Bridger-Teton National Forest contracts for risk analysis on Porcupine landslide
by Bridger-Teton National Forest
March 29, 2018
As part of its continuing efforts to monitor the ever-changing Porcupine landslide, 17 miles upstream the Greys River from the town of Alpine, Wyoming, the Bridger-Teton National Forest has contracted with Jorgensen Associates from Jackson, Wyoming to provide additional risk analysis to keep the Forest informed as it makes decisions about access to the area and next steps.
"We hired local engineering firm Jorgensen Associates to provide supplementary risk analysis with more of a focus on risk to recreating public," said Greys River District Ranger Justin Laycock. "The U.S. Army Corps of Engineers (USACE) risk analysis focused on both infrastructure and recreation. We are looking for additional information as our primary concern is the life safety of our recreating public," he said.
The 17-mile section of the Greys River Road, downstream of the Porcupine Landslide remains closed. "What we are dealing with is unstable earth. The only thing holding the rest of the hillside up right now is the unstable earthen mass at the toe of the slide. If you remove the toe then the rest of the slide will come down, making the situation worse," he stated.
The Forest doesn't have an estimate of when the area will be reopened or road work will begin. "The $3.2 million dollar award from the Emergency Funding for Federally Owned Roads (ERFO) that the Forest received earlier this week is the initial "get started" money and if additional funds are needed, we will apply for them again," Laycock said. Additionally the Forest also applied for and received $1 million dollars to repair the stabilized Blind Bull landslide of September, 2017, which is approximately 12-miles south east of the Porcupine Slide. "We will hire a contractor to get moving on the road work for both of these projects, but that hiring won’t occur until at least after spring runoff," he stated. "Until it melts off there is little we can do. The snow has to get out of there," he said.
The monitoring reports from the specialists watching the site did reveal new information this week. The water has cleared a bit as debris in the Wild & Scenic eligible Greys River had settled. "We were able to gain a more complete picture of the progress of the landslide," Laycock said. "On Friday for the first time we saw river bottom. The streambed was covered with angular rocks, not rounded cobble, and we could clearly see the river is already carving a new channel," he stated. There is a 10-ft drop off that has already been eroded away in this new channel where the river has cut into the opposite bank. "Along the affected roadway we see fractures more than 3-feet wide and 20-feet deep and those broken chunks of land will most certainly fall into the river and further add to the damming, increasing the amount of water being held back," he said.
The pooling of the river behind the debris has been occurring since early February. "Now we have 6-8 foot tall trees completely submerged under standing water," Laycock reported. "We can see what depths we are dealing with now and the angular material from the slide in the riverbed will continue to flush, likely with the increases in flow from the snowmelt," he said.
For more information, visit the Forest Website at https://www.fs.usda.gov/btnf/. For additional information, contact the Bridger-Teton National Forest at (307) 739-5500.
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