The Bridger-Teton National Forest hosted an informational meeting for the public on April 13, 2018 in Alpine, Wyoming regarding the Porcupine Landslide. Approximately 70 members of the public attended. The landslide, which is a quarter of a mile long, obliterated a portion of the Greys River Road when it slid on February 8, 2018.
The U.S. Army Corps of Engineers (USACE) completed a risk analysis for the Forest, modeling a potential flooding scenario as the landslide has dammed a portion of the Wild and Scenic eligible Greys River. While the model shows that the flooding risk to homes or property in Alpine, Wyoming is considered ‘low’ and limited to two structures, the risks in the river corridor itself are much different. The model analyzes a 40-foot high natural dam completely occluding the Greys River. In the model, if the dam backed up water, unlike an engineered dam it could spill over the top and breach the dam. The low lying areas of the Greys River corridor could experience up to a 10-foot wall of water. “We are monitoring the situation with river gauges, but at present we do not have a way to warn the public, in advance of a flood,” said Greys River District Ranger Justin Laycock. “In the interest of public safety we have closed the road. When we get new or different information we can revisit the decision. We are working with local engineering firm to examine other potential scenarios and resulting risk analysis,” he said.
With temperatures rising and snow turning to rain, the danger of the land slide continuing to move downhill and creating an even higher dam is increasing not decreasing. “The Forest is aware of the importance of the Greys River corridor for many uses, including hunting, and we are committed to working towards getting the Greys River Road repaired or exploring rerouting possibilities as soon as we can get in there safely to ground-truth possible options,” Laycock said.
Because the landslide is still moving, blasting, removing, or disturbing the tow of the landslide can cause the slide to destabilize further and create significant hazards downstream. Slope saturation by water is a primary cause of landslides. This effect can occur in the form of intense rainfall, snowmelt, changes in ground-water levels, and water-level changes along the Greys River. Land slides and flooding are closely allied because both are related to precipitation, runoff, and the saturation of ground by water. In addition, debris flows and mudflows usually occur in small, steep stream channels exactly the conditions along the Greys River and in the area of the Porcupine Landslide.
Landslides can cause flooding by forming a landslide dam that blocks the Greys River stream channel, allowing large amounts of water to back up. This causes backwater flooding and, if the dam fails, subsequent downstream flooding. Also, solid landslide debris can “bulk” or add volume and density to otherwise normal streamflow or cause channel blockages and diversions creating flood conditions or localized erosion.
For more information, visit the Forest Website at https://www.fs.usda.gov/btnf/ . For additional information, visit https://go.usa.gov/xnhY6 or https://www.weather.gov/riw/PorcupineSlideDSS or contact the Bridger-Teton National Forest at (307) 739-5500.