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

The California Department of Water Resources, Division of Safety of Dams (DWR-DSD), has determined that San Clemente Dam on the Carmel River in Monterey County, California, does not meet seismic safety standards. Several alternatives are being considered to bring the dam to a safe condition. These alternatives include thickening of the approximately 22-m-high, concrete arch dam, lowering of the dam to the extent necessary to meet seismic standards, and complete removal. At the present time, the reservoir upstream from the dam, which had an original storage capacity of about 1.76M m³, is nearly filled with sediment. An additional alternative has recently been suggested that would divert the Carmel River into San Clemente Creek near the head of the reservoir in a manner that would isolate about 80 percent of the sediment that is stored in the reservoir. The 31-km reach of the Carmel River downstream from the dam passes through urbanized areas within the Carmel Valley; thus, flooding and channel stability are matters of significant concern. In addition, the Carmel River contains habitat for the endangered steelhead and red-legged frog that could potentially be affected in both positive and negative ways by removal or lowering of the dam. Sediment-routing studies were conducted using the HEC-6T model to evaluate the response of the river to elevated sediment loads associated with the various removal/retrofit alternatives. The results are being used by the owners of the dam and the resource agencies to evaluate potential public safety impacts due to increased flooding and channel instability, as well as the potential for habitat restoration as the river recovers from the increased sediment loading. The modeling was performed in an interactive manner that allowed for management of complexities associated with the size-range and spatial distribution of the reservoir deposits and changes in the dam configuration. Results obtained from the modeling studies are believed to realistically describe the magnitude, and temporal and spatial distribution of erosion within the reservoir, and deposition and subsequent erosion in the river as the elevated sediment loads pass downstream.