Have we underestimated the West’s super-floods?
AGENDA 21 RADIO
BY PAUL PRESTON
So floods in California are NOT new. If there is one thing many in California are learning for the first time and others are having reinforced is that it can RAIN in California in buckets! As we have seen in the great rains and snow of the 2016-2017 season these storms that have brought water to a drought stricken state the massive amount of precipitation needed to end a drought but also to create an intense discussion on how to deal with a crumbling infrastructure that was originally designed to have greater capacity.
The failure to not fulfill the original plans of those planners in the 1950s and 1960s is now a haunting reality. This failure is best seen in the remains of the Auburn Dam along the American River in Auburn, CA which would have taken pressure off the Folsom Dam in Folsom CA was partially constructed in the 1960s and then stopped in the 1970s by California Gov Jerry Brown Jr. The Auburn Dam is today a testament in stupidity.
Auburn Dam was a proposed concrete arch dam on the North Fork of the American River east of the town of Auburn, California in the United States, on the border of Placer and El Dorado Counties. Slated to be completed in the 1970s by the U.S. Bureau of Reclamation, it would have been the tallest concrete dam in California and one of the tallest in the United States, at a height of 680 feet (210 m) and storing 2,300,000 acre feet (2.8 km3) of water. Straddling a gorge downstream of the confluence of the North and Middle Forks of the American River and upstream of Folsom Lake, it would have regulated water flow and provided flood control in the American River basin as part of Reclamation’s immense Central Valley Project.
The dam was first proposed in the 1950s; construction work commenced in 1968, involving the diversion of the North Fork American River through a tunnel and the construction of a massive earthen cofferdam. Following a nearby earthquake and the discovery of a seismic fault that underlay the dam site, work on the project was halted for fears that the dam’s design would not allow it to survive a major quake on the same fault zone. Although the dam was redesigned and a new proposal submitted by 1980, spiraling costs and limited economic justification put an end to the project until severe flooding in 1986 briefly renewed interest in Auburn’s flood control potential. The California State Water Resources Control Board denied water rights for the dam project in 2008 due to lack of construction progress.
Although new proposals surfaced from time to time after the 1980s, the dam was never built for a number of reasons, including limited water storage capacity, geologic hazards, and potential harm to recreation and the local environment. Much of the original groundwork at the Auburn Dam site still exists, and up to 2007, the North Fork American River still flowed through the diversion tunnel that had been constructed in preparation for the dam. Reclamation and Placer County Water Agency completed a pump station project that year which blocked the tunnel, returned the river to its original channel, and diverted water through another tunnel under Auburn to meet local needs. However, some groups continue to support construction of the dam, which they state would provide important water regulation and flood protection.
Scientists warn that enormous floods may be more likely than we thought — and the Oroville Dam and others weren’t built to withstand them.
In the late 1980s, a Japanese scientist named Koji Minoura stumbled on a medieval poem that described a tsunami so large it had swept away a castle and killed a thousand people. Intrigued, Minoura and his team began looking for paleontological evidence of the tsunami beneath rice paddies, and discovered not one but three massive, earthquake-triggered waves that had wracked the Sendai coast over the past three thousand years.
In a 2001 paper, Minoura concluded that the possibility of another tsunami was significant. But Tokyo Electric Power was slow to respond to the science, leaving the Fukushima Daiichi nuclear power plant unprepared for the 15-meter wave that inundated it in 2011. The wave resulted in a $188 billion natural disaster. More than 20,000 people died.
For the past several decades, paleo-hydrologist Victor Baker of the University of Arizona has been using techniques similar to Minoura’s to study the flood history of the Colorado Plateau. Like Minoura, he’s found that floods much larger than any in recorded history are routine occurrences. And like Minoura, he feels his research is being largely ignored by agencies and public utilities with infrastructure in the path of such floods.
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The damaged Oroville Dam spillway flows out of Lake Oroville at 100,000 cubic feet per second (cfs).
Dale Kolke / California Department of Water Resources -
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Earlier this month, when a spillway at the nation’s tallest dam in Oroville, California, nearly buckled under the pressure of record rainfall, the consequences of under-estimating flood risks were brought into sharp relief. Dams aren’t built to withstand every curveball nature can throw — only the weather events that engineers deem most likely to occur within the dam’s lifespan. When many Western dams were built in the mid-20th century, the best science to determine such probabilities came from historical records and stream gauges.
But that record only stretches back to the late 1800s, a timespan Baker calls “completely inadequate.” Today, technology allows scientists to reconstruct thousands of years of natural history, giving us a much clearer picture of how often super-floods occur. “The probability of rare things is best evaluated if your record is very long,” Baker explains.
By combing the Colorado River, the Green River and others in the Southwest for sediment deposits and other flood evidence and then carbon-dating the results, Baker has concluded the short-term record severely underestimates the size and frequency of large floods. On the Upper Colorado near Moab, Utah, Baker and his team estimated the average 500-year flood at roughly 246,000 cubic feet per second, more than double the 112,000 cfs that scientists had estimated drawing on the stream gage record alone. Baker’s calculations put the 100-year flood at 171,000 cfs, also much greater than the previous estimate of 96,000 cfs. In comparison, legendary flooding in 1983 and 1984 that nearly overwhelmed Arizona’s Glen Canyon Dam, just downstream, peaked at just 125,000 cfs. (The dam has been bolstered since then, and today engineers say it can handle flows up to 220,000 cfs.)
In California, too, super-floods may be more common than previously thought. United States Geological Survey hydrologist Michael Dettinger and UC Berkeley paleoclimatologist B. Lynn Ingram have studied the paleo-flood record across a broad swath of California and discovered that such floods happen at least every 200 years, and maybe more frequently. The last one was in 1862. Thousands of people died, towns were submerged and the state’s economy was devastated, yet it was nowhere near the worst: One flood in the 1600s was at least twice as big.
In 2013, Dettinger and Ingram wrote in Scientific American that California was due for another huge water year. Their prediction has proven prescient. So much rain and snow has pounded California this winter that as of Feb. 21, half the state was under flood, rain or snow warnings. Creeks are overflowing their banks and flooding homes, and water managers were forced to spill excess water over the Oroville Dam’s emergency spillway for the first time in the dam’s 49-year history. On the night of Feb. 12, the sediment-choked water began eroding a hole in the spillway, threatening to release a wall of water. More than 180,000 residents fled to higher ground.
Luckily, emergency crews were able to patch the spillway, and people trickled back home. But Oroville isn’t alone — across the country, some 2,000 dams whose failure could cause loss of life are in need of repair, according to the Association of State Dam Safety Officials. And in many ways, Californians dodged a bullet: this winter’s precipitation was nowhere near as heavy as the storms Dettinger and Ingram have studied, and yet if Oroville’s reservoir hadn’t been depleted by years of drought, floodwaters could have easily overwhelmed the dam.
Does this mean dams like Oroville and Glen Canyon need to be fortified to withstand bigger storms? Officials from the Bureau of Reclamation are confident that Glen Canyon, at least, is equipped to handle even “extremely large hydrologic events.” And The U.S. Army Corps of Engineers is reluctant to apply paleo-hydrology research to existing infrastructure, in part because we’ve altered rivers so much that some Corps’ scientists believe ancient flood records are no longer realistic indicators of current risks.
But Baker believes it would be foolhardy to not at least create contingency plans for the possible failure of some of the West’s biggest dams. That Japanese officials were warned about Fukushima and didn’t act is “an embarrassment,” Baker adds. “We may have some similar things occurring in the United States, if we don’t seriously pay attention to this science.”
Krista Langlois is a correspondent with High Country News
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