STAR VALLEY, WYO – Bridger-Teton National Forest authorities today announced they will loosen restrictions in the Greys River drainage, allowing for additional access now on Greys River Road downstream from the natural dam at Porcupine Creek. The news comes in the wake of the latest assessment by a Jackson-based engineering firm studying the landslide and potential flooding scenarios.
BTNF is reducing the closure area along the Greys River Road now beginning 10 miles downstream of the landslide at the junction with the Little Greys River Road. While the risk to recreationists still exists along the forest road, the new data and additional analysis has allowed the District Ranger to feel more comfortable with decreasing the area of restricted occupancy in the interest of public safety.
“The latest report issued by Jorgensen Associates at the request of the Bridger-Teton focused on specific risks to recreationists along the Greys River road, as opposed to our report we requested from the US Army Corps of Engineers (USACE), where we asked for a rapid assessment of potential risks along the entire downstream river corridor,” said Greys River District Ranger Justin Laycock.
The Porcupine Landslide has continued to move into the Greys River. The river has cut into the opposite (west) bank from the landslide and continues to flow and overtop a 13-foot-tall earthen dam.
“The Forest has installed remote monitoring in the form of two stream height gages, and a web camera which is providing real-time images to Forest engineers at regular intervals,” Laycock said. “These mitigations, along with a commitment from Lincoln County and their Search and Rescue team as well as the information presented in the latest technical report, are why we are reducing the area closure to everything upstream of the Little Greys River Road.”
There are two low-lying areas of concern that still remain in the first seven miles of the Greys River Road between the Town of Alpine, Wyoming and the Little Greys River Road, but District Ranger Laycock believes the Forest can safely mitigate these risks.
“Within the first 7 miles of road, Squaw Creek and Fire Trail Creek are still within the inundation areas at the worst case and at the lesser model, but we will sign all access entrances, and any areas predicted to flood will be closed to camping in this new Special Order,” Laycock said.
Some of the key difference between the two technical reports that helped inform the decision include a reduction in the size of the debris dam under a worst-case scenario from 40 feet to 33 feet. Additionally, the experts were able to adjust the predicted river flow (cubic feet per second or CFS) from 55,000cfs to 17,593cfs.
“The reason for these very significant differences are changes in conditions with runoff, slide movement, and most importantly the acquisition of post slide 3D topographic data,” Laycock said. “The USACE report was the best data we had at the immediate time and with the given uncertainties. Now with more time, more data, and a recent look at predicted warm-up and spring runoff, we have a new report which reflects the changed conditions with the dynamic landslide.”
Additionally, the Forest has plowed a user access route across McDougal Gap from Sublette County, Wyoming to the area above the Porcupine landslide.
“Since we now have access from above the slide, we can drive to the slide location and assess current conditions from above, without putting people at risk below the dam,” Laycock added.
