The San Clemente Dam, a 106 ft tall concrete arch dam constructed in 1921, has been found to be vulnerable to seismic activity and floods, its reservoir filled nearly to the brim with sediment. Courtesy of California American Water
The largest dam removal in the state of California will help restore the dwindling population of a threatened fish species and replenish beaches and dunes.
July 3, 2012—On June 21 the California Public Utilities Commission approved a plan to remove the 106 ft tall San Clemente Dam on the Carmel River in Monterey County, setting the stage for what is believed to be the largest dam removal project in the state’s history. Completed in 1921 approximately 18 mi upstream of the Pacific Ocean, the concrete arch dam has long impeded fish passage and the natural movement of sediment along the Carmel River. By removing the aging structure, the coalition of groups involved in the project aims to improve public safety and facilitate efforts to restore populations of the South–Central California Coast Steelhead, a fish species whose numbers have declined dramatically over the years.
Originally built to provide drinking water to residents of the Monterey Peninsula, the San Clemente Dam has been found to be inadequate in terms of seismic stability and flood safety. Seeking to rectify this problem, the dam’s owner—California American Water, a subsidiary of American Water, which has its headquarters in Voorhees, New Jersey—initially decided to strengthen the dam, rather than remove the structure. In this way, regulatory compliance would be achieved without having to address the expensive question of what to do with the massive quantities of sediment that have filled more than 90 percent of the dam’s reservoir. However, the prospect of restoring prime steelhead habitat to a more natural condition prompted a variety of groups—including local, state, and federal agencies—to work with California American Water to devise a practicable solution for removing the dam.
Originally constructed to provide drinking water to the residents
of Monterey, the San Clemente Dam has been impounding more
sediment than water for some time. Courtesy of California
The resulting plan calls for leaving most of the existing sediment in place and rerouting a 0.5 mi long section of the Carmel River away from the reservoir, so that it connects directly to San Clemente Creek, a nearby tributary. In this way the sediment can remain in place, obviating the need to haul away a volume of material that would require roughly 250,000 truckloads—a level of traffic that was deemed unacceptable for the rural area around the dam. “This concept of rerouting the river and leaving the sediment in its current location is critical,” says Richard Svindland, P.E., M.ASCE, the vice president of engineering for California American Water. “The reroute concept made the goal of dam removal feasible,” he says.
An approximately 400 ft long dike made of earth-fill material from the site will be used to divert the Carmel River. Expected to have a height of 45 to 50 ft, the dike will also include a seepage cutoff wall extending down to bedrock, according to the preliminary design developed by the owner’s engineer, the URS Corporation, of San Francisco. A massive wedge-shaped slope—approximately 700 ft long, 70 ft tall, and also made of earth-fill material—will be constructed upstream of the dam to stabilize the sediment in the reservoir and prevent it from moving downstream. Sediment removed from other project locations will be placed behind the so-called stabilized sediment slope. Once the dike and slope have been completed, the dam itself will be removed.
To improve conditions for fish, the rerouted section of the Carmel River will be restored to include a series of pools, riffles, and drop features. As a result, returning steelhead will be able to move upstream through the project site much more easily than is currently possible by means of the dam’s outdated fish ladder. For this reason, removing the aging dam will greatly improve access to 25 mi of spawning and rearing habitat upstream. At the same time, removal of the dam will enable sediment naturally present within the river to move downstream, helping to replenish supplies of sand to beach and dune areas near the river’s mouth.
Once the dam is removed, the river will be rerouted so that the
sediment impounded by the former dam can be left in place,
and the fish can find easier passage to spawning locations.
Courtesy of California American Water
Overall, the project is expected to cost $83 million; California American Water will pay $49 million, the amount that the utility would have paid to strengthen and retain the dam. The remaining $34 million will be provided by California’s State Coastal Conservancy. The partnership represents a “unique opportunity” in which California American Water is able to collaborate with public agencies to implement an “environmentally superior project,” Svindland says.
By year’s end, California American Water expects to select one of the following four short-listed firms to deliver the project by means of a design/build approach: ASI Constructors, Inc., of Pueblo West, Colorado; Granite Construction Inc., of Watsonville, California; a joint venture comprising the Kiewit Infrastructure West Company—a subsidiary of the Kiewit Corporation, of Omaha, Nebraska—and Parsons Water Infrastructure, Inc., of Pasadena, California; and Shimmick Construction Company, Inc., of Oakland. The design/build contract likely will be awarded in the first quarter of 2013, Svindland says, with construction scheduled to begin a few months later.