Residents near the King and Rim fire burn areas are bracing for what could be a long winter waiting to see if land burned by the recent wildfires will hold during the rainy season.
Intense rain from this first storm of December has elevated concerns for debris flows in areas where tree roots have been destroyed by fire. These tree roots are what hold the soil in place.
During intense rain, the soil has a difficult time moving water through and can become loose. Rock, water and soil can then slide down the slope. Dr. Jonathan Stock of the United States Geological Survey (USGS) said in these areas preexisting soil moisture doesn't matter as much as the intensity of the rain that falls. This means each weather system must be analyzed for the amount of rain potential it can deliver.
Outside of burn country, Stock said accumulating rain can have a bigger impact on landslide risk in steep terrain. He said it typically takes about 10 to 15 inches of rain before landslides occur due to rain. This is why January and February historically have the highest incidence of mudslides in California. Stock said two-thirds of California landslides happen during the first two months of the year.
Currently, the Bay Area has seasonal rain totals below 10 inches, but this is just the beginning of rain season. The USGS will continue to monitor rainfall accumulation at historically high risk areas to asses landslide concerns.
Stock said any area burned by wildfire will continue to be at risk through the rainy season with any excessive rain that falls. He said it typically takes three to five years for sufficient vegetation recovery to reduce the risk of debris flows.
Research is still being done on debris flow development, Stock said. Right now, there are still questions on where the debris flow will bulk up and if it will be large or small.
In the meantime, Stock said anyone downslope from burn areas need to be prepared to move to shelter quickly during intense rain and times of high risk.
