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Wolf Mountain landslide, Jackson, Wyoming--recent activity and the air temperature threshold.
The Wolf Mountain landslide, a large slump-debris flow with an estimated volume of 500,000 cubic yards, occurred on May 18, 1997 (Figure 1). The resulting debris flows overran a 400-ft stretch of U.S. Highway 26-89, a major arterial leading to the community of Jackson, and to Yellowstone National Park. The flow covered the highway with up to 15 feet of mud, rock, and trees. Subsequently, the slide was declared a disaster by the Governor of Wyoming, enabling a major clean-up and removal effort by the Wyoming Department of Transportation (WYDOT) and Wyoming National Guard. Because of concerns of possible future slide activity, WYDOT later constructed a large catch basin designed to prevent debris from reaching the roadway. On April 29, 1998 debris-flow activity occurred again, filling the 6000-cubic yard capacity basin to the brim (G. Michael Hager, WYDOT, written comm., 1998).
In 1996, prior to the occurrence of the Wolf Mountain landslide, a study was begun that resulted in the identification of an air temperature threshold for snowmelt-season landslides (Chleborad, 1997, 1998). The threshold (a 6-day moving average of daily maximum temperature of 58° F) was defined by the number and temporal distribution of associated landslide events. Based on the results of the study it was concluded that the air temperature threshold is a powerful index for anticipating or forecasting the onset of snowmelt-season landslides.
The validity of the temperature index is confirmed by the 1997 and 1998 Wolf Mountain landslide events. Examination and analyses of temperature data for the Jackson, Wyoming area for 1997 revealed that the May 18, 1997 slump-debris flow occurred 10 days after the first yearly occurrence of the 58°F threshold (Chleborad, 1998). Likewise, air temperature data for 1998 has revealed that the April 29, 1998 event is closely associated with the first yearly occurrence of the index (G. Michael Hager, WYDOT, written comm., 1998). Analysis of daily air temperatures provided by the National Climatic Data Center (U.S. Department of Commerce, unpub. data, 1998) indicates that the April 29, 1998 event occurred 5 days after the arrival of the threshold. The association of the 1997 and 1998 Wolf Mountain events and the temperature threshold is shown graphically in Figure 2.
Potential uses of the temperature index include timing observations and the deployment of field instrumentation to monitor hazardous landslides, timing avoidance or mitigation strategies, scheduling construction projects in sensitive areas, and anticipating highway maintenance needs. In addition, it appears useful as a basis for issuing public warnings of an increased potential for landslide activity in areas with known snowmelt-related landslide susceptiblity. As part of an ongoing study, additional data on snowmelt-season landslide events is being compiled to further test the validity of the threshold and determine its applicability in other areas.
Chleborad, A.F., 1997, Temperature, snowmelt, and the onset of spring season landslides in the central Rocky Mountains: U.S. Geological Survey Open-File Report 97-27, 35 p.
Chleborad, A.F., 1998, Use of air temperature data to anticipate the onset of snowmelt-season landslides: U.S. Geological Survey Open-File Report 98-124, 16p.
U.S. Department of Commerce, 1997, Wyoming Climatological Data: Daily temperature data, v. 102, No.s 1-5.
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