FORECASTING RAIN-INDUCED DEBRIS FLOWS ON MOUNT RAINIER, WASHINGTON

In recent decades, debris flows initiating from periglacial zones of Mount Rainier have inflicted severe damage to National Park infrastructure and downstream areas. It is generally recognized that debris flow-generating storms drop intense, warm rainfall which produces runoff on high elevation areas shrouded in glacial and volcanic sediment. To this point, however, few systematic approaches have been available to forecast debris flows in space or time. This study has developed a forecasting methodology with two major components: (1) hazard mapping according to initiation potential; and, (2) a characterization of storms by their potential to induce debris flows. Debris flow hazard mapping first involved mapping gullies that exceed thresholds in slope and drainage area, as identified in gullies recently responsible for debris flow initiation. These maps of initiation potential were then overlain on broader geomorphic mapping of sediment availability – for instance, recent glacial deposits were considered to have greater sediment availability than older (more stable) glacial deposits. To compare relative debris flow hazards of watersheds, a simple metric was developed based on the length of high sediment gullies exceeding the slope-drainage area threshold. The second component of the forecasting methodology involved a characterization of storms and their antecedent conditions relative to known rain-induced debris flows from 1980-2014. The methodology was developed by segregating storms with and without observed debris flows, and evaluating their precipitation intensities and durations, temperatures (freezing levels), and antecedent snowpacks. A rainfall threshold for landslides developed for Seattle by Chleborad et al. (2006) reasonably predicted debris flows, after eliminating storms with low elevation freezing levels or extensive antecedent snowpacks. Relatively small amounts of antecedent snowpack (~5-10 inches Snow Water Equivalent), as measured at the lower elevation limit of the mapped gully population, were found to significantly limit debris flow production. In addition to providing local managers a hazard forecasting tool, this study informs hazard evaluation more broadly in terrains with debris flows originating near annual snowlines and active glaciers.