Heightened potential for post-fire flash flooding and debris flows

Public officials have put thousands of Sonoma County homeowners on notice that October firestorms have left them at risk of powerful rain-related hazards such as flash floods and mud flows - threats heightened by a scarred landscape expected to absorb less rainfall than usual for the next few years.

Notifications mailed late last month and nearly three dozen road signs around the fire zones are intended to alert both residents and those passing through that even short bursts of heavy rain could cause sudden flooding or swollen rivers of soil, rock and other materials to materialize into mud or debris flows.

About 4,000 homes in the county are considered at moderate-to-high risk for such hazards, though post-fire mapping is still being refined to reflect the full range of areas vulnerable to potential threats, according to Barry Dugan, the program specialist with the Sonoma County Water Agency coordinating public outreach efforts on post-fire hazards.

The largest and most notable trouble spot is an area of the massive 56,556-acre Nuns fire in and around Hood Mountain Regional Park and Sugarloaf Ridge State Park, where flames charred steep hillsides above Pythian, Pierson and Adobe Canyon roads.

Heavy rainfall forecast for Monday afternoon and night was enough to put emergency personnel on the lookout for signs of trouble.

“We've got all hands on deck,” Shirlee Zane, chairwoman of the Sonoma County Board of Supervisors, said Monday. “I think we'll get through this storm OK, but people need to be prepared.”

Moisture-heavy storms known as atmospheric rivers are a greater concern than Monday and Tuesday's rain, because they produce denser precipitation, Dugan and others said.

A succession of atmospheric rivers, such as those that occurred in Sonoma County last winter, can easily cause flooding even absent post-fire landscape changes.

Wildfires substantially raise the risk of rain-triggered trouble by burning upper layers of soil so it repels water, allowing runoff to accumulate quickly and leaving the ground more vulnerable to erosion.

And fires denude hillsides of plant life that ordinarily would soak up rainfall and help stabilize the ground.

Experts say a half-inch of rain per hour over a fire-scarred area could be enough to prompt flash-flooding. Even a 15-minute downpour at a rate of about an inch an hour could lead to disaster, said Jason Kean, a research hydrologist with the U.S. Geological Survey, adding, and “that's a garden-variety storm.”

Runoff that speeds downslope also can pick up large loads of sediment, boulders, trees and other debris that “pack a bigger punch” than mere mud.

It happens so fast those in problem-prone areas need to be alert ready to get out of harm's way ahead of time, Kean said.

The Federal Emergency Management Agency recommends flood insurance for those at risk of post-fire flash floods.

“The tone of the messages that we're trying to convey,” Kean said, “is if you live along a creek or downstream of a creek that is prone to flooding any time, that there is a much greater risk of that creek flooding after the fire. If we were to ask people to remember one thing, it's just be aware of your surroundings if you live near the burn area or downstream of it.”

Multiple agencies have been working to assess post-fire vulnerability since devastating wildfires spread around the region beginning the night of Oct. 8, burning 137 square miles in Sonoma County.

The county also is working on an early-warning flood forecast and response system.

This new network, comprising 22 rain gauges and 11 stream gauges, is expected to be installed by the end of January within the Tubbs and Nuns fire areas, Dugan said.

Once completed, emergency officials and the National Weather Service can have real-time warnings of suddenly rising stream flows and rainfall and alert the public.

In general, Cal Fire experts believe the risk of landslide is lower than might be expected since there were large areas where the October fires burned at a high speed, doing less damage to the ground surface than if they'd moved more slowly, according to Lynne Tolmachoff, the agency's chief of public education.

But the National Weather Service suggests anyone who can look uphill and see a burned-out area is at risk.

That's the case for many residents of Adobe Canyon Road in Kenwood, most of whom have alarming tales of escaping the Nuns fire through long tunnels of flames close to Highway 12.

Miriam Casey gestured to the hill behind her home and said, “We've been wondering what does that mean for us.”

A neighbor, Gail Mazzucchelli, hasn't unpacked legal documents and family photos she boxed when evacuating three months ago.

She wants to be prepared in case the rain causes problems on the steep slope above her home, where multiple straw erosion-control wattles have been laid.

Maps assembled by the U.S. Geological Survey and, more recently, by the joint Sonoma County/Santa Rosa Recovery Operations Center show threat levels of low, moderate and high. Locally, staffers are still refining maps to reflect how large an area could be subject to flash floods or debris flows, given that those could originate inside the burn zone and spread downstream or downslope to areas outside the perimeter, Dugan said.

The relatively few areas considered at high risk are associated primarily with steep slopes and severely burned areas in and around Hood Mountain and Sugarloaf Ridge State Park, on the north side of Sonoma Valley. Much larger swaths in the fire perimeters are labeled moderate risk.

Scores of “watch streams” considered at heightened risk of flooding have been mapped, as well.

Kean, who is based in Colorado, said he observed water-repellent soils at Hood Mountain park during a recent visit to the Nuns fire zone and also saw signs of landslide left behind by record rainfall last winter.

“So we know that that area is susceptible to movement, and fire is only going to increase the likelihood of movement, meaning that it's going to take less rain to cause a flood than it would before the fire,” Kean said.

The county's December notifications, the first of at least two planned mailings, were sent to about 3,100 property owners, accounting for some who own multiple parcels, so renters would not have received them directly, Dugan said.

The notices advise residents to consult the post-fire hazard map at sonomacountyrecovers.org/rain-ready/, use erosion-control measures on their property and clear debris from nearby streams, storm drains and drainages.

It also urges residents sign up for emergency warnings through SoCoAlert.com, set their cellphones to accept alerts from the National Weather Service and create a household emergency evacuation plan.

“A critical part of our fire recovery efforts is continuing to protect the lives and property of those in the areas where the fires burned,” Zane said. “We're asking people to remain alert, be prepared and protect themselves from the increased risks from flooding after the fires.”

You can reach Staff Writer Mary Callahan at 707-521-5249. or mary.callahan@pressdemocrat.com. on Twitter @MaryCallahanB.