Pasadena’s Devil’s Gate Dam could be put to the test

Twelve years ago, the idea of a giant mudslide barreling down the Arroyo Seco in Pasadena was considered only a distant possibility.

Nevertheless, the Devil’s Gate Dam underwent a major enhancement that took two years to complete and enabled one of the oldest dams in Los Angeles County to withstand a potential massive debris flow.

At the time, engineers thought the improvements might be overkill because conditions would never be extreme enough to test the dam’s strength, at least not in their lifetimes.

“Not a chance,” recalled Brad Boman, engineering manager of water services at Pasadena Water and Power.

But since the Station fire burned more than 160,000 acres and nearly all of the Arroyo Seco in late August and September, making it the worst wildfire in recorded L.A. County history, the experts have changed their tune.

“Twelve years later, we’re confronted with that possibility,” Boman said.

L.A. County Public Works officials spent several days this month cleaning out floating debris that washed into the reservoir in mid-October during a brief rainstorm. Near the base of the dam, they also laid a “debris boom,” or a connected line of floating power poles designed to keep large debris away from the concrete structure.

Devil’s Gate, one of 13 dams in L.A. County, was built in 1920. The modifications made in the 1990s were needed to keep it up to code.

If a mudslide were to occur in the Arroyo Seco, the dam would take the brunt, and because of the area’s geography, a flow would probably move down the canyon when heavy rains hit, said Sue Cannon, a research geologist with the U.S. Geological Survey.

There is a concern with the amount of anticipated debris flow after the Station fire, said Rosa Laveaga, Arroyo Seco project supervisor for the Pasadena Public Works Department. “Nobody is for sure about what’s going to happen.”

The reservoir does not generate power or serve a water conservation function; instead it acts as a massive debris basin capable of storing 2 million cubic yards of material.

In comparison, the 29 debris basins in the foothill areas affected by the Station fire hold a combined 1.2 million cubic yards.

But the reservoir is nearly half full of sediment because it has not been emptied in years, and a large flow could push the level to 75% of capacity.

“We’re safe now, but once that fills up . . . “ Boman said.

It costs about $10 per cubic yard to empty sediment, and maneuvering equipment in the steep terrain can stretch the project to several weeks, if not longer.

Officials with the L.A. County Department of Public Works did not return calls seeking comment on why sediment has not been removed from the reservoir.

If a flow reached the dam, pipes carrying water underneath would probably be shut off to prevent them from becoming clogged with sediment and debris, Boman said. But because of the severity of the Station fire, experts are now concerned there will be too much debris to reopen the pipes.

“We’re talking 30 square miles of watershed that was completely burned,” Boman said. “We really don’t know how much sediment would be flowing down. No one really has experience with this.”

In addition to the underground pipes, there are three main gates through which water and debris can flow. The lowest is a 5-foot-diameter wash gate near the bottom of the dam’s east side. If it were to become blocked, water would flow into a 10-foot-wide outlet tunnel that sits 20 feet higher.

If that tunnel were to fail, the final passageway is a set of gates at the base of a spillway 40 feet higher. In a worst-case scenario, the flow would rise above the spillway, rushing at up to 14,000 cubic feet per second down the outlet on the other side of the dam, according to projections from the Pasadena Public Works Department.

Boman said residents have no reason to fear the dam would break or that areas behind the dam -- including the Rose Bowl and the Brookside Golf Club -- would be affected. The main area of concern, he said, is in the canyon.

“If we have a major rainstorm, we’ll have rocks the size of Volkswagens coming downstream,” he said.

Boman and Laveaga urged people to stay out of the canyon, which is a popular hiking destination.

“People are ignoring signs that are put up,” Laveaga said. “Do not go up into the canyon even on nice, sunny days.”

baxter.holmes@latimes.com