Evaluation of River Profile Adjustment Potential due to Sediment Management at Los Padres Dam, Carmel River, CA

Abstract:

Reservoirs interrupt the supply of granular sediment to river reaches and floodplains downstream of dams. Consequently, reservoirs are sediment sinks, with infill rates that reflect the combined effects of local and regional hydroclimate, landscape scale erosion rates, land use, and dam trap efficiency. Sediment supply interruption has profound implications for the quality of downstream aquatic habitat, river bed architecture and planform dynamics, and infrastructure and property safety. Resource managers attempt to address downstream impacts through a broad set of actions which include downstream habitat enhancement, sediment bypass or fish reintroduction downstream of dams, and complete dam removal.

Here, we evaluate the geomorphic consequences of a suite of potential actions at Los Padres Dam, Carmel River, California, that are intended to address sediment accumulation within the reservoir. Considered are dam removal, active or passive removal of sediment within the reservoir, and reservoir expansion. We focus on how the different actions may affect sediment transport and river profile adjustment in the Carmel River downstream of the dam to the Pacific Ocean, based on numerical simulations with the 1-D river profile evolution model BESMo (Müller & Hassan, 2018, ESurf). Simulations were performed at the daily time step for simulated 60-year time periods. Given uncertainty of the future climate and how this will affect river sediment transport and profile evolution, each scenario was simulated using 300 statistically derived hydrologic conditions. Our findings indicate that the removal of the dam can move the Carmel River sediment dynamics closer to natural conditions, primarily due to the re-establishment of connectivity of the upper watershed to the main stem, causing a fining of the channel bed surface. Management leading to prolonged sediment release is more effective in keeping the surface grain size distribution relatively fine. In contrast, the channel surface coarsens within a few years after a quick, uncontrolled dam removal was reworked. These findings are significant for the decision on which actions should be taken at Los Padres, as both patterns of channel aggradation/degradation and bed surface fining are important for both flood mitigation and the preservation of fish habitat.