This winter’s first major rainstorm is expected to bring 1 1/2 to 4 inches of necessary rain to Sonoma County, along with gusting winds. Local rivers are predicted to remain below critical stages, but areas affected by October’s wildfires are at high risk for flash flooding, mudslides and debris flow.
The National Weather Service’s general rule of thumb is that a half an inch of rainfall in an hour can cause flash flooding in areas affected by fires. Impacted areas include locations within and downstream of the Atlas, Tubbs, Nuns, and Pocket burn scars, including the Coffey Park and Fountaingrove neighborhoods.
Not only do these areas lack vegetation to slow runoff and help rainwater penetrate the earth, but wildfires have altered the soil, making it repel water. Water accumulation is unpredictable, moves rapidly and can carry boulders, trees, mud and other rubble. Unfortunately, flooding is common after fires. To prepare, the City of Santa Rosa and the County of Sonoma have cleaned and checked storm drains, and installed straw wattles, sandbags and gravel bags to prevent debris from entering storm water channels. Debris catching devices have also been added to culverts and ditches along rural roadways.
Sonoma County Supervisor Shirlee Zane says that the county sent out more than 4,000 cards to residents of high-risk areas advising people on how to prepare. Residents in those areas were also counseled to receive emergency warnings from the National Weather Service (NWS) and SoCo Alerts on their cell phones, and to have an emergency evacuation plan.
The Sonoma County Water Agency expects to install radar equipment to improve early warning forecasts for residents in high-risk areas in the next month or so. “We’ve got a site,” says Zane. “It’s just getting the final permitting.” The agency has been working in conjunction with NOAA and the University of Colorado, Fort Collins. The radar will be installed along the coast in cooperation with Santa Clara, Contra Costa, Marin and Sonoma counties.
Monday’s storm is predicted to bring 2-4 inches of rain to Sonoma County’s mountains and higher elevations, with 1 1/2 to 2 1/2 inches for most populated and valley locations.
Ash from the fire burn areas entering the watershed is also a concern. The EPA is close to finishing the cleanup of toxic remains from the fires, but not completely done. “The EPA is still here; they’re still working,” says Zane. “They’re monitoring the situation closely.”
Increased street sweeping and hydroseeding are additional ways the City of Santa Rosa and Sonoma County are working to prevent fire-related debris, pollutants and sediment from spreading into creeks and rivers. Hydroseeding is a planting process in which a mixture of seed, mulch and other ingredients are sprayed to promote plant growth and reduce erosion.
Hopefully these measures will be sufficient to prevent or mitigate any rainstorm-related disasters.
Meanwhile, the threat of mudslides and floods have led to evacuations in Southern California.