Of all the problems we hear about related to American infrastructure, the state of the nation’s dams is perhaps the scariest; a failure upstream of an urban area could cause more damage all at once than just about any other infrastructure-related event. The failure of the spillways at the Oroville Dam in northern California last February, which prompted the evacuation of 180,000 people, was a reminder of the potential danger.
Age, erosion, and poor design are some of the things threatening our existing dams, but another risk is earthquake. In California, the dam on the Calaveras Reservoir sits next to an active fault, and the dam’s failure could affect thousands of people. The reservoir’s owner is building a replacement. But it’s not the only one; according to a recent Wall Street Journal article, 30% of the more than 90,000 structures in the 2016 National Inventory of Dams have a “high” or “significant” hazard rating. More than 2,000 of them require immediate upgrade, according to the American Society of Civil Engineers, but doing so would cost tens of billions of dollars. (Take a look at the ASCE’s latest Infrastructure Report Card, which rates dams; bridges; water, energy, and transportation systems; and more, including breakouts by state).
In the case of the Calaveras Dam, which is more than 90 years old, the base was build on top of loose rock and soil, on a site where an earlier dam had in fact failed. Some of these very old structures were designed before engineers fully appreciated the effects of a strong earthquake; it’s now believed that the loose soil could liquefy, or lose strength and deform, during a quake. The new dam and similar replacements are built with a compacted foundation. In the meantime, workaround measures are sometimes used; for example, once the situation was discovered more than a decade ago, the state ordered the San Francisco Public Utilities Commission to limit water in the Calaveras Reservoir to no more than 40% of capacity until the replacement dam is complete.
We think of California as having the greatest earthquake risk, but many other states, particularly Oregon and Washington, also live with that threat. An article in the New Yorker a couple of years ago described the potential for a major (and perhaps overdue) earthquake in the Pacific Northwest in such vivid detail that some residents put their homes on the market.
Are you aware of dams or levees in your area that need attention? What’s the funding situation for inspections, upgrades, and replacements?