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Historical debris-flow occurrence in Rocky Mountain National Park, Colorado, USA
Rathburn, Sara L. Patton, Annette I. Bilderback, Eric L.
Rathburn, Sara L.
Patton, Annette I.
Bilderback, Eric L.
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2019
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Abstract
Debris-flow characteristics of initiation and runout are compared across the Continental Divide in Rocky Mountain National Park (RMNP), Colorado, USA to evaluate the influence of lithology, topography, and anthropogenic activities. Previous research indicates that 30 debris flows within the last century were large enough to transport sediment to valley bottoms and main stem channels and create new debris fans. Ten of the debris flows occurred on the east side of the Continental Divide and 20 on the west side. Those on the east side initiated after an extreme rainstorm in September 2013, whereas those on the west side occurred on the hillslope below an earthen ditch (12 flows) or on the natural hillslope (8 flows). Of those on the west side, four of the five largest debris flows initiated at the ditch. Comparing debris-fan area, transport distance, and elevation of initiation, we find greater variability in transport distance and elevation from east side debris flows, some of which entered alluvial channels and traveled >4 km under highly fluidized conditions. In contrast, the presence of the ditch on the west side limited the elevation of initiation, contributing area and hence, debris-flow transport distance. Lithologic differences between the east and west sides influences debris-supply conditions. On the east side, weathering-limited basins reduce source material and require a longer time to recharge convergent topographic source areas. Transport-limited hillslopes on the west side provide ample source material for debris flows, including till deposits and weathered volcanic rocks. Fan areas are comparable, and debris-flow morphology is generally similar across the Continental Divide. Recent anthropogenic and climatic disturbances, however, indicate that new patterns of debris-flow occurrence must be considered in the future. The increased occurrence of debris flows is an alert to park management who address hazards and risks to park staff, visitors and infrastructure. Furthermore, the high elevation of initiation for these modern debris flows expands the range of expected debris-flow hazards.
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