Study of burned USFS lands identifies risks
By Ann McCreary
The potential for erosion and flooding following a severe rainstorm increased by more than 10,000 percent on U.S. Forest Service lands burned by the Twisp River Fire, according to a post-fire analysis conducted by the Forest Service.
On national forest lands burned by the Black Canyon Fire, the potential for soil erosion after a severe rainstorm increased by 4,500-12,122 percent, the analysis found.
To evaluate potential hazards following this summer’s fires, the Forest Service assembled a Burned Area Emergency Response (BAER) team to conduct assessments and recommend emergency post-fire projects to reduce risks to people, property and natural resources.
The BAER report evaluated damage and potential risks on national forest lands burned by the Black Canyon and Twisp River fires, which includes Forest Service land in the Methow Valley. The report also assessed national forest property burned in the First Creek and Lime Belt Fires, which were part of the Okanogan Complex fire.
“The risk of flooding and other post-fire hazards is a real concern,” said Molly Hanson, a member of the Forest Service BAER team.
Methow Valley residents have witnessed first-hand the damage that results from severe erosion following the Carlton Complex Fire in 2014. Thunderstorms have brought down tons of mud and debris in several drainages including Benson, Canyon, Leecher, Texas and Frazer creeks on different occasions, causing extensive damage to homes, properties and roads.
The Forest Service last week approved $269,400 in funding for projects to reduce post-fire hazards. Of that amount, $163,300 was approved to mitigate risks on forest lands in the Black Canyon Fire area, and $23,400 was approved for projects in the Twisp River Fire area.
Black Canyon Fire
The Black Canyon Fire was ignited by lightning on Aug. 14 and burned across 32,735 acres of Forest Service land on the Chelan and Methow ranger districts. In the Methow Valley, it severely affected the Black Canyon, McFarland, Squaw and Gold Creek drainages.
Using satellite images and field assessments, the BAER team analyzed how severely Forest Service land was burned by the fire and where soils have become water resistant.
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.
That soil condition, called “hydrophobic soils,” 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.
In the Black Canyon Fire perimeter, the BAER team found that 8,662 acres — about one-fourth of the burned Forest Service lands — show a strong water-repellent tendency.
The team identified an estimated soil erosion potential of 22 tons per acre from a 25-year storm lasting one hour and delivering .73 inches of precipitation. That compares to a pre-fire potential of .18 to .47 tons per acre during the same storm, the report stated. That is an increase of 4,500-12,122 percent.
“This increased erosion can cause downstream sediment delivery that bulks flows, resulting in increased flooding effects,” according to the report.
The Black Canyon Fire burned 17 percent of the Forest Service lands in the Gold Creek watershed,5 percent in the South Fork of Gold Creek, 33 percent in McFarland Creek, 49 percent in Squaw Creek and 41 percent in the Black Canyon sub watersheds, according to the BAER report.
The team identified numerous threats to public safety, property, natural resources and threatened or endangered species. They include:
• Threats to public safety from flooding, falling trees, rocks and debris-laden flows in valley bottoms and in steep burned drainages throughout and downstream of the burned area.
• Threats to property including Forest Service roads, bridges, culverts and recreation sites, along with private homes and other structures in valley bottoms adjacent to or in flood prone areas or near stream channels.
• Potential negative effects on threatened or endangered fish from sediment or debris flow in Black Canyon Creek, South Fork Gold Creek, Squaw Creek or the lower Methow River.
To mitigate the risks, the BAER team recommended steps that include removing culverts that will not be able to contain anticipated debris flows and replacing them with drivable drainage dips.
The team also recommended installing gates to control access to some Forest Service roads to protect the public from hazards including flooding and falling trees; placing warning/closure signs for snowmobile trails and Sno-Parks at Echo Ridge, Antilon, Black Canyon and South Fork Gold Creek; temporarily closing the Echo Ridge trail; and patrolling areas at risk of flooding during and after storm events.
In both the Black Canyon and Twisp River Fire areas, the BAER team recommended working with the National Weather Service and Washington Department of Ecology to expedite installation of rain gauges to provide real-time warnings of potential flooding.
Twisp River Fire
The Twisp River Fire started Aug. 19 about five miles up the Twisp River Road. The human-caused fire extended up into the Little Bridge Creek drainage on its eastern flank and into the Cow Creek drainage on the western flank, according to the BAER report.
The burned area includes about 4,948 acres of national forest lands within the 11,220-acre fire perimeter. The fire also burned 71 acres of BLM land, 4,732 acres of state land and 1,469 acres of private land, the report said.
Satellite images and field assessments found about 1,000 acres of Forest Service lands were strongly water repellent after the fire.
The report estimated that Forest Service lands had a potential of 19 tons of erosion per acre from a 25-year storm lasting one hour and delivering .70 inches of precipitation. That compares with a pre-fire erosion rate of .18 tons per acre, and represents a 10,455 percent increase.
An area of particular concern within the Twisp River Fire perimeter is the Myer Creek drainage, the study found.
“The fire burned areas of high and moderate severity above private lands on Myer Creek,” posing a risk to public safety, the report found.
A catchment area of 900 acres, Myer Creek also poses the highest risk of erosion and sediment delivery to the Twisp River, the report said.
“This area could deliver measurable amounts of sediment and debris to the Twisp River if a 25-year/one-hour storm occurs,” which could damage habitat for threatened and endangered fish in the Twisp River, the report said.
Risk mitigation measures recommended by the report include removing an undersized culvert on the Forest Service’s Coal Creek Road that could fail in post-fire flows and installing control gates on some Forest Service roads within the burned area to protect public safety.
The report also recommends patrolling the area during and after storms to address potential problems with road drainage.
Work on risk mitigation measures is expected to begin as early as this week.
BAER team member Hanson recommended that residents and visitors in or near fire-affected areas keep a close watch on the weather and be prepared to move to higher ground in the event of a sudden or strong rainstorm.