SAN CLEMENTE AND MATILIJA DAM REMOVAL: ALTERNATIVE SEDIMENT MANAGEMENT SCENARIOS

摘要

San Clemente Dam on the Carmel River and Matilija Dam on Matilija Creek are located in Central and Southern California coastal watersheds. Both dams are thin concrete structures constructed in 1921 and 1948 respectively, and their reservoirs have been filled with sediments, effectively eliminating their water storage capacity and any incidental flood control function. As a result, the removal of both dams is under consideration. A major challenge to the removal of the San Clemente and Matilija Dams is the management of the approximately 2.5 and 6 million cubic yards of sediment impounded in their respective reservoirs. Mechanical removal of impounded sediments entails large economic and societal costs because of restricted access, distance to disposal sites, and the length of time to excavate and transport material. Natural flushing of sediments through the river system could adversely impact aquatic habitat, including their estuaries, as well as cause potential impacts to water supply systems and flooding of floodplain situated properties. The San Clemente and Matilija Dam projects illustrate two basic approaches to the management of impounded sediments: (1) permanent stabilization of sediment above San Clemente Dam and by-passing a short reach of the Carmel River through San Clemente Creek; and (2) temporary stabilization of coarse sediments within the reservoir behind Matilija Dam and controlled flushing of sediments through Matilija Creek and the Ventura River ( with slurrying finer fractions of sediments through a conduit to downstream storage sites). None of the sediment management alternatives is free of economic and societal costs, or environmental impacts; site specific circumstances will influence the best strategy for handling sediments to minimize adverse impacts and costs, and maximize benefits within an acceptable time-frame to achieve dam removal goals and objectives.