Two powerful storm systems known as atmospheric rivers are heading for northern California and Oregon, a region in the midst of an historic drought.
While the storms will bring much-needed water to a parched region and should significantly lower the wildfire risk, they also bring dangerous new flood and mudslide risks, particularly in areas recovering from wildfires.
Wildfires strip away vegetation and leave the soil less able to absorb water. A downpour on these vulnerable landscapes can quickly erode the ground as fast-moving water carries debris and mud with it.
I study cascading hazards like this, in which consecutive events lead to human disasters. Studies show climate change is raising the risk of multiple compound disasters, and it’s clear that communities and government agencies aren’t prepared.
Big-time precipitation is expected for the #WestCoast over the next 10 days. An atmospheric river will dump inches of rain and feet of snow across #California, #Oregon, and #Washington as numerous storm systems roll in off the Pacific…#CAwx #ORwx #WAwx pic.twitter.com/3jetdX9yRH
— Christopher Nunley, Ph.D. (@chrisnunley) October 18, 2021
When storms hit burn scars
California has experienced this kind of cascading disaster before.
In early 2017, following years of drought, the region had a wet winter that fueled dense growth of vegetation and shrubs. An unusually warm and dry spring and summer followed, and it dried out the vegetation, turning it into fuel ready to burn. That fall, extreme Santa Ana and Diablo winds—known for their sustained low humidity—created the perfect conditions for wildfires.
The Thomas Fire began near Santa Barbara in December 2017 and burned over 280,000 acres. The following January, extreme rainfall hit the region, including the burn scar left by the fire, and caused the deadliest mudslide-debris flow event in California’s history. More than 400 homes were destroyed in about two hours, and 23 people died.
These kinds of cascading events aren’t unique to California. Australia’s Millennium Drought (1997–2009) also ended with devastating floods that inundated urban areas and breached levees. A study linked some of the levee and dike failures to earlier drought conditions, such as cracks forming because of exposure to heat and dryness.
Individually, they might not have been disasters
When multiple hazards, such as droughts, heat waves, wildfires and extreme rainfall, interact, human disasters often result.