Fires increase risk of landslides, flooding
Landowners encouraged to mitigate effects, have emergency plans in place
Even before last month’s devastating fires, John MacLeod had already made plans to repair a heavily eroded drainage culvert in his family’s vineyard on Lawndale Road. But after fingers of the Nuns fire traveled across the property, burning MacLeod’s home, a few rows of grapevines, and leaving the once-grassy hillside charred black, MacLeod knew he had to act before winter storms caused any more damage.
Normally, runoff caused by rainfall is absorbed by vegetation and soil, but after a wildfire, damaged vegetation and soil may be unable to absorb water, creating flash flood conditions. Flood risk may remain until vegetation returns – up to five years after a wildfire.
At the MacLeod’s, the work includes spreading native seed, rice straw, and erosion control blankets to prevent erosion and promote re-vegetation. The work is being done in partnership with Sonoma Resource Conservation District (RCD) and the Conservation Corps North Bay (CCNB).
“We were able to move quickly on this property because we already had grant funds slated to be spent there, and were able to work with the funder (State Coastal Conservancy) to quickly redirect the funds toward emergency erosion control work,” wrote Sonoma RCD Executive Director Valerie Minton in an email. “In addition to the erosion control benefits, this project is also an opportunity to provide training to CCNB crews that may be used in the coming weeks to implement similar erosion control projects throughout the impacted areas.”
While the MacLeod’s erosion control project is relatively small, their vineyard has a prime view of Hood Mountain, which was hit hard by the fires and recently identified as a high-risk area for landslides and flooding in the event of heavy winter rain.
“The biggest risk is prolonged heavy rainfall where the soil saturates and mobilizes downhill. Those risks are in areas severely burned and with a steep slope,” said Cordel Stillman, director of programs for Sonoma Clean Power. Stillman was formerly the deputy chief engineer at the Sonoma County Water Agency and has been “borrowed” by the county to head up watershed hazard mitigation efforts.
Maps released by the county earlier this month highlight the Hood Mountain area and parts of Adobe Canyon as two of the fire-affected areas most likely to experience landslides in the event of heavy rain. Due to the confluence of topography, elevation slope, past debris flow patterns, rainfall data, and soil burn severity, USGS empirical data suggests that the area behind Graywood Ranch has an 80-100 percent likelihood of landslides during an episode 6mm of rain in 15 minutes (defined as “heavy rain” by the USGS). A larger portion of the Hood Mountain area (in both Sonoma and Napa counties) has a 60-80 percent chance, as does upper Adobe Canyon on the west side. The map is available at sonomavegmap.org/blog/2017/10/17/fires.
Recent rainstorms have brought nowhere near that level of rainfall intensity. Nonetheless, winter rains can carry ash, sediment and debris down into creeks and culverts, increasing the chances of flooding. “We have 8-10 Cal Fire crews out cutting debris and clearing drainage and culverts,” said Stillman in an interview last week.
Stillman said the county is actively working with Cal Fire, Office of Emergency Services, and FEMA to the identify areas of greatest risks, mitigate what they can, and then develop warning systems to better alert residents in case of emergency.
Additionally, the county hopes to install rainfall and stream gages from the USGS in creeks in the burned areas. This will provide important information about what is happening in the watershed, and whether there is risk of flooding.
The Sonoma County Water Agency is working with NOAA and Colorado State University on the loan of an X-Band Radar unit that will be installed on Sonoma Mountain. “This will give us much better information on incoming storms. This is important because where rainfall occurs can make a big difference in local flooding and debris flows,” said Ann DuBay of the Sonoma County Water Agency. The radar is expected to be in place by early January.
The county will be mailing a flier to affected homeowners in the area, highlighting “Preparedness for Rain After Wildfires.” Those should be showing up in mailboxes this week. It is also available on the Sonoma County Recovers website at www.sonomacountyrecovers.org/rain-ready/.
In addition to debris, toxic ash can wash into waterways during rainstorms. Although burned vegetation is non-toxic, burnt man-made structures leave behind toxins including sulfates, nitrates, asbestos, and heavy metals. In Kenwood, 151 parcels have been reported burned, with 100 homes destroyed.
Landowners who’ve lost a home or structure located near a water source are urged to contact the Sonoma Ecology Center (SEC). With landowner consent, volunteers and staff are hoping to install materials to help capture potentially toxic runoff into creeks and streams. Green hydromulch could be sprayed to contain the ash and prevent surface soil erosion. Sandbags and “wattles” might be used around burned structures and vehicles to contain toxics. Wattles are fiber tubes made of weed-free straw or coconut fiber. Wattles are staked to the ground, where they filter sediment and debris while letting water seep through. SEC is experimenting with making wattles out of biochar to increase the uptake of toxics. Those who feel their property poses a danger to the watershed are encouraged to contact SEC at email@example.com. Staff are trying to prioritize as best they can before any heavy rains come.
Landowners can also install their own wattles or sand bags, purchased from places like Freidman’s Home Improvement or The Wattle Guys (in Santa Rosa).
Sonoma RCD has grant funding to assist vineyard properties in Sonoma Valley with emergency high priority erosion control projects and is working to bring in additional funding for other land uses. The USDA Natural Resources Conservation Service has funding to help with certain post-fire work on ag lands.
Here are some other things property owners in burned areas, or downstream of burned areas, can do to prevent flooding or debris issues:
• Clear storm drains and culverts of debris to prevent flooding, redirecting sheet water into storm drains (and away from waterways).
• Clear out gutters and ditches.
• Repair fire breaks so they do not channel runoff, but blend evenly with the slope of the land.
• Minimize foot and equipment traffic as much as possible.
• If you see something (flooding, mud and debris flows), call 911.
• Monitor your surroundings, and have an emergency plan in place.
• Stay informed: Listen to local radio stations, and sign up for Nixle and SoCo Alerts. Make sure that the emergency alerts on your cell phone are activated (on smart phones, go to “Notifications” and make sure the Emergency Alerts notification is turned on).
ResourcesSonoma County Recovers
Sonoma Ecology Center
Sonoma Resource Conservation District
Sarah Phelps is an editor and reporter. She was raised in Kenwood and has a BA from Loyola Marymount University.