Los Angeles County Department of Public Works plans to remove 2.4 million cubic yards of sediment from the Hahamongna flood-control basin above Devil’s Gate Dam in Pasadena, Calif. during the next five years. But, the sediment-removal project is not scheduled to begin until 2016.
If forecasted heavy rain from El Nino occur in winter 2015, it would likely cause debris accumulated in the Hahamongna area from the 2009 Station Fire to threaten the dam’s structural integrity and cause flooding to neighborhoods downstream.
Devil’s Gate Dam is a 100-foot-high concrete gravity arch dam. Devil’s Gate Reservoir, located within the Hahamonga basin, has a capacity of 5,323 acre-feet. Using L.A. County DPW hydrology and sedimentation manuals, dam safety officials think failing to complete the project or conduct future sediment removal could result in overtopping at the dam. This could result in structural failure, which would inundate a significant area.
In 1920, flood engineers from the Los Angeles County Flood Control District built Devil’s Gate Dam in the Arroyo Seco, the first flood control dam in Los Angeles County. The Devil’s Gate Dam Arroyo is a tributary to the Los Angeles River.
The risk of flooding south of the dam would exist if the San Gabriel Mountains are soaked with a series of unrelenting storms from El Nino, sending large amounts of mud, rocks and burned trees into an already full Hahamongna basin, according to Los Angeles County dam safety officials.
“The dam, since it was constructed, was built in size to provide a certain level of flood protection to downstream communities,” officials said. “The quantity of sediment that we’re recommending to remove allows us to provide historical levels of flood protection to the downstream communities.”
Major sediment-removal projects are planned for other dams in Pacoima, Tujunga and three up in the San Gabriel Mountains. Those other reservoirs also have sediment buildup, but not to the same extent as Devil’s Gate.