Porcupine landslide restrictions pared back
A closure area around the Porcupine Landslide has been trimmed a little.
The new closure area begins 10 miles downstream of the landslide at the junction with the Little Greys River Road. A risk to trail users still exists along the forest road, but new data and analysis has convinced forest managers the restricted area can be decreased.
District Ranger Justin Laycock said the latest report was issued by Jorgensen Associates engineering firm in Jackson. It focused on specific risks along the Greys River Road. The first U.S. Army Corps of Engineers report assessed risks along the entire downstream river corridor.
Some of the key differences between the two technical reports 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 55,000 cfs to 17,593 cfs. “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,” said Laycock. 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,” Laycock said.
While the landslide has continued to move into the Greys River, the river has cut into the opposite west bank and continues to flow over the top of a 13 foot tall natural earthen dam.
And the forest is keeping a close eye on it. “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,” Laycock confirmed.
Forest officials are still concerned about 2 low-lying areas in the first 7 miles of the Greys River Road between Alpine, Wyoming and the Little Greys River Road. Laycock believes the forest can manage those risks with signs and area closures.
The forest has also established an emergency plan with Lincoln County dispatch. Under it, monitors could activate Star Valley Search and Rescue and the Department of Homeland Security. The agencies could then mobilize to evacuate the 7 miles of at-risk road and create a larger emergency closure area.
The Forest has also 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.
The Jorgensen Engineering study is available here.
