Monday, February 20, 2017
Associated Press
LOS ANGELES
Twelve years ago, widespread destruction from Hurricane Katrina on the Gulf Coast helped compel federal engineers 2,000 miles away in California to remake a 1950s-era dam by constructing a massive steel-and-concrete gutter that would manage surging waters in times of torrential storms.
The nearly $1 billion auxiliary spillway at Folsom Dam, scheduled to be completed later this year, stands in contrast to the troubles 75 miles away at the state-run Oroville Dam, where thousands of people fled last week after an eroded spillway threatened to collapse – a catastrophe that could 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 The structures also underscore the challenge of maintaining older dams with outdated designs.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.
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.
After years of drought, Northern California has become waterlogged this winter from heavy rain and snow. Oroville Lake is brimming, and water managers have been using the main spillway, which is lined with concrete, to lower the water level.
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.
An investigation into what went wrong could take months.
At 340-foot high Folsom Dam, operated by the U.S. Bureau of Reclamation, the huge concrete-and-steel chute being constructed by the Army Corps of Engineers is expected to add as much as 40 percent capacity to the main spillway that controls water flowing from the reservoir behind it.
It’s designed to allow safer releases during times of high water, precisely the challenge that led to fears of catastrophic flooding at Oroville.
Rick Poeppelman, chief of the Army Corps engineering division in the Sacramento district, said extensive data about probable maximum flood levels, not available decades ago, helped prompt the decision to build the auxiliary spillway at Folsom Dam.
When completed, the spillway can act like a second dam, allowing operators to release water through a series of gates and lower the reservoir level when a major storm is approaching. It includes a chute more than half a mile long. Another advantage: The new spillway gates will be 50 feet lower than those on the dam, allowing for earlier releases of water.