Team gauges risk of floods, mudslides in Lake Christine burn area
July 31, 2018
A special team assessing the potential for floods and mudslides from the scorched earth of the Lake Christine Fire is finding a mixed bag in the field.
Liz Schnackerberg, leader of the U.S. Forest Service's Burned Area Emergency Response Team, said Saturday terrain characteristics and the intensity of the fire on many of the lower slopes are potentially favorable for absorbing water while the opposite may be true on higher ground.
"This is a very broken, lumpy, dumpy landscape," Schnackerberg said while touring the lower slopes of Basalt Mountain in the Basalt State Wildlife Area.
It's not obvious how the water will flow because there are no major gullies and few perennial streams, she said.That means some of the water carrying ash and debris will settle on the vast undeveloped terrain of the mountain instead of tumbling down to the valley floor.
"In a lot of ways, the landscape is a good thing, a blessing." — Liz Schnackerberg, hydrologis.
The middle Roaring Fork Valley dodged its first bullet Saturday when torrential rain fell on the lower slopes of Basalt Mountain in two different waves. The second storm triggered the National Weather Service to issue a flash flood warning with the unnerving language that a flood was imminent.
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However, there were no reports of flooding or mudslides in Basalt, according to Police Chief Greg Knott. A minor debris flow was reported on Ace Lane's property, east of El Jebel Mobile Home Park.
Brooke Taber, the meteorologist for the Rocky Mountain Type 2 Incident Management Team Black, said about a 1/2 inch of rain fell over Basalt, El Jebel and the lower slopes of Basalt Mountain. Lesser amounts fell on the upper slopes.
Schnackerberg is working with Steve Hunter, a civil engineer and hydrologist for the White River National Forest, and Beth Anderson, a soil scientist based in Delta, on a rapid assessment of the Lake Christine burn.
Anderson said it appears the fire burned fast across the vast expanses of pinion, juniper and oak brush on the lower half of Basalt Mountain. She showed how the soil was black with ash in many places but it wasn't very deep. The soil was undisturbed 1/4 inch to 1/2 inch below. She dug down and pulled out roots that still bound the dirt. She also squirted water from a bottle and showed how the soil still absorbs it.
"This landscape still has a lot of stability and structure," she said. "The fire did not severely impact that soil."
Less than four weeks after the fire started July 3, vegetation is already returning to one of the first areas that burned. Anderson showed how suckers from oak brush roots have already popped up among the charred trunks above ground. Patches of almost fluorescent green grass have sprouted.
Schnackerberg said burn areas tend to be exceptionally beautiful three or four years after a blaze. Colorful fireweed typically proliferates. Oak brush will recover but it will take a considerable time for pinion and juniper to re-establish.
Within the vast blackened area where the team worked Saturday there were small pockets of white ash and soil where the fire burned more intense. Overall, the preliminary map showed at least 60 percent of the burn area in moderate burn severity.
Anderson said there was more evidence of intense fire and soil damage higher on the mountain. As the fire burned higher, it reached Douglas fir and other conifers. It burned slower and with greater intensity in that vegetation.
"In the spruce-fir, we really had deep ash," Schnackerberg said.
When the fire is intense it breaks down the structure and stability of the soil. It takes on properties or plastic, according to the team members. The water runs downhill quicker, boosting the potential for flooding.
"The soil is afraid of water. It's hydrophobic," Anderson said.
Schnackerberg and Anderson recently worked on a Burned Area Emergency Response team on the 416 Fire in southwestern Colorado. They accurately predicted where mudslides would occur when the rains came, Schnackerberg said. Each fire is different because of the variances in terrain and the rate the rain falls. The terrain at the 416 Fire is extremely steep, she said.
Schnackerberg said the best thing that could happen is getting slow, settling rainfall over a long period. Unfortunately, summer thunderstorms are often quick and violent.
The team is working with satellite images that indicate soil burn severity and now they're verifying conditions with fieldwork. They also flew over the 12,000-acre burn site to get the lay of the land.
Schnackerberg said it is inevitable that there will be some flooding and debris flow. They are working on a final burned severity map that will be turned over to the U.S. Geological Survey for additional work on locations of floods and mudslides, and the volume of debris that could move.
"We hope to have our assessment done the middle of next week," Schnackerberg said.
That will be submitted to the Forest Service's regional office for a review that will likely be finished within a month. That frees up funds to undertake mitigation.
Schnackerberg said it was too early Saturday to list specific mitigation steps that will be recommended. They could include installing larger culverts in strategic places, for example.
"A lot of time there's nothing we can do on the ground," she said.
So setting up early detection and warning for floods and mudslides will be a vital step.