On Tuesday, 21,000 people living near hillsides in Southern California had to be evacuated due to severe mudslides and flooding. These events were caused by torrential rain, with the Santa Barbara region experiencing rainfall in many areas at a rate of over an inch per hour. The most heavily impacted areas saw close to 1.5 inches per hour. This rainfall follows a record breaking fire season that left nearly two million acres of land burnt and barren, eliminating many trees and other vegetation that would usually absorb and slow the path of water. According to Robbie Monroe of the National Weather Service in Oxnard, half an inch of rain per hour is enough to trigger mudslides. With the rate of precipitation far exceeding that threshold and the increased vulnerability from the wildfires, the results have been devastating.
By January 11, 100 houses had been destroyed because of mudslides in the coastal-community of Montecito, causing 17 deaths. Many of the mudslides occurred in areas without evacuation orders or only with voluntary orders, increasing the impact to human life. Even in areas with mandatory evacuation orders, many residents reportedly refused to leave. Gas and power was shut off across the region, and Freeway 101, a major connector, was closed. Initial estimates indicated it might take as much as 48 hours to reopen.
Despite Santa Barbara’s Public Works Department’s floodplain management program which includes protocols for monitoring streams, clearing debris, and repairing flood damaged areas, responders were challenged by the speed at which the rain accumulated on the fire-scorched land and inundation of rescue calls. Dispatchers handled over 600 emergency calls regarding the mudslides. Thanks to the 500 responders, including air rescue teams from the U.S. Coast Guard and specialized search dogs, conducting operations to recover missing persons, dozens of residents were found and evacuated. However, as of Thursday afternoon, 17 people were still missing.
Southern California is not the only area in the Southwest that has experienced mudslides and flooding caused by torrential rain following intense fire seasons. In September of 2013, a 4,500 square-mile area in Colorado’s Front Range experienced up to 10 inches of accumulated rainfall overnight, which, coincidentally, followed its worst fire season to date at the time. Over the course of four days, Boulder County, the hardest hit in the state, experienced 17 inches of rain, which is near to Colorado’s average annual precipitation of 20.5 inches. The resulting flood forced the evacuation of over 12,118 people, destroyed 1,502 houses and damaged an additional 17,494, and caused six deaths.
Lessons on how communities can adapt to the threat of mudslides have been learned, though. In the 1980’s Gilbert White, the “father of floodplain management,” advised Boulder to resize bridges to allow for more water flow, and to remove structures along the creek. These revisions included incorporating multi-use underpasses to allow for excess water flow during a flood. In the 2013 floods, White’s recommendations were credited with mitigating much of the damages, as Boulder did not experience any bridge failures in 2013.
Sources and further reading:
Colorado Flood Facts and Figures – Floodlist
Big Thompson flood still Colorado’s worst – Denver Post
Officials: Boulder flooding could have been much worse – Daily Camera
Flood Control – The Santa Barbara County Flood Control District