El Nino Affects Sediment Removal Needed Above Devil’s Gate Dam

The El Nino phenomenon could delay plans by the Los Angeles County Department of Public Works to remove 2.4 million cubic yards of sediment from the Hahamongna flood-control basin above Devil’s Gate in Pasadena.

Hydro Review Magazine and its online edition, HydroWorld.Com, report that sediment removal is planned during the next five years, though actual work is not scheduled to begin until 2016.

Experts forecast that El Nino may produce heavy rains throughout the winter of 2015, which could cause more debris accumulating into the Hahamongna area from the 2009 Station Fire. This could threaten the dam’s structural integrity and cause flooding to neighborhoods downstream.

Areas under threat would include the Arroyo Seco American Viticultural Area (AVA) southeast of Monterey Bay.

Devil’s Gate, a 100-foot high concrete gravity arch dam, was built in 1920 in the Arroyo Seco as the first flood control dam in LA County. The dam supports a 5,323 acre-feet capacity reservoir within the Hahamongna Basin.

Dam safety officials say failing to complete the debris removal project, as well as future sediment removal, could result in overtopping the dam. Structural failure could inundate a significant area.

Officials say there is risk of flooding south of the dam if the San Gabriel Mountains are hit with a hard El Nino winter, sending large amounts of mud, rocks and burned trees into an already full Hahamongna basin.

“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 also planned for dams in Pacoima, Tujunga and three other locations in the San Gabriel Mountains.