By Ann McCreary
The arrival of autumn rain this week doesn’t carry the significant potential for floods and mudslides that comes with sudden summer thunderstorms in a burn-scarred environment, according to a National Weather Service (NWS) hydrologist.
Methow Valley residents discovered just how devastating thunderstorms following wildfires can be, when an intense storm on Aug. 21 brought down tons of water, mud and debris that damaged homes, properties and roads around the valley.
Katharine Rowden, a hydrologist with NWS in Spokane, said residents living in or near areas burned in the Carlton Complex Fire shouldn’t be overly worried that the longer lasting rains of fall could pose a similar threat.
“It’s not something we’re really concerned about in a typical fall rainy season,” Rowden said this week.
“The very intense rain in summer thunderstorms sends a large volume of water downstream very quickly, leading to the damaging flash floods and debris flows,” said Rowden.
“Fall/winter/spring rains are a different animal. We can get significant amounts of rain, but usually spread out over two or more days, not coming down all at once like in a summer thunderstorm,” she said.
“We generally don’t expect flash flooding and major debris flows with these types of events, unless it is a very extreme event, but we will see more runoff than usual, and as the freshly burned slopes get saturated, we could see some mudslides from the hillsides,” Rowden said.
Rowden worked on the Burned Area Emergency Response (BAER) report released last month that assessed potential risks to areas burned in the Carlton Complex Fire.
That report predicted water runoff and debris flows would increase dramatically in drainages where vegetation was destroyed and soils were damaged in the fire.
Those BAER projections were based on what would occur as a result of a significant summer thunderstorm, Rowden said.
“The concern for fall rains is if slopes were supersaturated and never got a chance to dry out. The soils on slopes can slide anyway, and without vegetation to hold the soil in place … that would be the concern,” Rowden said.
“I’m not talking an Oso-type landslide, but sloughing of soil. So far we’re not seeing that kind of rain,” she said.
“Even now though, once the soils are full of water from being rained on, we’ll see more runoff from burned areas. Without any vegetation there’s a higher chance of mudslides,” Rowden added.
The BAER report found the risk to homes and outbuildings from flooding, debris flows, runoff and sediment to be very high or high in Benson, Finley, Canyon, Cow, Texas, Davis Canyon, French, Frazer, Beaver, Squaw, Gold, McFarland and Black Canyon drainages.
Other risks in long duration rain events are the possibility of culverts or stream channels plugging from debris that is still being carried downstream, and possibly overflowing stream channels and flowing into homes and over roads, Rowden said.
“Additionally, Okanogan County has some unique risks where some of the stream beds are higher after this summer’s flash floods, which means that it won’t take as much water to impact private property and roads,” she said.
She advised local residents to report any plugged culverts or other potential risks to police or emergency management officials.
The higher risk of flooding and mudslides will return next summer, when thunderstorms return. In an effort to provide early warning of flash floods, NWS and the state Department of Ecology have installed 14 new rain gauges in the area burned by the Carlton Complex Fires, Rowden said.
The gauges are programmed to send reports to NWS in Spokane when rain accumulations present the risk of flash flooding, so that local residents can be warned of potential floods in their area, Rowden said.
The BAER report estimated that it will take up to five years before vegetation recovers enough to significantly reduce the risk of flooding and slides.
