Analysis of Potential Dam Removal/Retrofit Impacts to Habitat, Flooding andChannel Stability in the Carmel Valley, California

摘要

The California Department of Water Resources, Division of Safety of Dams (DWRDSD),rnhas determined that San Clemente Dam on the Carmel River in MontereyrnCounty, California, does not meet seismic safety standards. Several alternatives arernbeing considered to bring the dam to a safe condition. These alternatives includernthickening of the approximately 22-m-high, concrete arch dam, lowering of the damrnto the extent necessary to meet seismic standards, and complete removal. At thernpresent time, the reservoir upstream from the dam, which had an original storagerncapacity of about 1.76M m3, is nearly filled with sediment. An additional alternativernhas recently been suggested that would divert the Carmel River into San ClementernCreek near the head of the reservoir in a manner that would isolate about 80 percentrnof the sediment that is stored in the reservoir. The 31-km reach of the Carmel Riverrndownstream from the dam passes through urbanized areas within the Carmel Valley;rnthus, flooding and channel stability are matters of significant concern. In addition,rnthe Carmel River contains habitat for the endangered steelhead and red-legged frogrnthat could potentially be affected in both positive and negative ways by removal orrnlowering of the dam. Sediment-routing studies were conducted using the HEC-6Trnmodel to evaluate the response of the river to elevated sediment loads associated withrnthe various removal/retrofit alternatives. The results are being used by the owners ofrnthe dam and the resource agencies to evaluate potential public safety impacts due tornincreased flooding and channel instability, as well as the potential for habitatrnrestoration as the river recovers from the increased sediment loading. The modelingrnwas performed in an interactive manner that allowed for management of complexitiesrnassociated with the size-range and spatial distribution of the reservoir deposits andrnchanges in the dam configuration. Results obtained from the modeling studies arernbelieved to realistically describe the magnitude, and temporal and spatial distributionrnof erosion within the reservoir, and deposition and subsequent erosion in the river asrnthe elevated sediment loads pass downstream.