Project Description

SCE downloaded 1m DEM raster tiles from the Discover GIS Data NY website for the entire project area. The tiles were manipulated and merged in ESRI ArcGIS to produce a seamless elevation raster with units in feet. In post-processing, SCE used NYS Department of Transportation bridge and large culvert data to hydro flatten the elevation raster at important road-stream crossings. The final deliverable was a seamless, hydro flattened elevation raster, clipped to each HUC-8 or HUC-10 watershed that was to be modeled in HEC-RAS.

SCE obtained rainfall data from the NOAA Atlas 14 precipitation estimates at the centroid of the project watersheds and a single excess rainfall hyetograph for each of the seven studied return intervals was developed for the 10%, 4%, 2%, 1%, 1% plus, 1% minus, and 0.2% annual chance flood events. SCE also generated Manning’s n Values using NRCS USDA soil data and the National Land Cover Database.

SCE developed a total of ten HEC-RAS hydraulic models for the project. For all models, a 2D mesh was generated at a 200-foot grid spacing. Breaklines were added at FEMA Coordinated Needs Management Strategy (CNMS) stream junctions and along modeled levees and floodwalls. A total of 28 critical structures were selected to be modeled based on a Hydroshadow method developed by Dewberry engineers. SCE calibrated model flows using USGS stream gage data and StreamStats regression equations to FEMA Bulletin 17C standards. Results of the BLE HEC-RAS models were used to generate floodplains for the 1% and 0.2% annual chance exceedance events for the hydraulic models.