Risk of debris flows increases after fires
Now that the Cedar and Cub creek fires are out, residents in some areas within the burn scars may have more to worry about from flooding, state agencies are saying.
Representatives from the U.S. Forest Service, state Department of Natural Resources, Okanogan County Conservation District, National Weather Service and Okanogan County Department of Emergency Management held a virtual meeting Wednesday evening (Sept. 22), to discuss the results of the Forest Service’s Burned Area Emergency Response (BAER) report for the two fires, as well as reviews from the U.S. Geological Survey.
The BAER report found the Cedar and Cub Creek fires burned at a high intensity in many areas, causing high or moderate damage to soil in 48% and 41% of the fires’ footprints, respectively.
In those areas, much of the soil is now hydrophobic, meaning it tends to repel rather than absorb water. That is expected to lead to more runoff, and in areas with existing streams, could lead to debris flows after heavy rain.
The BAER report also includes information about damage to plant species, threats to recreation and threats to safety from dead, burned trees, but Wednesday’s meeting focused largely on threats to human safety from debris flows and flash flooding.
The Forest Service plans to close some roads, trails or campgrounds at serious risk, and post warning signs at others to protect visitors and Forest Service land and property. Areas listed as most at risk in the Cub Creek burn area include Falls Creek Road and surrounding areas, and at Cedar Creek include Wolf Creek, Gobbler’s Knob and Thompson Ridge.
However, agencies that work with private landowners, including the DNR, warned that those landowners will need to work to protect their own property.
“Don’t wait until it starts raining” said Joe Lange, of the state Natural Resources Conservation Service (NRCS). “Don’t think that it can’t happen just because you’ve never seen flooding at your house before.”
Residents of areas where recent fires have created a debris-flow or flooding risk are eligible for the Federal Emergency Management Agency’s (FEMA) post-fire flood insurance. Properties do not need to be in a floodplain to qualify.
The Okanogan County Conservation District plans to reach out to eligible landowners directly about the NRCS’s Emergency Watershed Protection Program. District manager Craig Nelson said the federal program will pay 75% and the state pays the remaining 25% for work on a person’s property to protect it from this kind of flooding. The program is voluntary, he said.
The BAER report for Cedar Creek found that a storm event of 32 millimeters per hour, or about 0.3 inches in 15 minutes — a fairly rare occurrence — has a higher than 80% chance of creating debris flows in many of the tributaries of Wolf Creek and tributaries to the Methow River, Early Winters Creek, Little Bridge Creek and the Twisp River Drainage.
However, a smaller rainfall in August showed the Forest Service that even a more common event could cause damage to watersheds.
“Over the fire area we had multiple rain gauges,” said Forest Service hydrologist Molly Hanson. “We got about 2/10 of an inch of rain in about an hour. That was a storm that we could go out as a BAER team and look for evidence of how did this landscape behave.”
Hanson said the rainfall created some localized erosion and runoff in badly burned areas, including specifically the Falls Creek area in the Cub Creek fire’s path.
The BAER report used the baseline 32mm/hr storm as a model to predict future runoff potential.
“We’re not seeing any small increases in flow. They’re all pretty big based on our model storm,” Hanson said.
Landslide hazards geologist Trevor Contreras with the DNR went into more detail about threats to privately owned populated areas versus recreational and wildlife areas covered by the Forest Service. Contreras also showed an interactive hazard assessment map overlaid with LIDAR — a kind of laser imaging that showed topography and previous stream paths — and the present location of homes in areas at risk of debris flows.
Some of the homes at most risk are those built on alluvial fans — geological debris fields along the path of a stream or other water source coming down from a mountain or hillside, which Contreras said can be particularly hazardous places after a wildfire upstream.
Alluvial fans are previous locations of debris flows and where a stream “hits the valley floor and deposits all of its sediment,” Contreras said.
“We look at the long-term effects and the evolution of these alluvial fans — we know they are very active places,” he said. “You might have a creek that hasn’t flowed in the time you’ve been there, but it might flow more vigorously and more often than before it burned.”
Alluvial fans are visible and distinctive — especially at a distance. Another way to tell you’re on the site of a past debris flow is to look around for large boulders, Contreras said.
“The indicators that 100 years ago there was a debris flow is these boulders everywhere,” he said.
The DNR’s map and Contreras point out areas of Wolf Creek, Mazama on the west side of Highway 20 near the fire station, areas around Lucky Jim Bluff and Thompson Ridge could be particularly vulnerable.
“I hope the area recovers just fine and you don’t get any of these events, but we wanted you to know it’s possible,” he said.
Okanogan County Emergency Management Director Maurice Goodall closed the meeting by urging people to keep an eye on National Weather Service alerts and make a plan in case of a debris flow.
“We need to be aware of our surroundings at all times. You’ve heard me say that a lot of times about fire and everything else … by the time you get one of our warnings, the flood might already be there,” he said. “You’ve got to have that plan, you’ve got to practice that plan, you’ve got to check on your neighbor. When in doubt, get out of there.”
For more information, go to http://www.centralwashingtonfirerecovery.info/
For more information on flood insurance, go to https://www.floodsmart.gov/.
Debris flow prediction map — https://landslides.usgs.gov/hazards/postfire_debrisflow/detail.php?objectid=374
DNR Lidar map — https://lidarportal.dnr.wa.gov/#48.52616:-120.07610:11