STAR VALLEY, WYO – The Bridger-Teton meeting last Friday in Alpine concerning the Porcupine landslide in Greys River drew about 70 interested members of the public.
As the days and weeks have passed since the event in early February, it has become increasingly clear that the magnitude of the slide and the amount of earth still moving is beyond initial impressions.
Aerial footage put together through a collaboration with the Bridger-Teton National Forest, Buckrail & Contour Aerials LLC shows a dramatic slope side cave-in and ongoing shifting of a massive amount of debris.
On the good side of the ledger is there is relatively few assets at risk in a potential flood scenario should the Greys River become completed dammed and later fail. On the minus side: a portion of the river has already become choked off and modeling shows a 40-foot high natural dam completely occluding the Greys River.
The landslide covers a quarter of a mile of obliterated Greys River Road. The US Army Corps of Engineers 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 indicates that if the dam backed up water, unlike an engineered dam it could spill over the top and breach the dam. 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.”
With temperatures on the rise and spring rains anticipated, the danger of the landslide 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 added.
Because the landslide is still moving, blasting, removing, or disturbing the toe 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 groundwater levels, and water-level changes along the Greys River.
Landslides 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.
