Fifty years after the 1971 San Fernando earthquake and the near failure of the Lower Van Norman Dam, the overwhelming majority of California dams are decades past their design life span. While earthquakes still loom as the greatest threat to California’s massive collection of dams, experts warn that these aging structures will be challenged further by a new and emerging hazard: “whiplashing shifts” in extreme weather due to climate change.
“The biggest issue facing dam safety in California is aging infrastructure and lack of money to fund repairs and retrofits of dams,” said Sharon K. Tapia, who leads the Division of Safety of Dams at the California Department of Water Resources. “Many older dams were built using construction methods considered outdated by today’s standards.”
Federal engineers have found that three major dams in Southern California — Whittier Narrows, Prado and Mojave River — are structurally unsafe and could collapse in a significant flood event and potentially inundate millions of people downstream.
Each has been reclassified as “high urgency structures” amid growing concerns that they were designed and built on 20th century assumptions and hydrological records that did not anticipate the region being hit more frequently by storms that were previously regarded as extremely rare events. Until only recently, it was thought that a flood of the magnitude similar to the one that hit California during the rainy season of 1861-62 when 36 inches of rain dropped on Los Angeles, could only occur every 1,000 to 10,000 years. Recent studies, however, suggest that the chances of seeing another flood of that size over the next 40 years are about 50-50.
“Even if engineers had made risk assessments that were accurate at the time these structures were built, they aren’t accurate now, and won’t be anymore due to climate change,” said Daniel Swain, a UCLA climatologist.
Today, about 75% of the 1,250 dams regulated by the State Department of Safety of Dams are over 50 years old. In addition, 250 are classified as “extremely high hazard,” indicating that their failure or misoperation is expected to result in loss of life and economic damage.
In 2019, the U.S. Army Corps of Engineers determined that a significant flood event could compromise the concrete slab of the spillway of the 80-year-old Prado Dam on the Santa Ana River, potentially flooding an estimated 1.4 million people in dozens of Orange County communities from Disneyland to Newport Beach.
The Army Corps also reclassified the Mojave River Dam, located in San Bernardino County, as “high urgency” after assessing that during an extreme flood event, water could exceed the design capacity of the dam and overtop it. Failure of the 50-year-old earthen dam would put 16,000 people and $1.5 billion in property at risk in the adjacent communities of Hesperia, Apple Valley, Victorville and Barstow.
Currently, the Army Corps’ top priorities in Southern California include spending an estimated $600 million to upgrade the 62-year-old Whittier Narrows Dam, east of downtown Los Angeles. The earthen dam was placed in the agency’s highest risk category when it was determined that three potential failure modes threatened more than 1 million people downstream, from Pico Rivera to Long Beach. The upgrade project is expected to be completed by 2026.
Source: Los Angeles Times