DEBRIS FLOWS AFFECTING HIGHWAYS: THE WYOMING EXPERIENCE

Debris flows affecting the highways in Wyoming have occurred at 2 locations in the past decade. In early 1996, a series of 15 debris flows plugged a concrete box culvert resulting in the overtopping of the roadway with slide debris. Each of these debris flows consisted of approximately 1000 cubic yards of material. The road closures resulting from these debris flows were significant because they occurred near the east entrance to Yellowstone National Park on a major highway into the park. The source of the material for the debris flows, was located .75 miles up the drainage, 1500' above the roadway. A series of circular failure landslides deposited material consisting of rocks, mud, and trees, into the drainage. The material mixed with the snow melt and was carried down the drainage as a debris flow.

To mitigate the effects of the debris flows, the existing concrete box culvert was removed, the roadway grade was raised, the channel was steepened as much as possible and a single span bridge was installed. Since the installation of the bridge, all subsequent debris flows have passed under the bridge and not affected the roadway.

The other major debris flow occurred on May 18, 1997 in the Snake River Canyon 18 miles southwest of Jackson, Wyoming. This debris flow was a result of a large landslide which occurred on a steep slope 1500' above the roadway. It is estimated that 100,000 cubic yards of rocks, mud and trees flowed down the slope, covered the highway with 15'-20' of material and created a delta 250' out into the Snake River. It took two weeks for the debris flow to stabilize enough to begin removal of material from the roadway. It took another month before full time traffic was restored.

To provide protection for the road from disturbed material in the existing channel, a large triangular catchment basin was cut into the slope above the roadway. This basin was designed to hold 8000 cubic yards of material. The next spring, during a period of rapid snow melt, a debris flow completely filled the basin within a 12-hour period. Maintenance forces were able clean out the culvert which drained the basin and allowed the slide debris to flow into the river without closing the road. In subsequent years, the amount of material flowing into the basin has decreased and a large concrete box culvert has been installed to allow material to pass under the road.