The California Coastal Conservancy reached agreement in December to bypass an obsolete dam on California�s Carmel River rather than dredging and buttressing the 90-year-old structure. The $84-million reroute and dam removal project will divert the river around the 106-ft high concrete arch San Clemente Dam built in 1921. The basin has since been swamped with 2.5 million cu yds of sediment, thereby reducing storage capacity from 1,425 acre-ft to 125 acre-ft. In 1992, the California Dept. of Water Resources Division of the Safety of Dams issued a safety order because of possible failure from a maximum flood event or an earthquake.

Photo: California American Water Co.
Silted-up San Clemente Dam, built in 1921, will remain in place, as Carmel River is diverted around it.

A 2008 Dept. of Water Resources study showed that the alternative of buttressing the dam would have cost $49 million, but not resulted in improved access for steelhead trout spawning or restoration of the lower river. California American Water, which draws supplies for its customers from the dam’s diversion point, agreed to pay the cost of buttressing with the California Coastal Conservancy and the National Oceanic and Atmospheric Administration picking up the rest of the price tag.

The diversion will require careful orchestration. Engineers took possible downstream flooding issues into consideration, and decided not to allow the accumulated sediment to wash downstream. The dam’s remote location also made transporting an estimated 200,000 truckloads of sediment impractical. Instead, contractors will reroute a half-mile of the Carmel River into San Clemente Creek by blasting and ripping through a narrow ridge and using the abandoned section of the river as sediment storage after creating a diversion dike from the excavated rock to maintain the new route.

The project will move into the approval stage through the Public Utilities Commission in 2011 and could begin construction as early as 2013. The project�s third phase includes dam removal and habitat stabilization.

This file replaces an earlier version posted 12/28/2010.