Modeling erosion and overbank deposition during extreme flood conditions on the Carson River, Nevada

The Carson River in west-central Nevada is one of the most mercury contaminated fluvial systems in North America. Most of its mercury is affiliated with channel bank material and floodplain deposits, with the movement of mercury through this system being highly dependent on sediment transport processes, particularly during overbank flows. To simulate these extreme situations, a United States Environmental Protection Agency (US EPA) hydrodynamic model (RIVMOD) was modified to include the ‘divided channel approach’ to estimate floodplain depths and velocities. The RIVMOD code was also augmented to allow dynamic width increases in the channel. Calibrated bank erosion functions, developed for the US EPA water quality model (WASP5), suggest that bank erosion is significantly greater at flows above bankfull discharge when compared to flows confined to the main channel. Verification of the bank erosion model matched observed width increases in 7 out of 10 reaches, with general trends matched in two of the remaining three reaches. Results also indicate that a single major flood event is responsible for nearly 87% of the total mass eroded during the period from 1991 to 1997. Overbank deposition was modeled using separate functions for coarse suspended sediment and washload material. Overbank deposition results are also in good agreement with observed values.