and ease of reference. The project reaches are as follows: South Meadows Parkway to Huffaker Narrows. The southern reach of the project extends from South Meadows Parkway to Alexander Lake Road at the Huffaker Narrows. Within this reach the roadway is elevated above the floodplain of Steamboat Creek. The Steamboat Creek channel occupies the eastern portion of the floodplain; a series of remnant irrigation drainage channels convey nuisance and stormwater flows from the western portion of the watershed towards Steamboat Creek. Within this reach the terminus of Thomas Creek, Alexander Lake discharge, and stormwater conveyances converge with Steamboat Creek at Alexander Lake Road (Huffaker Narrows). The flows from the west are adjacent to, but separate from, flows within Steamboat Creek at this location. There are no tributary inflows from east of the project in this reach. The USACE Truckee Meadows HEC RAS model does not extend through this reach. Huffaker Narrows to Mira Loma Drive. Steamboat Creek continues along the eastern perimeter of the floodplain adjacent to irrigated pastures. The pastures receive flows from the Thomas Creek systems, which are conveyed separate from Steamboat Creek through the Huffaker Narrows. These waters are used to flood irrigate the pasture of the north Butler Ranch. Tail waters from the irrigation eventually enter the Rosewood Lakes Golf Course through the western Mira Loma culverts or comingle with Steamboat Creek immediately south of Mira Loma Drive. Additional inflows from an ungauged conveyance of Rio Poco Drain enter the system in this reach. The USACE HEC RAS model extends through this reach to just north of the Huffaker Narrows bridge and inflow hydrographs in the HEC RAS model reflect these flow volumes and confluence locations. Mira Loma Drive to Pembroke Drive. The Mira Loma to Pembroke reach is dominated by the influence of the Rosewood Lakes Golf Course. The limited flood storage volume that can be obtained through this reach requires much of the flood volume mitigation to be generated along the northern limits of the storage areas and along Steamboat Creek north of Pembroke Drive. The high degree of connectivity to adjacent flood storage pools in this area makes it possible to achieve the design flood performance through this project reach. Boynton Slough is a major tributary influencing the storage and conveyance in this area. The data for the inflow hydrograph for Boynton Slough are identical to the data used in the USACE and TRFMA models. Pembroke Drive to Truckee River. Pembroke Drive to the confluence of the Truckee River is entirely within the Truckee River backwater effect of the 117 year peak event. This area allows for considerable 4 RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH)

5 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM flood volume storage development to occur due to the greater depth to groundwater than in the south, and creates the ability to develop additional flood storage volume through stabilization of Steamboat Creek by excavation of the incised creek banks. Additionally, a large wetland will be constructed on Yori Drain upstream of the confluence with Steamboat Creek. Yori Drain is a minor tributary that conveys stormwater, irrigation return flows, and urban drainage to Steamboat Creek; however, during flood events it is largely under the influence of the backwater from the Truckee River, as is common in this reach. Project Design Criteria The hydrologic design criteria includes conformance with all applicable local and state regulations. The project will reflect criteria, as previously presented in the design criteria memorandum, from the following: Low Impact Development Handbook, Truckee Meadows Stormwater Design Manual, and the applicable flood storage protection ordinances presented above. The project will be constructed to Orange Book and Silver Book Specifications. Roadway One lane in each direction will remain above the water surface elevation during a 117 year flood. Mitigation Wherever possible a one to one (1:1) volumetric mitigation will be developed for each impacted flood pool; where unavailable the use of the hydraulic model will demonstrate a net reduction in the water surface elevation of that flood pool through the creation of adjacent and connected flood volume storage. Bridges To allow for debris flows during flood events, all bridges have been set to a minimum elevation to allow a one foot freeboard over the 100 year water surface elevation. Culverts Culverts placed through the roadway embankment have been located to allow for a uniform rise and fall of the water surface on the east and west side of the embankment to eliminate the occurrence of significant differential water surface elevations across the embankment. Furthermore, the design seeks to minimize the amount of culverts to only those necessary to meet the hydraulic requirements, thereby reducing the need for long term maintenance. Methodology The following section discusses the methodology used to develop the hydraulic models and how those modeling decisions are reflected in the project design. Project Hydrology All project hydrology for the Truckee River, Steamboat Creek, and associated tributaries is identical to that presented in the 2012 USACE Truckee Meadows HEC RAS model. This inflow hydrology conforms to the statistical 117 year event on the Truckee River, and is consistent with the data used by the USACE and the TRFMA for hydrologic and hydraulic evaluation of flood events in the Truckee Meadows. Design Storm Event The design storm event for the Truckee Meadows reflects the the USACE hydrographs for the Truckee River, Steamboat Creek and its tributaries and was used to evaluated impacts to Critical Flood Zone 1 is the 117 year event. As a conservative approach the peak Steamboat Creek and Truckee River flows are slightly offset to reflect the observed difference of the peak hydrograph timing of the two systems as computed by the USACE. The timing of the hydrographs for the peak Steamboat Creek, Truckee River, and tributary flows is consistent with those used by the USACE and TRFMA 100 year and 117 year models. Tributary Inflow The tributary inflow through the project reach includes inputs at Alexander Lake Road (Thomas Creek Confluence), Mira Loma Drive (irrigation return flows and Rio Poco Drain), Boynton Slough (Dry Creek), and Yori Drain. All tributary inflows hydrographs are the same as those used in the USACE 2012 model. RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH) 5

6 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM Truckee River Backwater Effects The backwater during a 100 year and 117 year flood event on the Truckee River controls the downstream boundary conditions for the hydrology of Steamboat Creek. The backwater pool caused by the Truckee River flood pool influences the Steamboat Creek flows as far south as 1,200 feet south of Mira Loma Drive within the north Butler Ranch area. This backwater has a significant effect on the conditions affecting the Critical Flood Zone 1. Hydraulic Modeling The hydraulic modeling effort used to develop the proposed project design and to evaluate the existing and with project conditions required development of two hydraulic models. The commonly used HEC RAS model is in conformance with the existing hydraulic model work performed on the Truckee River and Steamboat Creek systems. The one dimensional (1D) HEC RAS model platform is the standard for presentation of hydraulic impacts associated with the proposed project design. To ensure an appropriate level of resolution for engineering analysis, the design team also developed a two dimensional (2D) model using the MIKE 21 modeling platform. This design level model provides for additional resolution and operational flexibility in assessing infrastructure design elements and has been used to guide design decisions that were then reflected in the project s HEC RAS models. HEC-RAS (One Dimensional Model) The no rise analysis for the Truckee Meadows Critical Flood Zone 1 is being completed using the USACE Truckee Meadows HEC RAS model completed in 2012 for use in planning the USACE Truckee River Project. The USACE model is based on 2006/2008 topography produced by Washoe County and includes current bathymetry for both the Truckee River and its tributaries including Steamboat Creek and Boynton Slough. On the Truckee River, the model extends from upstream of Glendale Bridge to approximately two miles east of Vista Boulevard. On Steamboat Creek, the model extends from just downstream of Huffaker Narrows to the confluence with the Truckee River. The model includes flow files for the 100 year and 117 year design events. The 117 year design flood is being used for the no rise analysis for the SEC because it is considered to be close to the flood of record on the Truckee River that occurred in December Recently, the TRFMA updated the geometry of the existing conditions 2012 Truckee Meadows USACE HEC RAS model by adjusting the geometry of two railroad bridges on the Truckee River downstream of its confluence with Steamboat Creek. The adjustments included additional bathymetry on Steamboat Creek just upstream of its confluence with the Truckee River and changes to the geometry to reflect the Pioneer Ditch Diversion recently reconstructed on the Truckee River. These few modifications to the 2012 USACE Truckee Meadows HEC RAS existing conditions model have been included in the SEC HEC RAS models, existing and proposed conditions. Design precision topography (+/ 0.5 foot) was collected for the SEC Project in D 83 Nevada State Plane Coordinate System West Zone, NDOT, modified with a basis of elevation of VD 88. It meets or exceeds ASPRS Class 1 and National Map Accuracy Standards for 1 =40 horizontal mapping scale with 1 foot contour intervals. The mapping includes the area surrounding the proposed roadway alignment east to the Steamboat Creek channel from Veterans Parkway to approximately 1,000 feet south of Clean Water Way as it crosses Steamboat Creek. The SEC project topography was corrected for dense vegetation but does not include continuous bathymetry on Steamboat Creek. For the SEC flood analysis, the USACE existing conditions model was updated to reflect this topography with the following process: The 3D breaklines and mass elevations points used to create the 1 to 2 foot 2006/2008 topography were obtained from Washoe County and edited to exclude all areas covered by the project topography. These data and the point and breakline data for the project topography (corrected with a ground to grid factor of ) were then used to create a composite Triangulated Irregular Network (TIN) within ArcMap GIS. 6 RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH)

7 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM The HEC RAS cross sections of Steamboat Creek, project adjacent cross sections of the Boynton Slough, and project adjacent storage areas were extracted from the geo referenced geometry of the 2012 USACE HEC RAS model using Geo RAS in ArcMap. Without changing the alignment or extent of the geometry, the USACE cross sections and storage areas were re cut against the updated TIN using Geo RAS. Lateral weirs and storage area connections adjacent to the project area were also re cut against the updated TIN. The updated cross sections and storage areas as well as updated lateral weirs and storage area connections were substituted into the geometry of the 2012 USACE HEC RAS model. To account for the lack of bathymetry in the project topography, the updated cross sections of Steamboat Creek and Boynton Slough were then edited to reflect the bathymetry in the 2012 USACE model. All parameters, including roughness, weir coefficients, and stability factors, were kept the same as the 2012 USACE model, with the exception of one area. An issue on the upper portion of the Truckee River near Station was encountered when running the 117 year event that caused the model to become unstable. To address the instability and allow the model to run to completion, the roughness of a 150 foot section of the Truckee River channel was increased from to 0.05 in both existing and proposed conditions. To create a proposed conditions HEC RAS Truckee Meadows model, the following procedure was followed: The existing point and breakline data outlined above (the merger of the project specific and Washoe County data) were edited to delete all information within the SEC Phase 1 and 2 areas of grading. These data and the breaklines representing the design of the SEC project were used to create the proposed conditions TIN. This TIN reflects the breaklines describing the mitigation volume areas, bridges, and floodplain geometry and roadway design of the Phase 1 and 2 of the SEC project. Storage areas that will be bisected by the proposed embankment, including Area24, Area25, Area10, Area6 and Area5, were divided within ArcMap along the embankment alignment, creating four new storage areas. These storage areas, along with storage areas Area5, Area52 and Area53, the cross sections of Steamboat Creek, project adjacent cross sections of Boynton Slough, and all lateral weirs and storage areas impacted by the project, were re cut in Geo RAS against the proposed conditions TIN. The proposed conditions SEC HEC RAS model was constructed using the existing conditions model geometry, updated with the proposed cross sections, lateral weirs, storage areas, and storage area connections. For the area just south of Mira Loma Drive to Alexander Lake Roadway, the Steamboat Creek floodplain was divided into two reaches to represent the flow around the proposed embankment. The flow split just north of the Alexander Lake Road Bridge is represented as storage area connections to improve model stability. The roadway is represented as a lateral structure with culvert connections according to the design. The SEC proposed conditions model was run with the 117 year design event to compare the results against the SEC existing conditions to ensure that the design does not cause a rise in peak water surface elevation or peak flood flows on the Truckee River downstream of the SEC project area. Multiple iterations have been evaluated to show that the post project model demonstrates the required no rise condition for the peak flood pool elevations. Because the original USACE HEC RAS Truckee Meadows model extended only to just north of Alexander Lake Road, additional HEC RAS model geometry files were constructed from Alexander Lake Road to the southern project boundary. Due to model instability issues at Alexander Lake Road, the southern portion of Steamboat Creek is represented in a stand alone model. The geometry for both existing and proposed RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH) 7

8 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM conditions was generated as described above with the Washoe County and project topography overlain with the proposed design. The areas just upstream and downstream of Alexander Lake Road are represented in the HEC RAS geometry as storage areas in both existing and proposed conditions to improve stability. In proposed conditions, Steamboat Creek is represented as two reaches for the east and west flow paths around the embankment. The embankment is represented as a lateral structure with culvert connections according to the design. The inflow hydrograph utilized for the design 117 year event is the USACE Truckee Meadows 117 year hydrograph for Steamboat Creek. The downstream boundary condition is based on the stage hydrograph computed by the Truckee Meadows HEC RAS models for both existing and proposed conditions. MIKE 21 (Two Dimensional Model) To determine the hydraulic impacts of constructing the SEC through the floodplain of the Steamboat Creek south of the flood pool and to assist in the design of bridges, culverts, and diversions structures associated with the roadway, two dimensional (2D) hydraulic models describing existing and proposed conditions were constructed. A 2D model was chosen for the area south of the flood pool to more accurately calculate flood depths and velocities for the wide, shallow flooding that occurs over the irregular floodplain of the Steamboat Creek during large events and to more accurately calculate the flow interception capacities of the proposed equalization culverts. One dimensional (1D) HEC RAS models completed in the last decade for various purposes throughout this region have shown significantly divergent and unreliable results. The 2D models are being used to evaluate flooding impacts, sediment transport impacts, and to verify 1D hydraulic modeling results in the Mira Loma Drive and Alexander Lake Road areas. CH2M HILL completed the 2D hydraulic modeling using the MIKE 21 F M software developed by DHI Water and Environment, Inc. MIKE 21 FM is a 2D finite element model that uses both quadrangular and triangular grid cells that vary in size and shape throughout the model domain (flexible mesh). Model development followed a series of specific steps as follows: 1. Generation of the digital terrain model 2. Delineation of the model limits (model domain) 3. Discretization (transforming the continuous surface into its discrete counterparts) of the domain and resolution (computational mesh) 4. Definition of hydraulic structures 5. Definition of the boundary conditions (flow and water levels) 6. Mapping of the flow resistance parameter Each step of the model development process is described in detail below. Generate Digital Terrain Model The existing conditions Digital Terrain Model (DTM) was constructed as described above. To account for below water surface invert data, survey data of the Steamboat Creek collected for the SEC project from Mira Loma south to South Meadows Parkway and invert data from the 2012 USACE HEC RAS existing conditions model for the Truckee Meadows were used. The invert/bathymetric data were used to create below water surface breaklines, which were then input into the existing conditions Triangular Irregular Network (TIN). The proposed conditions DTM was created by super imposing the 50 percent SEC design on the existing conditions DTM as described above. The resulting composite TIN surfaces, representing existing and proposed conditions, provided the basis for digital elevation models (DEM) used for the existing and proposed conditions 2D hydraulic model. The existing and proposed conditions DEMs were created by extracting 2 foot, 4 foot, and 6 foot gridded X, Y, and Z point data from the existing and proposed conditions TINs. The fine gridded data (1 foot cell size) were extracted from the DTM in areas within Steamboat Creek. Coarser gridded data (4 foot and 6 foot cell 8 RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH)

9 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM sizes) were used for all other hydraulic model domain extents where less resolution is required; for example, in floodplain areas where topography is less complex. Delineate Model Limits The horizontal datum of the TINs is North American Datum (D) 1983 (D83), Nevada West State Plane (FIPS 2703), U.S. Foot, adjusted to ground coordinates using a combined average scale factor of , and the vertical datum is North American Vertical Datum (VD) 1988 (VD88), U.S. Foot. All model inputs and outputs use English units. Discretize Domain and Resolution The model domain extends from its downstream boundary north of Mira Loma Drive, upstream to the existing Veterans Parkway, a distance of approximately 2.8 total river miles. CH2M HILL discretized the model domain using both quadrangular and triangular elements. Quadrangular elements were used wherever possible to define the floodplain area and specific features that require tight boundary fitting (i.e., box culvert). Triangular elements were used to define the channel. Elements in the main channel range 3 to 10 feet (length scale), with the higher resolution scale applied in the areas around the bridge sites. The floodplain elements range from 50 to 100 feet, with the larger elements used in elevated floodplain areas that are not expected to become inundated. The final computational mesh included 48,500 elements. Elevation data were interpolated to the mesh using gridded X, Y, and Z point data extracted from the TINs. Bridge decks were excluded from the TINs during the TIN creation process, as all bridge decks are above the peak flood elevation. Model Hydraulic Structures The model domain includes a single corrugated metal culvert at Alexander Lake Road. This structure was modeled as a composite structure (a weir and a culvert in a single structure) using MIKE 21 FM to account for overtopping flows during flooding events. Because structures can only be defined between two cells within the modeling software, we placed the composite structure near the upstream end of the culvert s true location and then modeled the remainder of the culvert as an open channel. This approach maintained the correct headwater elevations and flow rates. All bridge openings and equalization culverts were modeled as open channels. Define Boundary Conditions The model solves partial differential equations (St. Venant equations) that require defined boundary conditions at all exterior open boundaries and initial conditions for all internal elements. Both existing and proposed conditions models have two open boundaries: one upstream and one downstream. The upstream boundary was defined with the Steamboat Creek hydrograph from the 117 year design event of the USACE 2012 Truckee Meadows HEC RAS model. The downstream boundary was defined with the stage hydrograph at the location of the downstream boundary (11,000 feet north of Mira Loma Drive) from the same HEC RAS model at Steamboat Creek Cross Section The initial conditions were either set as dry or wet to begin with results from a previous model run. The initial conditions do not affect the final steady state results; however, using results from previous model runs helps reduce model run times. The 2D models are being run to demonstrate that the proposed mitigation excavation and equalization culverts through the embankment are adequate to keep the proposed maximum water surfaces resulting from the design event flood (117 year) at or below the existing conditions to the greatest extent possible. Use and Limitations of the Two-Dimensional Model The CH2M HILL team developed the 2D model to generate hydraulic design solutions that were unobtainable using a 1D model, due to computational limitations of the HEC RAS platform. The MIKE 21 model was used to optimize the sizing and placement of infrastructure, evaluate velocity and sediment scour parameters, and assess overbank flow performance at a finer resolution than what is possible with the RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH) 9

10 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM HEC RAS 1D modeling software. The standardization of the use of the HEC RAS platform has prompted CH2M HILL to develop parallel HEC RAS and MIKE 21 models. This approach has allowed for improvements to be developed and tested in the 2D platform and to be incorporated into the HEC RAS model with greater efficiency and confidence. For data presentation purposes the 1D, HEC RAS model is used to maintain consistency with the technology employed by the flood plain managers and regulatory agencies. All final flood management analysis data is presented using the existing conditions and proposed project HEC RAS models. Results The hydraulic design of the roadway and flood mitigation features was an iterative process using the two modeling platforms described above. The main goals of the process were to demonstrate no rise in the project area and over the larger Critical Flood Zone 1 using the existing and proposed conditions HEC RAS 117 year Truckee Meadows model, and to show the no rise conditions within the Steamboat Creek floodplain south of Mira Loma Drive and west of the SEC embankment using the 117 year peak MIKE 21 unsteady model and the extended HEC RAS model, and only a slight rise within the Steamboat Creek floodplain east of the SEC embankment south of Mira Loma Drive. The resulting design includes placement of culverts within the SEC embankment, the addition of conveyance culverts within Alexander Lake Road, Pembroke Drive and Clean Water Way, excavation for flood volume between the western channel limit of Steamboat Creek and the proposed embankment, grading within the Rosewood Lakes Golf Course driving range area and grading up and downstream of the Clean Water Way bridge over Steamboat Creek. Multiple combinations of features were tested within the modeling platforms to demonstrate the benefit of the various features and combinations of features. After each run, the maximum water surface results were compared to the existing conditions. Figure 2 shows the differences in maximum water surface elevation between the existing conditions and proposed conditions models for a sample of the different design scenarios that were run for the Truckee Meadows area between 1,200 feet south of Mira Loma north to the model boundary. The elements of the scenarios that are above and beyond the 50% design plans along with the estimated costs for each scenario are shown in Table 1. The proposed design optimizes hydraulic efficiency and cost at the most beneficial locations. The design includes: Five bridges located at 1) Alexander Lake Road crossing (Huffaker Narrows), 2) south of Mira Loma Drive, 3) Rosewood Lakes drainage, 4) Boynton Slough and 5) Yori Drain. These bridges were designed with the soffit of each bridge at least 1 foot above the maximum 100 year water surface elevation. The bridges were designed to maximize hydraulic benefit in large infrequent events on the Truckee River and maintain existing flow patterns in frequent events. Multi celled box culvert structures are located at 13 locations within the proposed roadway embankment. The culverts are sized to allow the required conveyance with a cost effective design with heights that put the soffits above the maximum 117 year water surface elevation. Excavation of available floodplain areas to allow for mitigation of flood storage and conveyance is proposed for the area between the roadway embankment and the Steamboat Creek channel. This excavation is above ordinary ground water levels as determined through monitoring of the groundwater levels over the past year along the roadway alignment. The volume of excavation exceeds that required for the project but due to limitations with right of way is not one to one (1:1) in every storage area as outlined in the Washoe County and City of Reno Flood Ordinances, due to the limitation of the existing terrain. However, the hydraulic modeling was done in order to show the sufficiency of the mitigation. Representation of this flood volume trade off and performance approach is allowed by and described in the flood ordinances presented above. Overall, the ratio of excavation to embankment within the floodplain for the project is greater than 1.7:1. 10 RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH)

11 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM To simplify the presentation of the HEC RAS modeling results the project area is presented as three project reaches: 1. Truckee River Flood Pool the area from 1,200 feet south of Mira Loma Drive to the north, inclusive of the Truckee River systems and those areas of the model north of the Truckee River in the City of Sparks. 2. North Butler Ranch The reach from Mira Loma Drive south to Alexander Lake Road which includes the undeveloped Butler Ranch pasture, and is bounded on the east by upland terrain. This area is south of the Truckee River flood pool and conveys flood flows from the Huffaker Narrows to the southern limits of the Truckee River flood pool near Mira Loma as open channel flow with a hydraulic grade line of 0.3% (S=0.003 ft/ft). 3. South Butler Ranch includes all areas within the project south of Alexander Lake Road (Huffaker Narrows), which includes the confluence of Steamboat Creek tributaries entering from the west. As discussed above the existing 2012 USACE HEC RAS model, as amended by the SEC team, extended to the Huffaker Narrows. The project area south of the Huffaker Narrows was initially developed using the MIKE 21 platform then translated to the HEC RAS modeling platform for consistency of presentation and conformance with the requests of the floodplain manager and in anticipation of future FEMA modeling efforts. Truckee River Flood Pool Reach Construction of the SEC within the Truckee River Flood Pool reach of the project results in a decrease of the water surface elevations of the designated Flood Storage Pools, with the exception of Flood Storage Areas 3, 30, and 36 A, C, and D as shown in Table 2. Reduction of the water surface elevations within the Critical Flood Zone 1 was accomplished through excavation of the floodplain, improved conveyance at Clean Water Way and cross conveyance connectivity through culverts and bridges. The resulting difference in maximum water surface elevations between existing and proposed project conditions are shown in Figure 3. The only water surface increase of significant depth is within a very small storage area near Peoples Drain which is due to a modeling anomaly. North Butler Ranch The design of the SEC within the North Butler Ranch project reach focused on providing the greatest water surface elevation reduction nearest to areas of future development and existing homes (all in the western portion of the Steamboat Creek floodplain), while limiting the increase in maximum WSEL in the design storm to under 1 foot. Therefore, the reduction of the water surface elevation is not uniform across the floodplain with the western portions experiencing a greater benefit from the project than the areas east of the SEC, where flood waters are constrained between the roadway and the undeveloped hills along the eastern edge of the floodplain. The difference in water surface elevations within North Butler Ranch is illustrated in Figure 3. It is possible to balance the water surface elevation between the east and west portions of the floodplain. However, to do so not only requires additional considerable infrastructure at significant costs, but also would result in less reduction proximate to developable areas. The estimated rise in water surface elevation above the existing conditions occurs in two small areas east of the SEC and entirely within areas that will be conserved from future development. The impacted lands, shown on Figure 3, fall within future RTC controlled lands and do not impact homes or other development. Allowance of these isolated water surface elevation increases is appropriate at these locations considering the greater benefit near the developed areas, and throughout developable areas, including Planed Use Development within North Butler Ranch, approved by the City of Reno. The duration of the water surface imbalance between the east and west side of the SEC occurs for a brief period (approximately 1 day) at the peak of the 117 year flood event. During this limited duration, velocities and hydraulic forces are within normal operating ranges, as documented in the HEC RAS model output files. RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH) 11

12 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM South Butler Ranch South Butler Ranch receives the majority of the flow from the Steamboat Creek along the eastern perimeter of the floodplain adjacent to undeveloped lands. The tributary flows from Thomas and Whites Creek systems enter the South Butler Ranch floodplain from the west and converge with Steamboat Creek near the northern end of the reach at Huffaker Narrows. Inflow hydrographs for these inflow sources reflect those data used by the USACE and TRFMA. The flood volume mitigation was created by excavation of the Steamboat Creek floodplain east of the SEC and west of Steamboat Creek in the same manner as the North Butler Ranch reach. This excavation allows for the establishment of a seasonal overbank condition of Steamboat Creek to dissipate the energy of the overbank flood events. Cross connection culverts are located in several areas through the SEC in the South Butler Ranch reach to equalize water surface elevations throughout the floodplain and to convey tributary flows from the west to the east where they combine with Steamboat Creek. Similar to conditions in North Butler Ranch, a water surface differential of less than one foot between existing and proposed conditions occurs in several areas within the South Butler Ranch floodplain east of the SEC (Figure 3). The elevated water surface is confined to areas east of the SEC that are prohibited from future development and are not adjacent to developable lands. The areas showing an increase in water surface elevation over the existing condition do not pose a flood risk to existing or future development sites, as the area is entirely within the future conservation easement of the Steamboat Creek floodplain ease of the SEC. Project modeling of the proposed design illustrates impacts to the existing condition floodplain including a water surface reduction throughout the floodplain west of the SEC, and isolated areas of less than one foot increase during the peak of the flood event east of the SEC in both the North and South Butler Ranch reaches. Conclusions The hydraulic modeling efforts to assess the impacts to existing condition water surface elevations at the 117 year flood event illustrate that the proposed SEC roadway design will have a net positive impact resulting in a reduction of the existing flood storage pool(s) elevation from no change to 0.73 feet, with an average reduction of the flood pools of approximately 0.06 feet. With the exception of storage area #3, which shows an increase of feet, the entire area north of Mira Loma Drive and south of the Truckee River complies with the no rise or No Adverse Effect of the City and County flood management ordinances. As shown in Figure 3, there are a few flood pools north of I 80 that exhibit a slight increase in the water surface elevation, less than feet (less than half an inch), and in one small area south of I 80 and north of the railroad tracks; through discussion with City of Sparks staff it is agreed these reflect computational limitations of the model. The impact of the proposed SEC south of Mira Loma and south of the Truckee River affected flood storage pools (Critical Flood Zone 1;North and South Butler Ranch), including a water surface elevation reduction ranging from 0.04 to 0.60 feet over the entire area west of the proposed SEC roadway. Isolated areas of water surface increase east of the SEC are projected in both the North and South Butler Ranch project reaches. These areas exhibit an increase of less than one foot and are contained entirely within the floodplain parcels that are designated to become part of the Steamboat Creek conservation easement. While these areas are not in specific conformance with the City of Reno Ordinance 6164, they lie south of the Critical Flood Zone 1, have no bearing on the proposed Truckee Meadows Flood Project, and do not impact lands containing or adjacent to developable parcels. The extensive level of hydraulic modeling of the conservative 117 year flood event undertaken to evaluate the impacts of the SEC showed that it is possible to achieve a no rise condition over the entire project area, including those areas south of Mira Loma Drive. However, designing to that level of no rise conformance will reduce the advantage gained through water surface reduction closest to development (i.e. less reduction of 12 RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH)

13 SOUTHEAST CONNECTOR FLOOD ALYSIS MEMORANDUM the flood elevation) and is cost prohibitive, as the infrastructure required to achieve a holistic no rise is considerable. Inclusion of infrastructure to achieve the holistic no rise will increase the visual, cultural resource, financial, and aesthetic impacts associated with the roadway and will reduce the benefit gained from construction of the proposed design. The conclusions of the flood analysis and associated hydraulic modeling efforts are summarized by the following points: The proposed design demonstrates no rise in water surface elevations within the Critical Flood Storage Pool(s). The proposed design demonstrates no rise west of the roadway in all areas south of the Critical Flood Storage Pool, within North and South Butler Ranch. All area east of the SEC roadway indicating an increase in the water surface elevation are entirely isolated from developable lands, are located in areas to be deed restricted through the application of a Conservation Easement, and are limited to the property boundaries controlled by RTC. Reducing the rise east of the roadway will also decrease the benefits gained along the western portion of the floodplain adjacent to developed areas, and will increase cost in a manner disproportionate to the flood protection benefit. When considered as a whole, the proposed design demonstrates the overall compliance with the No Adverse Impact intent of City of Reno Ordinance 6164 and Washoe County Development Code and demonstrates that the impacts associated with fill placed within a major drainage way has been properly evaluated and mitigated for the protection of public health and safety. RDD/ (FLOOD ALYSIS MEMORANDUM_SEPT2013_DRAFT_FILMCH) 13

14 Tables

15 Phase 1: Culv at Sta Phase 1: Culv at Sta Culv at Culv under Clnwtr Way (e of SEC) Culv at Culv at Culv at Culv at Grading in Golf Course Culv under Pembroke Culvert in SEC south of Boynton Slough Replacement of Cleanwater Bridge Grading up and downstream of Cleanwater Estimated Change in Costs from 50% (millions) Profile Change from 50% Plans Culverts under Cleanwater east of Steamboat Location A B C D E F G H I J K L M N 1 10'x10' Alternative 'x12' 'x8' (50% Plans) Alternative 2 Alternative 3 Alternative 4 Alternative 5 Alternative 5a Alternative 6 Alternative 6a Alternative 7 Alternative 8 Alternative 8a Alternative 8b Alternative 9 Alternative 9a Alternative 9b Proposed Design 3 12'x8' Denotes Change From Phase 2 50% Plands Denotes Change from Phase 1 Plans 6 10'x10' 'x8' 10'x8' 10'x8' 5 10'x10' 6 10'x10' 7 10'x10' 'x8' 6 10'x10' 7 10'x10' 'x8' 10'x8' 10'x8' 10'x8' 10'x8' 7 10'x8' 7 10'x8' 10'x10' 10'x10' 6 10'x10' 'x8' 6 10'x10' 'x8' 6 10'x10' 'x8' 6 10'x10' 'x8' 5 10'x10' 7 10'x10' 7 10'x10' 10'x10' 10'x10' 10'x10' 10'x10' 3 12'x12' 3 12'x12' 7 12'x12' 5 12'x12' 5 12'x12' 12'x12' 12'x12' 12'x12' 12'x12' 12'x12' 12'x12' 3 12'x12' 3 12'x12' 3 12'x12' 3 12'x12' No None Driving Range and Hole 'x6' Yes None $ 'x6' Driving Range + 5 and Hole 9 2'RCP and 8 Yes None $ 'x6' Driving Range x 10 and Hole 9 2'RCP and 8 Yes Slight $ 'x6' Driving Range + 5 and Hole 9 2'RCP and 4 Yes Yes $ 'x6' Driving Range + 5 2'RCP and 4 Yes Yes $ 'x6' x 10 2'RCP and 8 Yes $ 'x6' x 10 2'RCP and 8 Yes Yes $ 'x6' Driving Range + 5 and Hole 9 2'RCP and 8 Yes None $ 'x6' Driving Range foot Yes and Hole 9 2'RCP and 8 Expansion None $ 'x6' Driving Range foot Yes 2'RCP and 8 Expansion None $ 'x6' Driving Range foot Yes 2'RCP and 8 Expansion None $ 'x6' Driving Range ' x 10' Yes and Hole 9 2'RCP and 4 None $ 'x6' Driving Range ' x 10' Yes 2'RCP and 4 None $1.70 Driving Range Driving Range 4 10'x6' 4 10'x6' Yes Yes 3 10' x 10' 5 10' x 10' None $1.30 None ~$1.3 Table 1: Alternative Scenarios for SEC Design Flood Analsysis Memorandum SouthEast Connector Project

16 CFZ1 Flood Storage Pool Max WSEL Existing Conditions Max WSEL Proposed Conditions Change in WSEL (Proposed Existing) Area Area Area Area Area Area Area Area Area Area Area Area Area1B Area Area Area Area Area Area Area Area Area27A Area27B Area Area Area Area Area30B Area Area Area Area Area36 B Area36 C Area36 D Area36A Area36E Area36F Area Area37A Page 1 of 2 Table 2 Predicted Change in Max WSEL (Proposed Existing Conditions) Flood Analysis Memorandum SouthEast Connector Project

17 Area Area Area Area Area Area Area Area45A Area Area Area Area4B Area Area Area Area Area Area5B Area Area Area Area SAPeoples SAPeoples SAPeoples SAPeoples TreatmentPlant TreatmentPlant Page 2 of 2 Table 2 Predicted Change in Max WSEL (Proposed Existing Conditions) Flood Analysis Memorandum SouthEast Connector Project

18 Figures

19 SPARKS BLVD N SPARKS CITY TRUCKEE LIMITS RIVER RENO CITY LIMITS WASHOE COUNTY LIMITS McCARRAN BLVD W ASHOE COUNTY SPARKS LIMITS SOUTHEAST CITY LIMITS CLEANW ATER W AY CONNECTOR SPARKS CITY LIMITS SPARKS TRUCKEE CITY LIMITS RIVER PROJECT LIM ITS RENO CITY LIMITS McCARRAN BLVD MIRA LOMA DRIVE W ASHOE COUNTY PEMBROKE DRIVE WASHOECOUNTY LIMITS 580 SPARKS RENO CITY LIMITS RENO SOUTHEAST CONNECTOR ALEXANDER LAKE RD WASHOE COUNTY LIMITS VICINITY M AP RENO CITY LIMITS FLOODALYSIS-figure1.dgn PROJECT LIM ITS SOUTH MEADOW S PKW Y WASHOE COUNTY LIMITS FIGURE 1 PROJECT LOCATION M AP SOUTHEAST CONNECTOR FLOOD ALYSIS M EM ORANDUM

20 Alt 1 Alt 2 Alt 3 Alt 4 Alt 5 Alt 5a Alt 6 Alt 6 Alt 7 Alt 8 FIGURE 2 Truckee Meadows Model Results of Alternatives Proposed Minus Existing Computed Max WSEL Flood Analysis Memorandum SouthEast Connector Project Alt 8a Alt 8b Alt9 Alt 9a Alt 9b

21 Giroux St Airway Dr Sullivan Ln 18th St N Truckee Ln Clear Acre Ln Queen Way E Queen Way UV 651 Baring Blvd E 9th St Silverada Blvd E 4th St Wedekind Rd Oddie Blvd G St I St Victorian Ave Rock Blvd 15th St 80 Greenbrae Dr Prater Way York Way UV 445 4th St I St Stanford Way Probasco Way E York Way E Greenbrae Dr Howard Dr E Lincoln Way O Callaghan Dr E Prater Way Sparks Blvd E Lincoln Way Lillard Dr Vista Blvd Wabash Cir E 2nd St Galletti Way S Rock Blvd Glendale Ave UV 650 Mill St UV 650 E Greg St Vassar St Mill St Villanova Dr UV 653 UV 653 Matley Ln 395 S McCarran Blvd Mira Loma Dr E Peckham Ln Boynton Ln Spring Dr S McCarran Blvd UV 667 S Virginia St Airway Dr UV 667 Longley Ln VICINITY MAP Double Diamond Pkwy S Meadows Pkwy Double R Blvd S Meadows Pkwy Veterans Pkwy LEGEND Max WSEL of Proposed - Existing Proposed SouthEast Connector Alignment $ 0 1,000 2,000 UNK C:\USERS\MHORVAT1\DESKTOP\SOUTHEAST CONNECTOR\HYDRAULICMODELFIGURES\FIGURE4.MXD MHORVAT1 10/2/2013 9:33:43 AM Feet FIGURE 3 Hydraulic Model Results Predicted Change in Max WSEL from Existing to Proposed Conditions Flood Analysis Memorandum SouthEast Connector Project