Two California dams illustrate challenges faced

have sent a 30-foot wall of floodwater gushing into three counties.

Together, the two dams illustrate widely diverging conditions at the more than 1,000 dams across California, most of them decades old. The structures also underscore the challenge of maintaining older dams with outdated designs.

“Fifty years ago, when we were evaluating flood risk, the fundamental assessment was the climate was stable, not changing. We now know that is no longer true,” said Peter Gleick, chief scientist with the Pacific Institute, a California-based think tank specializing in water issues.

“We need to look at the existing infrastructure with new eyes,” he warned.

Back in 2005, Katrina’s deadly path became an arguing point for U.S. Rep. Doris Matsui, a California Democrat who was among those pushing Wash- ington for improvements at Folsom Dam, perched 25 miles from 500,000 people living in Sacramento, the state capital.

“I used that, vigorously, to say we are the second-most at-risk river city in the nation,” Matsui said.

State officials now face questions about maintenance at Oroville Dam, the nation’s tallest at 770 feet, and why a decade ago they dismissed warnings from environmentalists that more needed to be done to strengthen its earthen emergency spillway.

The emergency spillway is a brush-covered hillside below a masonry lip and had never been used until last weekend. When water gushed onto it, the ground began eroding, and it was feared the intake lip could collapse and water would surge down the hill.

John France, vice president and technical expert on dams for the engineering consulting firm AECOM, said the problems at Oroville should raise alarms across the country.

“Most of the dams in the United States are over 50 years old, when we didn’t understand floods as well as we do now. So we have a number of dams in the U.S. that have spillways that aren’t large enough for the floods that they should be designed for,” France said.

In a place known for alternating cycles of prolonged drought and biblical rainfall, the idea of asking reservoirs to store extra water for dry times frequently clashes with environmentalists, who want to see rivers flow freely.

At Oroville, opened in 1968, construction crews recently patched cracks on the main spillway, and a state inspector judged the repairs “sound” in a February 2015 report.