With the close of the winter storm season, Los Angeles County Department of Public Works (DPW) will resume work on an essential infrastructure project at Devil’s Gate Reservoir in the City of Pasadena, CA. The Devil’s Gate Reservoir Restoration Project is a four-year effort to increase vital flood protection for communities downstream of Devil’s Gate Dam.

“Critical infrastructure projects, like the work being done at Devil’s Gate Dam, are considered essential services by the State of California, and LA County Public Works is dedicated to a community-first approach as it moves forward in this multi-year effort. Workers on the project site will adhere to state and local guidelines regarding COVID-19,” said the DPW announcement.

Sediment removal at Devil’s Gate Reservoir is vital to prevent a potentially catastrophic flood event that would affect northeast Los Angeles, Pasadena and South Pasadena; the urbanized downstream communities that the dam is supposed to keep safe.

Completed in 1920, Devil’s Gate is the oldest dam constructed by the LA County Flood Control District, built to protect communities from floodwaters and fast-moving debris pouring from the San Gabriel Mountains. Yet Devil’s Gate Reservoir is so full of debris that it “no longer has the capacity to safely contain another major debris event” and “there is risk of significant flooding and debris flow below the dam,” says a DPW report.

The dam is situated immediately north of Interstate Highway 210, near the Rose Bowl, and under a worst-case scenario, torrential rains could send mud, rocks and water over the dam and flooding into the Rose Bowl, the 110 Freeway, Pasadena, South Pasadena and northeast Los Angeles in less than 40 minutes — and a complete dam failure could cause flooding at the Rose Bowl in less than 10 minutes.

After the 2009 Station Fire burned more than 160,000 acres of the San Gabriel Mountains and subsequent storms brought more than 1.3 million cubic yards of soil and debris down from the denuded tributary area into Devil’s Gate Reservoir (enough to fill the Rose Bowl 3 times), a Flood Control District analysis of the dam found that “Due to the volume of sediment deposited within the dam reservoir, the dam discharge structures are under direct threat of inundation.”

To mitigate this threat to the dam — and the communities below it — DPW plans to remove up to 1.7 million cubic yards of sediment from the reservoir. Known as the ‘Big Dig’, the four-year project is a massive undertaking requiring hundreds of truck trips in and out of the Arroyo Seco Channel daily.

Once the ‘Big Dig’ is complete, the reservoir will have 3 million cubic yards of capacity to better manage flood risk, and DPW will establish a permanent stormwater maintenance area that is cleaned annually to avoid large-scale sediment removal projects in the future. The permanent maintenance area allows for the creation of 70 acres of enhanced habitat for wildlife and recreational opportunities for local communities.

Source: Various