Last week, back-to-back potent winter storms hit Northern California. These storms were an impressive kind of precipitation event called an atmospheric river or “Pineapple Express.” Such storms are characterized by a narrow band of moisture extending from the tropical Pacific Ocean to the coast of California. As the moisture collides with California’s Sierra Nevada Mountain Range, heavy precipitation is produced.
The storms brought widespread rainfall to California, with Northern California experiencing extensive flooding, saturated soils and mudslides. As a flood control measure, California state officials opened the Sacramento Weir. Last opened in December 2005, the 100-year-old weir relieves water pressure along levees in the Sacramento and American Rivers by diverting water into the Yolo Bypass, a floodplain of farmland and wildlife habitats.
Opening the 1,920 foot-long Sacramento Weir is a thrilling show for weather and water nerds, as the 48 floodgates are operated manually. All other weirs in California pass floodwaters on their own once the river level reaches the weir's overflow depth. The decision to open the Sacramento Weir is both thorny and complicated, based on river measurements/forecasts and outdated Army Corps of Engineers criteria. These protocols, along with much of California’s water resources infrastructure, were based on the assumption that winter precipitation in the Sierra Nevada falls mostly as snow and melts gradually throughout the spring/summer, serving as a key water resource for the state. Observations have already revealed warming-driven changes to California's snowpack and storms.
Research and fundamental physics suggest continued future warming will intensify and moisten atmospheric river events, and precipitation will shift more toward rainfall than snowfall. This is likely to bring about larger floods in flood-prone areas like Sacramento. Legendary Western US inundations like the Great Flood of 1862 can occur again, and may even be more intense. Stronger and contemporary flood control measures will certainly be necessary to prevent disastrous floods in this region.
The open Sacramento Weir is currently saving California's capital from major flooding, but perhaps this barrier isn't a long-term solution to California flood risk. Will the timeworn weir be able to accommodate stronger and more intense floods in the upcoming century? What are the long-term effects of changing flood characteristics on the state's intricate system of reservoirs, aqueducts and dams? How will human interaction with flood hazard in this region impact other hazards, including drought and wildfire risk?
The answers to these questions aren't clear, and highlight the complex and multifaceted nature of flood risk/insurance. This is especially true in regions like California, where aging water management systems are coupled with projected changes in storm/flood characteristics. More precipitation is forecast for the already-wet Western US later this week, prolonging California's risk of significant flooding and emphasizing the gravity of California's flood issues.
Category: Climate/natural disasters: Climate change, Floods/storms