Investigators Discover Key Cause of 2017 Oroville Dam Spillways Mishap
In the heart of California, the warming climate is expected to bring about changes in atmospheric rivers, phenomena that have only been widely understood in the past 20 years. These powerful weather systems, responsible for delivering a significant portion of the state's precipitation, are projected to become more variable and play larger roles in extreme flood events.
The research into this matter was spearheaded by the "Atmospheric River Reconnaissance" (AR Recon) team, based at Scripps Oceanography, a centre for global earth science research and education at the University of California San Diego. Scripps Oceanography is also home to Birch Aquarium at Scripps, a public exploration centre that welcomes 500,000 visitors each year.
The team's analysis focused on the potential role of intense "atmospheric river" storms in the large inflow of water into the reservoir behind the Oroville Dam. In February 2017, a series of atmospheric river-driven storms led to breaches in Lake Oroville's primary concrete spillway, prompting evacuation orders for 188,000 people and causing $1 billion in damage repairs.
The incident was also analyzed by researchers from the University of California San Diego and the University of Colorado. The study, which appeared in the journal Geophysical Research Letters, suggests that without the unusual warmth that caused extreme snowmelt from the atmospheric river, the inflows to Lake Oroville would have been less, and the situation around the spillway failures may have been less critical.
University of Colorado researcher Keith Musselman stated that climate change is expected to increase the intensity of storm events in the Sierra Nevada, including extreme melt of deep mountain snowpack. During the atmospheric river, several records were set for how much snowmelt occurred, increasing water available for runoff by 37 percent over rain alone. The snowmelt in the Lake Oroville watershed reached 200-400 millimeters (8-16 inches) in some areas that additionally received up to 500 millimeters of rain (20 inches).
The costs of repairs to the spillways at Lake Oroville, completed in November 2018, totaled $1.1 billion. The study received support from NASA and the California Department of Water Resources.
The February 2017 atmospheric river sequence was not extraordinary in terms of the amount of rain and snow it delivered, but the unusually deep snow recorded in the northern Sierra Nevada mountains before the storm event was extraordinary. The large influx of water overwhelmed soil storage and resulted in tremendous runoff, straining the capacity of California's second-largest reservoir.
Co-author of the study, Keith Musselman, highlighted the potential threats to public safety and infrastructure associated with a warmer and more variable climate. The study underscores the importance of understanding and preparing for the impacts of climate change on extreme weather events in California.
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