|Damaged spillway with eroded hillside near Oroville, Calif. |
(California Department of Water Resources photo)
As dams get older, experts say a growing number of people downstream are at risk, USA Today reports: "Among those high-hazard dams, nearly 1 in 5 lack an emergency action plan," and "the average age of the United States' 84,000 dams is 52, according to the American Society of Civil Engineers' report." The Federal Emergency Management Administration explains how dams are rated for hazards, and states have lists of dams and their ratings.
The Oroville episode is an example of how changing weather patterns are affecting dam safety, Anne C. Mulkern reports for Climatewire. Daniel Swain, a climate scientist at UCLA, said "a series of storms powered by a phenomenon known as the atmospheric river hit Northern California this winter. Those filled Oroville, prompting the release of water onto its spillway. Then that structure suffered a sinkhole that became apparent last week."
Dam operators last weekend "stopped sending water down the spillway, and flows crested the alternate 'emergency' spillway, essentially a hillside," Mulkern writes. "When that caused soil erosion headward, or in the direction toward the structure, dam officials feared the concrete spillway would collapse, sending a 30-foot wall of water downstream, causing 'catastrophic flooding,' Democratic Gov. Jerry Brown said in a letter to President Trump." The 770-foot-high dam is the tallest in the U.S.
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A July 27, 2006 memo from John Onderdonk, a senior civil engineer with the Federal Energy Regulatory Commission's Dam Safety and Inspections, "said that if the emergency spillway were to be used, it 'would not affect reservoir control or endanger the dam,'' reports Greenwire. "He also said 'in the rare event of a discharge, the emergency spillway would perform as designed' and could conceivably handle a flow of water at a rate of 350,000 cubic feet per second. But last weekend, water flowed down the emergency hillside spillway at a maximum rate of about 12,000 cubic feet per second—a far lower rate—and significant erosion occurred."