T156 Debris Flows: From Hazard Mitigation to Landscape Evolution

Tuesday, 2 November 2010
8:00 AM - 12:00 PM
Mile High Ballroom 4AB

CHALK CREEK VALLEY, COLORADO: A NATURAL DEBRIS FLOW LABORATORY

Paper number: T156-3
Program: Technical Sessions
Time: 8:20 AM – 8:35 AM

Authors

Abstract

Debris flows that begin from surface-water runoff and grow in volume by entraining sediment can be extremely destructive. Such runoff-initiated debris flows occur in steepland basins where vegetation is sparse, exposed bedrock is abundant, and moderate-to-high intensity rainfall is common. An outstanding need exists to understand the physics of sediment entrainment and quantify the entrainment potential of debris flows for hazard assessments.

In 2002, during an emergency response visit to the Chalk Creek Valley by the USGS, we observed multiple generations of recent debris-flow deposits that originated from steep, bedrock-dominated basins; particularly at Chalk Cliffs. These observations indicated that the Valley would be an ideal laboratory for research on runoff-initiated debris flows. Monitoring at Chalk Cliffs between 2004 and 2006 showed an average of two debris flows each summer. This high rate of occurrence led to an expansion of the monitoring effort in 2008 and 2009, when the University of Colorado and East Carolina University began working at the site. Monitoring is designed to capture flow stage, pore-fluid pressure, bed-normal stress, rates of channel erosion, soil moisture, rainfall, and video and still photography during debris flows. Terrestrial laser scanning is used to monitor topographic changes caused by flows.

Debris flows initiated via a range of mechanisms at, or between, two end members: a) a firehose effect whereby overland flow becomes concentrated, pours over steps or cliffs onto sediment and rapidly erodes and mobilizes the sediment, and b) progressive entrainment of sediment by water in topographically smooth rills and channels. Topography and channel bed moisture played a role in the growth of debris flows. Erosion and entrainment were enhanced by step height, deepening and upslope migration of plunge pools, and discrete failures of channel and bank sediment. However, locations of erosion and deposition were variable, dependent on densities and pore-fluid pressures of individual flows, and could not be predicted from channel topography alone. High moisture levels in channel sediments were not required for debris-flows, but high levels increased rates of erosion and entrainment. Modeling of surface-water runoff shows good potential for debris-flow prediction at Chalk Cliffs.