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Sonoma County braces for rain after wildfires

By: Katherine Minkiewicz
November 24, 2017

After the North Bay saw its first string of storms last week and with more rain expected, the burned areas of Sonoma County and a slew of local, state and federal agencies are preparing for potential flash flooding and larger than normal mud and debris flow — one of the many consequences following the deadly October fires.

In a statement posted in early November on the website, the City of Santa Rosa, Sonoma County, Cal Fire and the Sonoma County Water Agency, announced that they would be taking steps to prepare and warn property owners in the burned out areas of an increased risk for flooding

According to the statement, in urban areas near Santa Rosa the city will periodically check and clean storm drains and install straw wattles — an erosion and flood control device and sandbags to prevent flooding and excess debris from clogging drains. Near rural roadways throughout the area, the county will be installing “debris capturing devices in culverts and ditches.” The Cal Fire Incident Management team will be working to clear debris to reduce the risk of flooding along water channels and streams.

Susi Brady, a press information officer for Cal Fire, said there will be 16 workers from the Cal Fire incident management team working with the city and county to help clear any debris caused by the rain. Brady said the crews will mainly be working in, “Mark West Springs, Fountaingrove and Glen Ellen.”

In addition to Cal Fire and the county working together to prepare for the upcoming rainy season, typically December through February, the Sonoma County Water Agency will also be “installing rainfall and stream gauges in watersheds in burned areas and working to install radar equipment to improve early warning forecasts for residents in high-risk areas.”

Mike Thompson, interim general manager of the SCWA said the agency is working to “obtain and install” X Band radar on Sonoma Mountain, a common radar system that is frequently used by air traffic control and police agencies for vehicle speed detection and can also help track weather. With the help of the radar, SCWA will be able to track precipitation totals in real time, which may be helpful helping in acting as an early warning system for flooding.

“In Sonoma County this will be critical,” Thompson said of the ability to forecast and have a warning mechanism. “The majority of the burn areas are in what we call a radar shadow of the existing radars we have in the Bay Area. They just don’t pick up all the rainfall in that area so we are working to get this X Band radar installed… This will show the intensity of the rainfall through the burn areas and that data would then be provided to the National Weather Service, which would then be able to issue flood or debris flow warnings.”

The agency is also working to obtain stream gages in places such as Adobe Creek and Nathanson Creek, which would also help predict flooding.

Thompson said of the overarching goal of the radar and stream gauge system, “The idea of this whole system is to be able to get people living in that area advanced warning and evacuation warnings if they believe there is going to be a high chance of flooding. What we understand, the first couple months after a fire, runoff from the burn areas is significantly higher than normal, so there are significant concerns about flooding in these particular areas.”

But why is flash flooding such a high risk particularly after wildfires? Owen Anfinson, an assistant professor at Sonoma State University who studies geosciences and teaches sedimentary geology, says depending on the severity of the fires, the loss of the vegetation canopy and trees can help induce the risk of flooding.

“There are a few factors that come into play. Without a canopy you essentially have more raindrops hitting the ground, so without tree cover you have more raindrops hitting the surface and with the lack of vegetation you have a decrease in slope stability and you also have greater surface runoff. Some of the really fine ash can fill up the pores in the soil so we can have an increased amount of runoff for that,” Anfinson explained. “But the biggest thing is the plants that are dead are not using up that water so that water either infiltrates into the ground or becomes runoff. So it’s many factors and tends to add up to higher erosion rates the first year after that fire.”

Even though Cal Fire crews and local agencies are already working to prepare high risk areas for rain, the county is also urging property owners of burned out areas to prepare and stay informed regarding flood warnings.

Carly Cabrera, media contact for SCWA and for information on rain preparedness, stated that straw wattle installation is recommended to help prevent erosion and flooding and residents should also stay tuned to Nixle to get weather updates. She said rain will be the biggest concern for property owners in burned out areas this winter.

“People can buy them at local hardware stores,” Cabrera said on where wattles can be purchased. “We’re also currently working on more messaging and alert systems, but Nixle is so far the best way to keep updated on flooding.”

Last years’ rainy season brought in records amount of rainfall — a whopping 57.8 inches for Santa Rosa according to National Weather Service data, however, Anfinson said this year is expected to not be as wet, a plus for the burned out areas of drainage basins in Sonoma County that won’t be able to absorb as much water as usual.

“I would expect it to not be as wet, however, that is a large assumption… and it is difficult to predict that we would have another large rain event, but I doubt it would be quite as large as last year,” Anfinson said.

And while flooding is a concern, both Anfinson and Thompson said rain runoff carrying toxins from the fires is a larger concern since it can negatively affect the watershed and the salmon who use Mark West Creek as a spawning ground.

“A lot of houses that burned may have toxins or chemicals; paints, batteries and all that kind of stuff that burns up in a house and when the rains come and hit that, it’s going to wash that stuff into the creek at the same time of year that the salmon are coming up to spawn from the Russian River. The toxins can get into the water supply and I think that is a big concern,” Anfinson said.

Currently the SCWA is working with the San Francisco Regional Water Quality Control Board and the North Coast Regional Water Quality Control Board to continuously monitor the quality of the water.

“But the main concerns with regard to the contaminants in the runoff are in the urban areas and the heavily urbanized areas. So the City of Santa Rosa and the County of Sonoma put in erosion control measures to try to prevent that contaminated ash getting into the waterways,” Thompson explained. “But from a basic flood control incentive with regard to debris and sediment, we think we are in pretty good shape.”

As a precaution FEMA and Cal OES are urging those in areas affected by the fires to consider getting flood insurance. In a news desk statement they wrote, “What starts as normal rainfall can turn quickly into a costly and potentially deadly flood. Residents need to protect their assets with flood insurance… before a weather event occurs.”

For more information on flood insurance, visit: To access the SCWA flood forecast hotline, call: (707)526-4768.