Use of air temperature data to anticipate the onset of snowmelt-season landslides

New data on landslides induced by snowmelt, or a combination of snowmelt and rainfall, confirms previous findings that an air temperature threshold is a powerful index for anticipating or forecasting the onset of snowmelt-season landslides. Analyses of the data shows that a high percentage (85%) of the landslide events occurred within 2 weeks after the first yearly occurrence of an optimal temperature threshold of 58°F (a 6-day moving average of daily maximum temperature). Thus, the analyses indicate the threshold can be useful as an empirical basis for issuing warnings, in a narrow time window, of an increased potential for landslide activity in areas highly susceptible to snowmelt-season landslides. Other potential uses for the threshold 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. INTRODUCTION Landslides induced by snowmelt or a combination of snowmelt and rainfall occur in many regions of the world each year and are often dangerous and destructive. Snowmelt-related landslides, especially debris flows (sometimes referred to as "mudslides" in media news reports), may be numerous in years with heavy snowfall and associated deep snowpack. Snow meltwater (snowmelt) that seeps into the subsurface contributes to slope instability and the development of landslides by saturating the soil or rock mass, elevating pore pressures, and increasing shear stress. In 1996, I conducted a preliminary examination and analysis of climatic data associated with historical siiowmelt-related landslide events that occurred in the central Rocky Mountains (Chleborad, 1997). The study revealed an air temperature threshold that appears useful for anticipating or forecasting the onset of landslides during the spring snowmelt season. 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. Historical, daily air temperature data recorded at National Weather Service substations located near the landslide sites was used to estimate site temperatures, to characterize temperature trends, and to identify the temperature threshold (U.S. Dept. of Agriculture, 1925; U.S. Dept. of Commerce, 19791997). The purpose of this report is to present additional data and analyses that tests the validity of the threshold and demonstrates its applicability to other areas of the western U.S. For this report, 11 additional landslide events, including four from Utah, three from Wyoming, three from Colorado and one from Nevada (the eastern Sierras), have been added to the database, increasing the total number to 27 landslide events. Basic information on the 27 landslide events is presented in Table 1. Several of the new additions are well known historic landslides. These include the 1925 lower Gros Ventre landslide, Wyoming (event no. 1, Table 1; Voight, 1978), the 1983 Rudd Canyon debris flow in Utah (event no. 9; Kaliser, 1983), the 1983 Slide Mountain rock and soil slide in Nevada (event no. 8; Waiters, 1983), and the 1997 Wolf Mountain slump/debris flow, Wyoming (event no. 25, G. Michael Hager, Wyoming Department of Transportation, personal comm.,1997). Coincidentally, each of those landslide events, directly or indirectly, resulted in loss of life and (or) costly property damage.