Analysis of Potential Dam Removal 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. Alternatives that are being considered to bring the dam to a safe condition range from thickening of the approximately 22-m-high, concrete arch dam to complete removal. The 31-km reach of the Carmel River downstream from the dam passes through urbanized areas within the Carmel Valley; thus, flooding, channel stability, and in-channel habitat are a matter of significant concern. Several alternative dam retrofit/removal scenarios were considered in this study. Sediment-routing studies of erosion that would occur in the reservoir were performed for each of the scenarios. The subsequent movement of the released sediment through the downstream reach of the river between the dam and the Pacific Coast, was modeled to assist the owners of the dam and the resource agencies evaluating the 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. Sediment data collected throughout the reservoir deposits, in conjunction with available topographic information and simulated hydrologic data, was used as input to the model. Execution of the HEC-6T model was performed in an interactive manner that allowed for management of complexities associated with the size and distribution of the reservoir deposits and control of 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. This information provided a basis for evaluating changes in flood potential, channel stability, and the ultimate potential for restoring instream habitat.