Flooding and other environmental effects are possible
By Ann McCreary
The potential for flooding and subsequent damage has significantly increased in areas burned by last summer’s Twisp River Fire, according to findings of a post-fire analysis.
A Burned Area Emergency Response (BAER) report found that roads, homes, irrigation facilities, utilities, fish and wildlife are at risk as a result of damage caused by the Twisp River Fire and other fires that ravaged Okanogan County last summer.
The BAER report identifies post-fire risks to people, property and natural resources, and recommends emergency measures as well as long-term actions to reduce potential damage on state, private and other non-federal lands. The report was conducted by a multi-agency team led by the Okanogan Conservation District.
The Twisp River Fire is one of five Okanogan County fires included in the analysis, which also examined the Lime Belt Fire, Nine Mile Fire, Tunk Block Fire and North Star Fire.
The Twisp River Fire began Aug. 19, burned 11,222 acres, destroyed four homes, and resulted in the deaths of three firefighters who were fighting the fire.
Two drainages burned by the Twisp River Fire — Woods Canyon and Myer Creek — both showed significantly increased potential for runoff after a storm.
Peak water flow would increase 6,772 percent compared to pre-fire flow in the Woods Canyon drainage as a result of a one-hour storm that deposits .75 inches of precipitation — a thunderstorm type of event.
During a storm lasting 24 hours that deposited 2.7 inches of rain — a winter type of rainstorm — the predicted increase in flow for Woods Canyon is 478 percent.
For Myer Creek the post-fire flow from the one-hour storm would increase 2,136 percent from pre-fire amounts, and the 24-hour storm flow would increase 437 percent.
As Methow Valley residents witnessed during floods in areas burned by the Carlton Complex Fire in 2014, runoff in burned areas is capable of bringing down huge volumes of ash, mud, rocks and vegetation.
The report identifies risks to human safety and property and the likelihood of damage that could result from flooding and debris flows associated with post-fire storm runoff from burned watersheds. For circumstances deemed to be very high or high risk, the report developed site-specific recommendations to prevent or reduce damage.
The Twisp River Fire was “not (a) bad burn overall” but had some severe areas in the fire perimeter, according to the BAER findings.
Potential damage to Myer Creek Road, Twisp River Road and an irrigation canal along Twisp River was rated “very high” with “major” consequences. The report noted “potential to lose ingress/egress for approximately five homes on Myer Creek Road.”
The report also described the “Woods Canyon road system at high risk” with “many undersized/misplaced culverts.” Debris flow could also affect access to 10-12 homes on Twisp River Road, the report said.
Risk of potential damage to a home on Twisp River Road was also found to be very high, noting that a diversion built by the homeowner “may not withstand flows.”
The report found a “high” risk of flooding and debris damage for Woods Canyon Road. There is also flood risk to a private driveway, house, barn and well as a result of a bulldozer line that altered a natural drainage path in an area of moderate to severe burn.
The report also evaluated the possibility of the Aspen Lake dam failing, and determined the risk to be low.
Recommendations for protecting the at-risk property included armoring driveways and roads with rocks; installing soil berms or ecology blocks to divert flows; reseeding a bulldozer line; and restoring a natural drainage pattern. Total estimated costs for the recommended treatment in the Twisp River Fire area is $43,000.
About 564 acres burned in the Woods Canyon drainage, with about 44 percent of that acreage burned at moderate or high severity. On Myer Creek drainage 1,131 acres burned, with about 62 percent considered moderate or high severity.
An evaluation of effects on fish and wildlife predicted “a potential for sediment to reach the Twisp River from fire-scarred slopes and drainages where both the fire intensity and soil burn severity were high (Myer Creek and Woods Canyon).”
That could damage habitat for fish in the river, including some endangered species. Excess sediment can increase water temperatures, reduce light penetration and plant growth, reduce the population of aquatic insects, and adversely affect spawning and migration, the report stated.
Loss of forage and cover damages the mule deer winter range and fawning habitat, and loss of trees will affect western gray squirrels, the BAER study found. Riparian areas that burned also resulted in lost habitat for birds, amphibians and other species.
The report recommended numerous steps to restore habitat including stabilizing soils to prevent slides, replanting vegetation, educating homeowners on how to clean fire retardant off their homes to avoid washing retardant into waterways, controlling invasive and noxious weeds, and replanting gray squirrel habitat.
In total, the five fires analyzed in the BAER report burned more than 520,000 acres on public, private and tribal lands in Okanogan County, and destroyed about 200 homes last summer. Towns and cities throughout the area were threatened and residents evacuated.
The report suggested about $6 million in emergency stabilization treatments and up to $1.9 million in long-term rehabilitation work.
Many homeowners have already been contacted about the possibility of participating in the federal Emergency Watershed Protection program, which provides funds to build diversion structures to protect properties from flooding, said Kirsten Cook, outreach coordinator for the Okanogan Conservation District.
About a dozen property owners in the Methow Valley participated in the program after the Carlton Complex Fire.
The Conservation District has been contacting property owners and conducting site visits in the wake of last summer’s fires, Cook said.
“This [BAER] report may seem a little overwhelming,” Cook said. “It’s important for landowners to know that if they are concerned, they should call us to come do a site visit.” District staff can evaluate risk and recommend flood prevention approaches, she said.
The BAER report was conducted by a team of about 20 people including soil scientists, hydrologists, engineers, foresters, biologists and archeologists. Team members used satellite imagery, on-the-ground evaluations and computer modeling to develop their analyses and recommendations.
The full report is available on the Okanogan Conservation District website at www.okanogancd.org/sites/default/files/Okanogan_BAER_FULL2015Report.pdf.