LOS ANGELES - The U.S. Army Corps of Engineers Whittier Narrows Dam in Pico Rivera performed as originally designed during Southern California’s most recent storm event March 6.
The dam is one component of a complex Los Angeles County Drainage Area flood risk management system that consists of 5 dams, 33 debris basins and 180 miles of main and tributary channels.
The Corps recently completed a draft for the Whittier Narrows Dam Safety Modification Study that recommends important modifications to the dam to ensure it continues to reduce flood risk for the public.
“Life safety is the number one priority for the Corps,” said U.S. Army Corps of Engineers Los Angeles District Commander Col. Aaron Barta. “The Corps is taking a proactive approach with the recommended modifications to ensure it continues to function well into the future.”
Whittier Narrows Dam was risk characterized Dam Safety Action Classification 1 by the Corps, due to the combination of loss of life with a very high likelihood of failure only when filled by a rare flood event. The study shows should this rare flood occur, the dam could fail through erosion underneath or even overtopping, putting the very large population downstream (in excess of 1 million people) at risk.
“The dam was designed consistent with the standards of the day and it has succeeded in reducing flood damages,” said Lead Project Engineer Doug Chitwood. “However, floods larger than we have experienced since the dam was built will occur at some point. The proposed project and community preparation are key components in reducing the flood risk.”
The Corps will continue to seek opportunities to communicate with the public and its partners the importance of this study and the value of this dam to Southern California. Downstream communities have begun actively preparing evacuation plans to minimize risk should a rare flood event occur in the future.
“There are no guarantees when it comes to Mother Nature,” Barta said. “Residents downstream from the dam should be aware of evacuation routes, local emergency communication procedures and be prepared for any flood event whether big or small.”
Whittier Narrows Dam is the only DSAC 1 dam in the nation still in the study phase, and getting it to construction is a Corps priority. The dam modifications do not require congressional authorization as the project is already an authorized federal project. The goal is to complete the study in 2019, initiate design and be ready to start construction in 2021.
From 1982 to 2009, flood damages prevented from the dam are valued at $4.7 billion, with an average annual benefit of $169 million. The recreation area provides an estimated annual visitation of 2.1 million people.
1) Why did the U.S. Army Corps of Engineers conduct the study?
Following the devastating life loss that resulted from the levee failures which occurred during Hurricane Katrina, the Corps of Engineers undertook the massive task of re-evaluating all dams and levees within its portfolio, utilizing a multi-phased process to identify and prioritize those structures which needed modifications. The issues identified at Whittier Narrows during this process led to the dam safety modification study.
2) How much will the modifications cost and how long will it take?
Assuming that we can initiate detailed design this summer (2019), the first construction contract would be awarded in the fall of 2021. Construction is anticipated to take roughly four years. The project cost for design and construction is anticipated to be in the ballpark of $500 million.
3) Is Whittier Narrows Dam Safe?
There are always risks with any dam and the downstream communities always need to be prepared. That said, Whittier Narrows has successfully provided flood protection for over 60 years and we anticipate that it will continue to do so.
4) Why don’t the recommended modifications require congressional authorization?
Whittier Narrows Dam is already authorized for flood risk management. This modification study does not change the project purpose, so it does not require congressional authorization. Under the Dam Safety Assurance program, safety improvements warranted upon detailed studies are generally within the discretionary authority of the Chief of Engineers. The existing project authority is sufficient to permit project improvements for the flood risk management function of the project.
5) What modifications are recommended?
Risk Management Plan 5, has been identified as the recommended plan (also referred to as the Tentatively Selected Plan, or TSP) because it is the most efficient and effective plan at reducing incremental risk while minimizing environmental impacts. Plan 5 prevents the earthen dam from eroding and failing when overtopped by placing roller compacted concrete on the crest and downstream slope of the embankment. The plan prevents failure due to erosion underneath dam by adding additional seepage control features (e.g., trench drains, relief wells and graded filter/drainage blankets) to allow the safe dissipation of subsurface pressures which would be the result of unusually large pools.
For more information or to read the draft Environmental Impact Statement visit go.usa.gov/xEA5A