Report assesses mud, flood and hazardous tree threats in aftermath of Lake Christine Fire
The Aspen Times
The biggest threats in the aftermath of the Lake Christine Fire are falling trees on national forest lands, floods and debris flows, according to an assessment released Thursday by the U.S. Forest Service.
A partial report by a Burned Area Emergency Response (BAER) team said the Forest Service will maintain a closure of national forest on Basalt Mountain for an undetermined amount of time because of the risk of hazard trees falling.
“Even trees that still appear green may have a weakened root system due to smoldering at the base of the tree,” the report said. “For this reason, the Forest will maintain an area closure for the burned area until the risk of hazard trees subsides.”
The closure affects the popular Mill Creek Trail and another trail referred to as the “big loop” that connects Basalt Mountain Road to the Cattle Creek drainage. Roads on Basalt Mountain also will be closed while the Forest Service monitors hazard trees, performance of culverts and the ability of the routes to shed water.
A more immediate concern is the potential for floods and debris slides. The report assessed the “soil burn severity” for much of the affected area and how that could increase the probability for debris flows.
In essence, the report said areas that were susceptible to floods and debris flows prior to the fire will have larger magnitude events. Floods and mudslides will be more frequent in areas where they used to occur occasionally. Even areas that didn’t have any events prior to the fire could now experience them.
The report said there are limited steps the agency can take on national forest to limit floods, debris flows and soil erosion because of “the nature of the burn and slope characteristics.” Instead, the report recommended keeping people out of the at-risk parts of the national forest and implementing an early-alert system.
“These mitigations include area closures, warning signs, and public safety approaches such as installation of an early warning system to notify area residents and users of when damaging storms may be approaching,” the report said.
The Forest Service said in a statement it shared its report with other federal, state and local government agencies to help them determine their next steps. Much of the land immediately above the midvalley floor is the Basalt State Wildlife Area, owned by the state, and the Bureau of Land Management.
“Over the next few days the Forest Service will review the recommendations by the BAER Team and request funding to move into implementation on national forest lands,” Aspen-Sopris District Ranger Karen Schroyer said in a statement.
Basalt Town Manager Ryan Mahoney said there are limited physical steps the town can take to ease the risk of floods and mudslides. Jersey barriers have been placed where drainages from Basalt Mountain enter the town’s Hill District, he said. Those will stop debris but not water, he said.
Mahoney said it is unlikely the town can move in heavy equipment onto the steep slopes of state-owned land above town in the Basalt State Wildlife Area to try to stop or limit floods and debris flows.
“I don’t have an answer today about what we’re going to deploy or if we can deploy anything,” he said.
Flooding and debris flows have already affected the land of Ace Lane just east of El Jebel Mobile Home Park. Two waves of torrential rain on Saturday sent water and debris cascading from the steep slopes above, which are BLM and state-owned land. His team created diversions that prevented debris from hitting a barn and apartments. Since then, workers have created check dams, catch basis and berms to fend off future threats.
Dave Marrs, who works with the Lane family on business issues, said he is attempting to contact personnel with the Natural Resources Conservation Service and other agencies to see if any water diversions can be placed on public land above Lane’s property to ease the threat.
The BAER report showed extensive risk of flood and debris slide onto Lane’s property from the steep slope above. Other high-risk areas are above the town of Basalt, though it wasn’t affected Saturday.
The BAER team assessed the southern portion of the burn. The northern section needs further assessment once more when the fire is extinguished. Therefore, the report covered about 9,800 acres rather than the 12,588 acres within the affected area.
The team determined 1,189 acres or 12 percent of the soil burned at a high severity rate. Another 5,195 acres or 53 percent experienced a moderate burn severity.
On the other end of the spectrum, 2,507 acres or 26 percent of the soil burned a low severity while 906 acres or 9 percent was unburned.
“An estimated 65 percent of the area within the Lake Christine Fire perimeter had high or moderate Soil Burn Severity and may have developed water repellent soils as a result of the fire,” the report said.
“Water repellent soils develop when organic material — dead plant debris — on the soil surface burns during a fire, releasing waxy substances that coat soil particles, basically ‘shrink wrapping’ the soil and filling in the pores that allow water to soak in during rain events. This condition is referred to as hydrophobicity,” the report continued. “When water can’t infiltrate into the soil because the pores are blocked, water runs off over the surface causing erosion and increased flood flow potential.”
That means an increase in flood flows and a more rapid time for the flood flows to reach downstream areas, according to the report.
It also means enhanced chances for destructive debris flows. The BAER team worked with the U.S. Geological Survey to determine the probability of debris flows initiating. They found pockets where the probability of occurrence was at 60 to 80 percent.
The report noted that “several unique physiographic characteristics” of the burned area contribute to an atypical degree of uncertainty of the debris-flow hazard. The potential hazard is likely overestimated, according to the report.
“However, hazardous flash flooding and debris flow are still possible during intense rainfall, and residents of downstream communities should heed all warnings issued by local officials,” the report said.
The potential for soil erosion is as high as 3.3 tons per acre, the report said. “For perspective, one acre of soil equal to the thickness of one sheet of paper is equal to one ton of sediment,” it said.
In the bigger picture, natural processes will result in re-vegetation of the soil, a process that’s already apparent on lower ridges affected by the fire. Green shoots have already sprouted from the blackened trunks of oak brush above Lane’s property and in the Basalt State Wildlife Area. Grass also is growing in some areas.
“While soil loss may be greater in localized patches, these impacts are not considered significant and will not result in permanent impairment of soil productivity in the long term (10 years),” the report said.