Solving Floodplain Management Challenges Using a Collaborative Approach

The Carson Water Subconservancy District is a water resources management agency with a multi-jurisdictional and bi-state jurisdiction. The members have elected to solve their floodplain management challenges in a collaborative approach that considers systemic impacts of land use decisions on the entire river corridor.

In 2005, CWSD signed a Cooperating Technical Partner Agreement with the Federal Emergency Management Agency. In 2012, various municipalities located along the Carson River, State and Federal agencies entered into a Risk Mapping, Assessment and Planning  charter agreement with FEMA. The charter agreement enables FEMA and CWSD to work with counties on flood hazard mitigation projects on a watershed basis. This was the first charter agreement in FEMA Region 9 and is a blueprint for other regional cooperation programs.

It is the intent of the Risk MAP charter members to track floodplain impacts in a cumulative manner throughout the watershed. With this goal in mind, the CWSD initiated a series FEMA flood risk remapping efforts using state-of-the-art modeling. The goal of this long-term effort was to remap the entire Carson River in Nevada, including a small portion in California.  

Beginning in 2011, and continuing to 2022, the CWSD has contracted with HDR to conduct detailed hydrologic analysis, hydraulic modeling and flood hazard mapping effort for more than 118 miles of the Carson River’s East Fork, West Fork and Main Stem, spanning across five counties and two states. The result will produce an updated flood hazard mapping that can also serve to quantify the cumulative impacts of future encroachments within the jurisdictional floodplain corridor.

This project, which followed traditional FEMA Physical Map Revision guidelines and specifications, employed the use of unsteady-state 1D and 2D dimensional modeling. This breakthrough modeling solution allowed for the assessment of spatial, temporal and volumetric impacts of encroachments within the river’s flood hazard zones, while addressing the unique challenges of using unsteady 1D and 2D solutions to define special flood hazards and regulatory floodways. 

To conduct the modeling of the overall study area, the project utilized the U.S. Army Corps of Engineers’ Hydrologic Engineering Center, HEC-RAS 5.0.X. This technology provides the CWSD with a public domain software tool that can be used for floodplain management and impact analysis of future floodplain modifications.

In September 2022, FEMA accepted our map revision package for the Carson River in Douglas County and has completed the public comment period. This acceptance of the Douglas County Portion is the 4th of 5 mapping submittals to be accepted by FEMA. When finalized, this project will have remapped approximately 29,000 acres of new regulatory floodplain.