The Carmel River Reroute and San Clemente Dam Project is the largest dam removal project ever to occur in California and one of the largest to occur on the West Coast. This groundbreaking project sets precedence for other dam removal and river restoration projects in California and nationwide. The project involved removal of the 106 foot high antiquated dam and implementation of a watershed restoration process that will bring the Carmel River back to life. The goals of the Project were to:
- Provide a long-term solution to the public safety risk posed by the potential collapse of the outdated San Clemente Dam in the event of a large flood or earthquake which would have threatened 1,500 homes and other public buildings
- Provide unimpaired access to over 25 miles of essential spawning and rearing habitat, thereby aiding in the recovery of threatened South-Central California Coast steelhead.
- Restore the river’s natural sediment flow, helping replenish sand on Carmel Beach and improve habitat downstream of the dam for steelhead.
- Reduce beach erosion that now contributes to destabilization of homes, roads, and infrastructure.
- Re-establish a healthy connection between the lower Carmel River and the watershed above San Clemente Dam.
- Improve habitat for threatened California red-legged frogs.
With any dam removal project in the western U.S., one of the most difficult issues is determining what to do with the sediment which has accumulated behind the dam. It is estimated that there are 2.5 million cubic yards of sediment behind San Clemente Dam. Due to limited and difficult access to the dam site, trucking the sediment out was deemed infeasible, both environmentally and economically. Likewise, due to the current significant flooding issue along the lower Carmel River, allowing the sediment to erode downstream was ruled out because it would likely worsen downstream flooding. Therefore, the project design involved an innovative engineering approach to re-route a half-mile portion of the Carmel River into San Clemente Creek and use the abandoned reach as a sediment storage area.
San Clemente Dam was located just downstream of the confluence of the Carmel River and San Clemente Creek. The two waterways were separated by a narrow ridge. The majority of the sediment which had accumulated behind the dam was located along the Carmel River side of the reservoir. The design of the Reroute and Removal project took advantage of this situation by transforming the Carmel River arm of the lower reservoir (already full of sediment) into a permanent sediment storage area. This design minimized the amount of sediment which had to be excavated and moved, thereby reducing the project cost as well as some of the environmental impacts. Other components of the project included constructing the access roads and the future transfer of CAW property around San Clemente Dam to the U.S. Department of Interior Bureau of Land Management.
Reroute of the Carmel River into San Clemente Creek To establish the Carmel River arm of the reservoir as a permanent sediment storage area, the river was rerouted into the adjacent San Clemente Creek, upstream of this area. This was accomplished by cutting a “Reroute Channel” through the narrow ridge separating the two waterways, approximately one-half mile upstream of the dam (More).
Excavation and Stabilization of the Accumulated Sediment Although the majority of accumulated sediment was already on the Carmel River side of the reservoir, approximately 380,000 cubic yards of sediment that had accumulated in the San Clemente Creek arm had to be excavated and added to the Carmel River sediment storage area (More).
Restoration of San Clemente Creek and the now Combined Flow Reach On the San Clemente Creek side of the reservoir, the half-mile reach between the dam and the downstream end of the Reroute Channel was reconstructed to carry the combined flows of both the river and the creek, and to allow for fish passage. First, the sediments were excavated down to the pre-dam elevations (More).
Dam Removal San Clemente Dam was removed after all of the other projects elements were complete are in the final stages of construction. (More).