Designing flood mitigation alternatives in coastal northern california: Hydraulic modeling and geomorphic analysis of sedimentation and fish habitat

Abstract

On the Pacific Coast of northern California in the United States, coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss) are protected fish species, and habitat requirements must be considered in analysis and design of flood mitigation projects. Development of viable flood mitigation strategies requires detailed hydrologic and hydraulic analyses to evaluate processes that control flooding and ecosystem conditions. Morphodynamics of the stream channel and floodplain are governed largely by sediment transport and deposition, hence these processes must also be analysed. We present analyses of flooding and geomorphology, including consideration of fish habitat constraints, for developing flood mitigation conceptual designs in two field areas. Easkoot Creek flooding affects the community of Stinson Beach located near sea level on the coast 25 km north of San Francisco. Green Valley Creek flooding affects a principal road and vineyards near the community of Graton about 95 km north of San Francisco in the Russian River watershed. Watershed-scale hydrologic models and twodimensional hydraulic models (MIKE FLOOD) supported by LiDAR-derived topographic data and field surveys were used to estimate flows and flood extent during design storms for existing conditions and for alternative potential flood mitigation measures. Bed load sediment transport was evaluated using hydraulic model predictions of shear stress and appropriate bed load transport algorithms. Conceptual designs for flood mitigation were identified that reduced potential flood impacts while also enhancing fish habitat and managing sedimentation.