From The High Country News (Krista Langlois):
For the past several decades, paleo-hydrologist Victor Baker of the University of Arizona [has been studying the] flood history of the Colorado Plateau. [And] he’s found that floods much larger than any in recorded history are routine occurrences [and] he feels his research is being largely ignored by agencies and public utilities with infrastructure in the path of such floods.
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 112,00 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.)
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.”