Carson City is part of the Great Basin Watershed, a high-desert valley that experiences flash floods with alluvial fan flooding and riverine flooding. The FEMA flood boundaries for two tributaries have been altered by the Nevada Department of Transportation’s extension of the interstate freeway.
Cardno’s team used spatially varied precipitation depth grids to calculate area-weighted hyetographs for each subbasin. This allowed the team to account for varying rainfall patterns and better define the hydrology across the watershed.
Cardno utilized the U.S. Environmental Protection Agency’s SWMM5 model for the hydrologic analysis and calculated area-weighted input parameters for the model using ArcGIS software tools.
Cardno selected the FLO-2D hydraulic model due to the program’s ability to simulate complex hydraulic interactions and diversions associated with the flat topography and shallow flooding within the project area. The FLO-2D model output was used to develop proposed floodplain mapping that accurately represents flood risk and revises the effective Flood Insurance Rate Map (FIRM) for the study area.
Staff are currently developing Google Earth KMZ files to educate the public on future floodplain extents and how FIRM changes might affect properties and insurance rates in their area.