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Supervisors approve big dig at Devil's Gate Dam

Devil's Gate Dam is in Pasadena, located near the 210 Freeway and Oak Grove Drive. Sharon McNary/KPCC

The Los Angeles County Board of Supervisors on Tuesday approved a scaled-back version of a plan to clear sediment from behind the Devil's Gate Dam in Pasadena to reduce the flood risk for homes downstream along the Arroyo Seco.

At Tuesday's hearing, Supervisor Kathryn Barger proposed the county's plan to dig out 2.4 milliion cubic yards be cut back to 1.7 million cubic yards. That's still more than the 1.1 million the City of Pasadena and two environmental groups wanted.

The sediment removal was made necessary by the heavy rains following the 2009 Station Fire that caused about 1.3 million cubic yards of sediment to flow from the burned mountain slopes into the reservoir behind Devil's Gate Dam. That reduced the dam's water-holding capacity.

The dam collects water from a portion of the San Gabriel Mountains watershed that flows down through Pasadena, South Pasadena and Los Angeles on the concrete Arroyo Seco channel, eventually flowing into the Los Angeles River and the Pacific Ocean.

Several years ago, the county commissioned a hydrology study that concluded that the capacity of the dam was so limited that an extreme series of rainstorms could overtop the dam and flood the 110 Freeway and hundreds of homes on either side of the Arroyo Seco.

To restore the dam's capacity and protect the downstream homes, L.A. County Public Works Department wrote a plan to remove  2.4 million cubic yards of sediment over several years, but it was challenged in court.

The Arroyo Seco Foundation and the Pasadena chapter of the Audubon Society sued in Superior Court to pressure the county to enact a less extreme sediment-removal plan that would not destroy as much wildlife habitat as the county's plan. The two groups prefer an alternate plan, one also endorsed by the Pasadena City Council, to take out only 1.1 million cubic yards of sediment over about five years.

Before  Tuesday's unanimous vote, Barger spoke of the need to balance the flood safety concerns for homes along the 110 Freeway against the objections of upstream residents who dread the emissions from hundreds of trucks hauling out dirt each day.

The supervisors voted on a revision to an environmental impact report that had been approved by the board several years ago.

The EIR had been revised to correct areas a Superior Court judge found deficient. The judge wanted the county to more precisely describe how it planned to replace destroyed habitat. The county Public Works Department proposes replacing one acre of habitat for every acre taken out, Director Mark Pestrella said in an interview last week. Also, the county plans to keep the footprint for the area that is to be dug out consistent from year to year, so that the new habitat can grow in undisturbed.

The Pasadena chapter of the Audubon Society would prefer to see the county replace a larger ratio of habitat, said chapter conservation lead Mark Hunter.

In the revision, the county committed to using 2010-vintage or later diesel trucks to haul dirt. Hunter said the group preferred trucks that run on compressed natural gas, to reduce the emissions coming from the project.

Pasadena Mayor Terry Tornek thanked Barger for proposing the lower amount of dirt removal, but cautioned that the proposal would send as many as 400 dirt-hauling trucks per day in and out of the dam basin.

The project still requires approvals from state Department of Fish and Wildlife and from the Superior Court, Pestrella said.