From The Landslide Blog (Dave Petley):
The scale of the erosion problem at the Oroville Dam site
As the fairly desperate attempt continue to shore up the spillways at the Oroville Dam site, and to lower the water level ahead of the next rainstorm, better images are emerging of the scale of the problems on both spillways. If we start with the main spillway, which suffered the original erosion events last week, it is hard to appreciate the serious nature of the problems. However, this image (via the Sacramento Bee) from the weekend shows workers in the hole eroded in the main channel:-
Of course the high flows down the spillway since then are likely to have exacerbated the problem significantly. Meanwhile, the headward erosion problem in the emergency spillway is now garnering most of the attention, primarily because of the potential for a major collapse. This image, from Hamodia, shows the nature of the erosion that has developed at multiple locations immediately down slope of the emergency spillway at the Oroville dam site:
The most serious problem appears to be a gully towards the bottom of the image, but there is also substantial amounts of erosion occurring on the other side too. The danger is of course that the these gullies will suffer headward erosion until they undermine the spillway lip. whereupon collapse may occur. One challenge is that the quality of the rock does not appear to be high, which accounts for the rapid erosion in both cases. This image, via Twitter, shows the nature of the bedrock with which the crews are dealing:-
The challenges are substantial. Fortunately, as before, there is no threat to the dam itself.
From the Associated Press via The U.S. News & World Report:
HOW IMPORTANT IS LAKE OROVILLE?
Lake Oroville is the starting point for California’s State Water Project, which provides drinking water to 23 million of the state’s 39 million people and irrigates 750,000 acres of farms. It is the largest reservoir in the system, which was built in the 1960s and early 1970s to carry rain and snowpack from the Sierra Nevada mountains to parts of the San Francisco Bay area, Central Valley and Southern California.
Lake Oroville, completed in 1967, is a cornerstone of the system of 34 reservoirs, lakes and storage facilities, built and operated by California’s Department of Water Resources. It feeds into the Feather River — about 70 miles north of Sacramento — as well as the Sacramento River and the Sacramento-San Joaquin River Delta. From there it travels south on the 444-mile California Aqueduct.
Oroville’s storage capacity of 3.5 million acre-feet of water is enough to supply urban California for up to six months, said Peter Gleick, president emeritus of the Pacific Institute, a water research organization based in Oakland, California.
“The risk of losing Oroville is very, very low” he said. “The consequences would be catastrophic.”
WHAT ARE SPILLWAYS?
When reservoirs get too full, their operators release extra water down long channels, or spillways, designed to carry it downstream in a safe, controlled way.
Oroville Dam has a main concrete spillway that normally is used to release floodwaters into the Feather River downstream. A second spillway mainly made of earth serves as an emergency backup. It also was supposed to be able to handle high flows from the dam, but it had never been used before Saturday.
The force of water [released] from the lake has damaged both spillways.
WHAT CAUSED THE FLOOD THREAT?
After five years of drought, a wet winter has strained the system at Lake Oroville, which is receiving runoff from melting snow in the Sierra Nevada as well as from the latest in a series of heavy storms.
Dam operators noticed chunks of concrete in the main spillway on Feb. 7. When workers stopped releasing water to investigate…With the reservoir nearing the top of the 770-foot-high dam, dam operators were forced to keep using the main spillway despite increasing damage to it from the rushing water.
The dam reached capacity Saturday, sending water surging over the second, emergency spillway. Operators on Sunday noticed water was gouging a hole in the earthen emergency spillway as well. Fearing that the emergency spillway could fail and send torrents of water rushing downstream uncontrolled, authorities ordered the evacuation Sunday evening.
BESIDES OROVILLE, WHERE DOES CALIFORNIA GET ITS WATER?
The Central Valley Project, operated by the U.S. Bureau of Reclamation, irrigates more than 3 million acres of farms and provides enough drinking water for more than 1 million people. The system of 22 reservoirs was built from 1937 to the 1950s, extending about 400 miles from the Cascade Mountains near Redding to the Tehachapi Mountains near Bakersfield. It includes Shasta Lake, the only reservoir in California that’s larger than Oroville.
The Colorado River supplies 19 million urban dwellers in Southern California through a 242-mile aqueduct from Lake Havasu, Arizona, to the state’s coastal regions that was completed by Metropolitan Water District of Southern California in 1941. The Colorado also farms California’s Imperial Valley — a major source of the nation’s winter vegetables — through the 80-mile All-American Canal that hugs the state’s border with Mexico.
Other significant pieces of the state system include the Los Angeles Aqueduct, which carries water from Mono Lake to the city of Los Angeles, and the Hetch Hetchy reservoir in Yosemite National Park, which supplies the San Francisco Bay area.
Gov. Jerry Brown is asking the Trump administration for federal assistance in responding to a potential failure of a spillway at the Oroville Dam in Northern California.
In a letter to President Donald Trump released Monday, Brown asks for help for the three Northern California counties affected.
Brown says aid is needed to assist the 188,000 residents of Butte, Sutter and Yuba counties who were ordered to evacuate Sunday after concerns an emergency spillway could give way, unleashing a gush of water to downstream towns.
Brown has criticized Trump on many of his initiatives, but at a news conference Monday he lauded the president’s plan to invest $1 trillion on infrastructure.
The governor says California and Washington will work “in a constructive way” to repair failing infrastructure in the state…
The California Department of Water Resources says helicopters are dropping loads of rock on a hole at the lip of Oroville Dam’s emergency spillway.
Workers are hoisting giant white bags filled with rocks and at least two helicopters flying them and releasing them in the spillway’s erosion. Dump trucks full of boulders also are on their way to dump their cargo on the damaged spillway.
The barrier at the nation’s tallest dam is being repaired a day after authorities ordered mass evacuations for everyone living below the lake out of concerns the spillway could fail and send a 30-foot wall of water roaring downstream…
California’s U.S. senators are calling on President Donald Trump to approve a disaster declaration for the state in response to damage from recent storms.
Sens. Dianne Feinstein and Kamala Harris wrote in a letter Monday that the situation is especially dire downriver from Oroville Dam, where damage has threatened flooding and forced nearly 200,000 people to evacuate.
The senators are asking the president to provide $162.3 million in disaster assistance that California requested.
The lake behind Oroville Dam swelled significantly with this winter’s rains and the collapse of its damaged spillway threatens to flood downstream communities.
From the Associated Press (Olga R. Rodriguez and Don Thompson) via The Colorado Springs Gazette:
Nearly 200,000 people who were ordered to leave their homes out of fear that a spillway could collapse may not be able to return until the barrier at the nation’s tallest dam is repaired, a sheriff said Monday.
Butte County Sheriff Kory Honea gave no timetable for the work. Officials from the California Department of Water Resources were considering using helicopters to drop loads of rock on the eroded spillway at Lake Oroville, about 150 miles northeast of San Francisco.
Meanwhile, the water level behind the dam dropped, easing slightly the fears of a catastrophic spillway collapse. But with more rain expected later in the week, time was running short to fix the damage ahead of the storms.