MONITORING INITIATION CONDITIONS FOR RUNOFF-GENERATED DEBRIS FLOWS IN UNBURNED BASINS, CENTRAL COLORADO

Runoff and sediment entrainment are important mechanisms of debris-flow initiation in many burned and unburned basins in the semi-arid, western U.S. A better understanding of initiation conditions for runoff-generated debris flows is needed for assessing debris-flow hazards in the region. Common problems that inhibit such an understanding include: a lack of rain gages in small basins where debris flows most commonly occur; rain gages that poorly measure rainfall from short, intense thunderstorms; little or no knowledge of antecedent rainfall and soil conditions; and poor constraints on times of occurrence. To examine initiation conditions and processes for runoff-generated debris flows in unburned areas, we instrumented four basins in central Colorado. The four basins are at Georgetown, Gypsum, and at Chalk Cliffs and Cottonwood Creek near Buena Vista. All of the basins are small (< 0.5 sq. km), steep (25-40°), sparsely vegetated, at least partially south facing, and have had recent runoff debris-flow activity. In each basin, we installed event-recording rain gages and dielectric soil-moisture sensors. We also measured hydraulic conductivity and grain-size characteristics of colluvium in debris-flow initiation zones.

Preliminary results suggest that rainfall thresholds for debris flows are inversely related to a bedrock ratio (area of exposed bedrock/basin area) and positively related to the degree of soil development on colluvium. For example, Chalk Cliffs, with a bedrock ratio of about 0.6 and no soil development on colluvium, experienced three debris flows in 2004. A rainfall threshold defined on the basis of storms that produced these flows, as well as non-debris-flow storms, is I=0.69D^-0.73, where I = rainfall intensity (mm/hr) and D = rainfall duration (hrs). Other basins with smaller bedrock ratios and better developed soils had storms that exceeded this threshold, but experienced no debris flows. Initiation zones often contain patchy bedrock mantled by silty, gravelly sand that has hydraulic conductivities from 30 to 115 mm/hr. Patterns of erosion (washing, rilling, scour) suggest that bedrock plays a key role in initiating runoff and subsequent debris flows in some areas. Soil moisture data (available in the summer of 2005) should provide insights into antecedent conditions prior to debris flows.