OUTDATED Concrete arch dam is a 57-year-old silt catcher. (Photo courtesy of Matilija Coalition /Paul Jenkins)

Five years after the breaching of Edwards Dam in Maine, dam removal continues steadily. A report released July 21 by American Rivers estimates that 60 dams will be removed by year’s end, says Eric Eckl, a spokesman for the Washington, D.C.-based environmental group.

A feasibility study released within days of the survey details a program that would dwarf those removals. A $110-million proposal to restore a watershed in Ventura County, Calif., includes removing the largest dam ever to be demolished in the U.S.

"Nothing like this has been done on this scale in this type of environment," claims Paul Jenkin, coordinator for the Matilija Coalition, a longtime advocate of removing the 190-ft-tall, 620-ft-long Matilija Dam. The plan has "the potential for a real case study in watershed restoration," Jenkin says.

Located on the Ventura River in Los Padres National Forest, about 16 miles from the Pacific Ocean, the 57-year-old concrete arch dam blocks the southern steelhead, an endangered species of trout, from prime spawning grounds. By trapping sediments, the dam has eroded breeding areas and deprived beaches of sand. Notching and sedimentation have reduced the reservoir’s capacity to an estimated 500 acre-ft, only 7% of the dam’s original capacity.

The study identifies extensive measures required before demolition. The key challenge "is how to deal with moving the 6 million cu yd of sediment that are trapped behind the dam," says Jonathan Vivanti, feasibility study manager for the U.S. Army Corps of Engineers’ Los Angeles District.

"The big issue is going to be downstream water supply," says Jeff Pratt, director of the Ventura County Watershed Protection District, the dam’s owner. The proposal includes provisions to protect Lake Casitas and other surface water sources. About 2 million cu yd of fine material trapped behind the dam would be removed through a temporary 2-mile-long slurry pipeline and deposited at a 118-acre site. An 8-mile-long pipeline from Lake Casitas will supply water for the slurry.

To allow the steelhead and other fish to reach spawning areas, about 1.1 million cu yd of material would be excavated to create a 100-ft-wide, 2-mile-long, soil-cement-lined channel. Eroding material now held back by the dam will increase the volume of water during floods, so a new downstream levee would be constructed and two existing levees raised about 5 ft.

Demolition methods would be up to the contractor but Vivanti says blasting is the most likely technique. The program could start as early as 2008 and take about three years for completion, says Vivanti.