County readies for Devil’s Gate cleanout

The Los Angeles County Department of Public Works will start work in July to remove 25,000 cubic yards of sediment from the basin behind Devil’s Gate Dam in Hahamongna Watershed Park.

The project is a stopgap measure to prevent valves and other dam works from becoming clogged this coming winter while the county studies options for the eventual removal of roughly 1.5 million cubic yards of mud and debris, most of it deposited after the August 2009 Station fire.

Excavation work, which could begin as early as July 5, would be limited to within about 100 feet of the face of the dam — leaving all wooded areas in the basin intact and eliminating the need for dirt-hauling trucks to pass through La Cañada Flintridge, said Chris Stone, assistant deputy director of Public Works’ Water Resources Division.

“We need to remove sediment that’s accumulated around intake structures to make sure they remain operational over the next one or two winters while we pursue [a study] for the larger project. If we couldn’t operate the valves, we could no longer discharge water downstream, and then it could build up and go over the stow-way” — flooding unchecked into areas below the dam, Stone said.

Dirt removed from the dam face this summer will either be stored temporarily at Johnson Field, an unused Pasadena Water and Power groundwater restoration basin nearby, or hauled out past the southern end of the JPL parking lot, down Windsor Avenue in Altadena and along the Foothill (210) Freeway to a fill area in Irwindale.

Pasadena City Council members are expected to decide on June 20 whether to allow use of Johnson Field, which along with keeping trucks off Windsor would not interfere with maintenance of the park, said Dan Rix, a Pasadena city engineer.

Stone gave a project update on Monday to La Cañada Flintridge City Council members, who expressed concern that one of several potential truck routes still in play for future work involves using Oak Grove Drive to access the freeway.

“In my eyes, you guys are some of the heroes of our debris flow problem, only my biggest concern is the haul route when you start cleaning it out,” said Councilwoman Laura Olhasso, who called for the city to hold a future project scoping meeting.

The eventual removal of millions of cubic yards of sediment would require 300 to 400 truck trips per day out of the basin over several months, Stone said.

The larger project would also involve the destruction of some 50 acres of woodland that’s grown on top of years of mud deposits behind the dam, creating a habitat for wildlife and a popular spot for local hikers, dog walkers and equestrians.

“This is an escape from the congestion of the city. We should slow down and think about this,” said downtown Los Angeles resident James Emrich, who visited the basin last weekend.

Though Public Works officials initially sought to begin removing all 1.5 million cubic yards this summer, concerns prompted the Los Angeles County Board of Supervisors — acting on a motion of Supervisor Michael Antonovich in March — to order an environmental impact study that delays the larger sediment removal project up to two years.

“Not only were there impacts to the environment, the project was going to have a significant impact on the community,” said Edel Vizcarra, planning deputy for Antonovich.

Vizcarra praised the partial summer cleanout for at least temporarily preserving recreational space while also restricting truck traffic.

Stone said that work in July would occur from 7:30 a.m. to 4 p.m. on weekdays for three to four weeks, leaving adjacent areas open to the public on weekends.

Following sediment removal, county officials will make improvements to the dam aimed at preventing potential debris blockages.

Built in 1920, Devil’s Gate Dam had not required a basin cleanout for decades, but rainstorms in the months following the August 2009 Station fire in the Angeles National Forest deposited about one million cubic yards of mud and debris there.

Another 300,000 to 500,000 cubic yards flowed into the basin last winter, said Stone.

“In the first storm season after the fire, the sediment level went up 25 feet,” he said. “That’s a million cubic yards in one season. Without a burned watershed, we only got 100,000 cubic yards in 10 years.”