Climatic and Geomorphic Triggering Mechanisms of Cascadian Periglacial Debris Flows

Oregon State University, Corvallis OR

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Debris flows initiating in the upper reaches of glaciated valleys of the Cascade stratovolcanoes are typically associated with intense, multi-day rain events being brought in by the subtropical jet stream. Recent investigations suggest that the number and size of such rain-induced debris flows may be on the increase, which indicates that they may be affected by climate change. The overall goal of the research is to characterize the triggering mechanisms of these periglacial debris flows in the Pacific Northwest using both climate and geomorphic perspectives. The targeted study areas are Mount Hood, Oregon and Mount Rainier, Washington but the research is broadly applicable to debris flows on other composite volcanoes in the Pacific Northwest. The specific objectives are to: 1. Map and characterize the initiation sites of rain-induced periglacial debris flows; 2. Map and quantify annual changes in glacier area on Mount Rainier and Mount Hood over the past several decades; 3. Characterize and classify the storms that have caused recent periglacial debris flows over the past several decades; 4. Quantify antecedent conditions for debris flows including rainfall and snow cover; 5. Understand recent debris flows in the context of debris flows over the historic record; 6. Develop and evaluate prognostic schemes for rain-induced periglacial debris flows. This research employs an empirical approach based on a wide range of observational evidence. The initiation sites will be mapped and characterized using field surveys, airborne LiDAR imagery, and aerial photos. Annual changes in glacier extent and area will be mapped using 30-m Landsat satellite imagery (1984-present) and previously published glacier maps. The inventory of debris flows will be used to match with specific storm events and antecedent conditions. Meteorological data from various sources will be used to characterize storm direction, integrated water vapor transport, rainfall intensity, storm freezing altitude, snow cover, and previous rainfall amounts. Historic context of debris flows will be explored using chronologies developed from aerial photos and geomorphic and dendrochronologic data from field surveys. Using our quantitative characterizations of storms and initiation sites we will then develop a typology of sites and circumstances that are important in triggering rain-induced debris flows. Such periglacial debris flows ("landslides") are devastating to infrastructure in the region as they wipe out roads, bridges, buildings, campgrounds, trails, and irrigation facilities. After a particularly large number of these in November 2006 event, Mount Rainier National Park had to be closed for an unprecedented six months; the park alone suffered nearly $30M in damage. At Mount Hood, debris flows from this storm destroyed a major bridge and road in the White River drainage causing a multi-week delay in opening one of the premier ski areas in the region; in the Eliot drainage, the Middle Fork Irrigation District is no longer able to insure their water diversion structures because of the high risk of debris flows. Of longer-term concern, the sediment carried by these debris flows and floods is deposited in the streambed, which raisies the streambed elevation and subsequent flows will more readily result in major flooding. In addition, the receding glaciers on expose more sediment capable of erosion. This raises such questions as: will debris flows become more common and will they carry larger volumes of sediment longer distances? Characterizing both the storm type and the initiation sites will aid decision makers by providing a scientific basis for risk assessment.