Have we underestimated the West’s super-floods?

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.

  • 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
  • Rock, aggregate, and cement slurry continue are be placed in areas affected by erosion from the Oroville Dam emergency spillway.

    Florence Low / California Department of Water Resources
  • A helicopter prepares to deliver another bag of rock and material to the base of the Oroville Dam emergency spillway to help repair the erosion below the Oroville Dam Emergency Spillway.

    Brian Baer / California Department of Water Resources
  • The California Department of Water Resources suspended flows from the Oroville Dam spillway after a concrete section eroded on the spillway.

    Kelly M. Grow / California Department of Water Resources

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.

A member of Cal Fire, right, talks to workers on the Oroville Dam project in front of the main spillway in Oroville, California, on Feb. 20, as forecasters issue flash flood warnings for the San Francisco Bay Area.
Hector Amezcua/The Sacramento Bee via AP

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

High Country News Classifieds
  • OWN A THRIVING MOUNTAIN GUIDE SERVICE.
    Eastern Sierra guide service for sale to person with vision & expertise to take it onwards. Since 1995 with USFS & NPS permits. Ideal for...
  • MEMBERSHIP DIRECTOR
    Job Announcement: Membership Director Job Title: Membership Director Supervisor: Executive Director Classification: Full-time exempt Location: Boise, ID Job Overview Winter Wildlands Alliance is seeking a...
  • IMPROVED LOT
    Private road, hillside, views. Well, pad, septic, 99 sq.ft. hut. Dryland permaculture orchard. Wildlife. San Diego--long growing season
  • STEWARDSHIP SPECIALIST
    San Isabel Land Protection Trust seeks experienced person to manage its 133 conservation easements in south-central Colorado.
  • EXECUTIVE DIRECTOR
    Executive Director Position Announcement POSITION TITLE: Executive Director ORGANIZATION: Friends of Cascade-Siskiyou National Monument REPORTING TO: Board of Directors LOCATION: Ashland, OR POSTING CLOSES: March...
  • ARIZONA CONSERVATION CORPS PROGRAM DIRECTOR
    Arizona Conservation Corps is seeking a Program Director in Flagstaff or Tucson
  • UNIQUE INVESTMENT OPPORTUNITY
    Profitable off-the-grid business located 2 miles from Glacier National Park. Owner has 6 years operating experience. Seeking investor or partner for business expansion and enhancement....
  • NEW MEXICO PROPERTY - SILVER CITY
    20 acres, $80,000. Owner financing, well, driveway, fencing possible, very private, sensible covenants, broker owned. Contact - 575-534-7955 or [email protected]
  • GRANT WRITER
    "We all love this place we call Montana. We believe that land and water and air are not ours to despoil, but ours to steward...
  • COMMUNICATIONS COORDINATOR
    ABOUT US: "This thriving citizens organization exemplifies the ideal of public involvement in public processes." - Billings Gazette At Northern Plains, we believe that true...
  • ST. LAWRENCE UNIVERSITY - ASSISTANT DIRECTOR IN THE OUTDOOR PROGRAM
    To view the complete position description please visit: http://employment.stlawu.edu. St. Lawrence University is an Equal Opportunity Employer.
  • SEASONAL SAN JUAN RANGERS
    Seeking experienced crew members to patrol Colorado's most iconic mountain wilderness.
  • REMOTE SITKA ALASKA FLOAT HOUSE VACATION RENTAL
    Vacation rental located in calm protected waters 8 miles from Sitka, AK via boat with opportunities to fish and view wildlife. Skiff rental also available.
  • DEVELOPMENT AND ADVOCACY DIRECTOR
    Provide stewardship and protection for the Great Burn wildlands along the Montana-Idaho stateline. This position is based in Missoula, MT, where a river runs through...
  • LISA MACKEY PHOTOGRAPHY
    Fine Art Gicle Printing. Photo papers, fine art papers, canvas. Widths up to 44". Art printing by an artist.
  • WILDERNESS CONSERVATION CORPS - OREGON
    The Siskiyou Mountain Club is hiring interns for the 2020 Field Season. Interns utilize non-mechanized tools to complete trail restoration and maintenance while gaining job...
  • EXPERT LAND STEWART
    Available for site conservator, property manager. View resume at http://skills.ojadigital.net.
  • CONSERVATIONIST? IRRIGABLE LAND?
    Stellar seed-saving NGO is available to serious partner. Package must include financial support. Details: http://seeds.ojaidigital.net.
  • CANYONLANDS FIELD INSTITUTE
    Colorado Plateau Natural & Human History Field Seminars. Lodge, river, hiking options. Small groups, guest experts.
  • WESTERN NATIVE SEED
    Specializing in native seeds and seed mixes for western states.