LOS ANGELES --
A highly anticipated modification study of a Southern California dam has been finalized by the U.S. Army Corps of Engineers and is now in the project’s design phase.
The Whittier Narrows Dam Safety Modification Study was completed in June 2019 and approved by the Corps’ Headquarters in August. The design phase of the project began in October.
“We are excited about the study completion and are looking forward to working with our local, state and federal partners moving forward into the design phase,” said Col. Aaron Barta, commander of the Corps’ Los Angeles District. “Whittier Narrows Dam was the only ‘very high urgency’ dam in the Corps of Engineers’ portfolio that was still in the study phase. That chapter is now closed. Our top priority is getting this project into the construction phase and completed.”
Whittier Narrows Dam was constructed in 1957 by the Corps as a flood-risk management project and is located 11 miles east of downtown Los Angeles. The dam reduces the risk of flooding to more than 1 million people.
Following Hurricane Katrina, the Corps dramatically overhauled its process for prioritizing dam and levee evaluations and modifications to better account for potential impacts to the population downstream of the structures. The modification study was subsequently initiated to evaluate the risk to the population, which extends from Pico Rivera to Long Beach.
In May 2016, the Corps placed Whittier Narrows Dam in the agency’s highest risk category, “very high urgency,” when it became clear three potential failure modes threatened the downstream population. The three failure modes are premature opening of the San Gabriel River Spillway gates, erosion resulting from water piping through the foundation of the earthen dam and overtopping during an extreme flood. While the first would result in downstream flooding as the levees were overtopped, the latter two could result in a catastrophic failure of the dam.
In 2019, the Corps awarded a $5-million contract to Metro Builders of California to complete construction on the spillway gates. Estimated completion is February 2020.
The modifications to prevent the earthen dam from eroding or failing if overtopped, include placing roller-compacted concrete on the crest and downstream slope of the embankment and improving the seepage control system with features like trench drains and graded filter/drainage blankets.
Now in the design phase, the Corps is working with its local and state partners to refine the modification plan and ensure the design meets requirements. The first construction contract for utility relocations is scheduled to be awarded in September 2021; the larger dam modification contract is scheduled to be awarded a year later. Construction of the project is expected to be completed by 2026, with environmental monitoring extending through 2031.
From 1982 to 2009, flood damages prevented by the dam were valued at $4.7 billion, with an average annual benefit of $169 million.
“The dam was designed consistent with the standards of the day and it has succeeded in reducing flood damages,” said Doug Chitwood, lead project engineer. “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 flood risk.”
ABOUT CORPS’ DAMS
The U.S. Army Corps of Engineers operates more than 700 dams that serve a variety of purposes, including navigation, flood damage reduction, water supply, irrigation, hydropower, recreation, environmental enhancement and combinations of these purposes. As part of the Corps’ responsibility in managing these dams, the agency has a comprehensive Dam Safety Program that has public safety as its primary objective. Corps’ dams are routinely inspected and continually evaluated for safety in accordance with the Federal Guidelines for Dam Safety issued in 1979.