ResolutiontoApproveTO7WSE - Laserfiche WebLink

Home Browse Search

�A► WATERSHED SCIENCE & ENGINEERING October 9, 2018 • AutoCAD file showing location and elevation of each surveyed point • ASCII file of survey points (PNEZD) Task 4. Hydrologic Analysis — Flood Discharges WSE will estimate the magnitude of the 10-, 50-, 100-, and 500 -year annual instantaneous peak flood discharges using regional relationships and will develop flood inflow hydrographs for Reecer Creek. Task 5. Hydraulic Modeling WSE will create two separate two-dimensional (2D) hydraulic models to cover the two Reecer Creek study reaches: One model will cover Reecer Creek from the EWC Canal to the KRD canal (approximately 5.5 miles), and the second will cover Reecer Creek between University Way and Interstate 90 (approximately 1.5 miles). Both models will be completed using the U.S. Army Corps of Engineer's Hydraulic Engineering Center - River Analysis System (HEC -RAS). The 2D models will be used to estimate flood inundation limits, flow paths and FEMA specific data including floodplain boundaries. The models will depict channel and floodplain geometry using field surveys and 2011 LiDAR. The geometry of road crossings will be coded using surveys, as -built plan sets, and/or dimensions collected by WSE during the field reconnaissance. Required model variables including Manning's roughness values and culvert entrance and exit loss coefficients will be determined through model calibration or estimated using engineering judgement. Downstream boundary conditions will be established using normal depth method internal to the model. If reliable high water mark and discharge data can be collected, WSE will calibrate the model to reproduce the event(s). If data is not available, WSE will use engineering judgement to refine the model to generally reproduce flood characteristics observed by County staff and floodplain residents. Once the model is complete, it will be run for the 10-, 50-, 100-, and 500 -year events. 100 -year results will be used to generate the flood inundation boundaries and base flood elevations necessary to update FEMA mapping. Task 6. Field Verification of Hydraulic Modeling WSE will conduct a one -day field reconnaissance of the study reach to verify initial modeling results and assumptions. This will include collection of additional field measurements as necessary to update the model to accurately reflect observed conditions. Task 7. Flood Hazard Work Map WSE will use the 2011 LiDAR, field survey data, and County GIS layers to create the base map upon which the flood data will be added. WSE will map 100 -year flood inundation boundaries and base flood elevations (100 -year flood water surface elevations) based upon the hydraulic modeling completed in Task 5. WSE will meet with the County to present and discuss the preliminary maps. WSE will refine the maps upon receipt of County comments. Assumptions: Study maps will be submitted as "Approximate Zone A" mapping with base flood elevations.