Areas denuded of vegetation were severely affected by last week’s heavy rain, which blocked culverts with sediment and caused small creeks to surge. Scientists expect increased risks of flooding and mudslides to persist for years. Photo by Marcy Stamper

Areas denuded of vegetation were severely affected by last week’s heavy rain, which blocked culverts with sediment and caused small creeks to surge. Scientists expect increased risks of flooding and mudslides to persist for years. Photo by Marcy Stamper

By Ann McCreary and Marcy Stamper

Changes in soil from the intense heat of fire, along with a treacherous mix of ash and water, create a heightened risk of floods and mudslides in fire-damaged areas.

When fires move across the surface of the ground, burning grasses and plants, the combustion of vegetation creates a gas that penetrates the soil and then condenses into a substance that repels water, said Craig Nelson, manager for the Okanogan Conservation District.

“It creates almost a waxy-type substance on the surface of the soils, and that’s what makes the rain so dangerous,” Nelson said. “It’s like pouring water on a crayon.”

That condition, called “hydrophobic soil,” makes it difficult for the soil to absorb water and increases the rate of runoff.

The lack of grasses, bushes and trees to hold soil in place also contributes to the risk of erosion and mudslides, Nelson said.

A third factor that intensifies flood and erosion risk is the combination of ash and water, which produces a mixture that is “almost like a runny cement,” Nelson said.

“The ash will change the specific density of water, meaning that what would never float on water before will float on water. Water that’s mixed with ash now has even greater power because it can pick up and float things that normally wouldn’t float,” Nelson said.

Following wildfires in the foothills around Wenatchee two years ago, spring floods carried “small boulders that were hundreds of pounds. The water went into one person’s house, blew out a wall, and came out carrying a full hot-water tank,” Nelson said.

Motorists should never attempt to drive through runoff, especially below a burned area, Nelson said. “It makes it literally easier to float your car,” he said.

Mitigating risk

Unfortunately, the potential for “massive erosive events” can be expected to continue for three years in most burned areas, and up to five in some watersheds, Nelson said.

To help mitigate risk in the near future, the conservation district is working with Okanogan County Emergency Management, the National Weather Service (NWS) and the Washington Department of Ecology to improve flash-flood warning systems in the burned areas of the Methow Valley.

“We’re working diligently right now to get 17 more rain gauges up in burned areas,” Nelson said.

Flash flood warnings for the Methow Valley are issued by a

NWS station near Spokane, based on radar and rain gauge readings, Nelson said. Currently only a couple of rain gauges are located in the area affected by the fire, and they report only once an hour.

The new rain gauges will be installed in the areas burned by the Carlton Complex Fire and will provide reports every few minutes. Forecasters will be able to evaluate the risk of flash flooding with more-specific, local information, Nelson said.

“We’re trying to improve that response … hopefully as early as this week,” he said. The additional gauges will also provide county emergency management officials more accurate information about the need for people to evacuate.

The Conservation District plans to spread grass seed on burned areas in the fall, but it will take a year or two for root structures to develop enough to stabilize the soil. Nelson said they have to wait until fall because seeds won’t germinate if planted now.

The district is also working on a program operated by the U.S. Department of Agriculture (USDA) to protect some at-risk homes. The Emergency Watershed Protection Program identifies residences at the bottom of watersheds or draws that are considered “defensible” and provides earthen structures and sandbags to divert flash floods away from the home.

After assessing potential properties using maps and aerial photography, Conservation District staff members and USDA officials were in the field last week evaluating homes in the burned areas, Nelson said. Approximately 30 to 40 homes were identified as being at-risk and defensible, and the homeowners will be contacted next week, he said.

Assessing burns on federal lands

As part of the process of preventing erosion and mudslides, a Burned Area Emergency Response (BAER) team began assessing U.S. Forest Service land burned in the Carlton Complex this week, according to Central Washington BAER public information officer Dolores Maese.

Working from maps projecting burn severity, the teams assess actual conditions on the ground, which they use to project the effects of precipitation, said Maese.

The teams, composed of hydrologists, soil scientists, foresters and engineers, evaluate soil condition, loss of vegetation, and the steepness of the slope. They also assess the potential for increased sediment carried by streams and rivers to areas downstream of a burn, as well as impacts on fish.

The specialists will compile their findings and recommendations and submit them to the Forest Service for funding. The initial assessment and recommendations take from two to five days to complete, meaning their report could be done by early next week for the National Forest lands in the southwestern section of the fire, the first they are inspecting, said Maese. The teams will also evaluate the northeastern portion of the burned area.

Prioritized recommendations—those most important for protecting life and property—will be submitted to the regional forester in Portland for approval. Money for erosion protection and intervention comes from the same funding pool as fire suppression and is typically made available after a wildfire, said Tommy John, assistant team leader for BAER.

While the forester’s decision is usually received within a few days, bidding and procurement take longer, so the earliest the BAER team’s preventive work could begin is the end of September, said Maese.

Among the preventive measures often applied are mulch, such as straw or ground wood, and straw bales placed as barriers near a road or river, said John. They also recommend removing or blocking smaller culverts (under 2 feet in diameter) so that they can’t become plugged.

In some instances, a slope may be too steep or rocky to benefit from treatments, said John. “And some areas don’t need treatment. Not every area of fires is at imminent risk of problems,” Maese said.

BAER’s goal is to implement emergency stabilization and flood-control measures before heavy rains occur. “Last week’s heavy rains gave the team some good information about how the area can be expected to behave in another event,” using data about the amount and duration of rain, said Maese.

While BAER only assesses federal lands, the teams coordinate with the National Resources Conservation Service and the Conservation District, who use the information to advise state land managers and private-property owners.

Because the risk of erosion and flooding will remain for three years or more, valley residents need to take flash-flood warnings seriously, said the Conservation District’s Nelson.

“Don’t assume the entire area of watershed above you is recovered. Until the entire watershed is healed, you’re at risk. If it starts to rain, don’t wait for the rumble. Get to high ground,” he advised.

The 30-day weather outlook for the Methow Valley calls for above-normal temperatures and near-normal precipitation, said Miller of the NWS.

Spring, with its precipitation and snowmelt, may present a challenge, predicted Okanogan County Sheriff Frank Rogers.

“I don’t want to be doom and gloom, but my big concern is spring. If we get a good snowpack and a wet spring … my gut is telling me, it’s not going to be good,” Rogers said.