DEBRIS-FLOW RESPONSE OF BASINS BURNED BY THE 2002 COAL SEAM AND MISSIONARY RIDGE FIRES, COLORADO

Abstract

Debris flows can be one of the most hazardous consequences of rainfall on recently burned hillslopes. Understanding the conditions under which debris flows can occur, and characterization of the magnitude of the debris-flow response are critical elements in post-fire hazard assessments. In this study, we document the debris-flow response of basins burned by the 2002 Coal Seam and Missionary Ridge Fires. The Coal Seam Fire burned 12,229 acres in the steep terrain immediately west of Glenwood Springs and the Missionary Ridge Fire burned 72,962 acres just north of Durango. Eyewitness and newspaper accounts of the rainfall-induced runoff events, measurements of channel cross sections, maps of burn severity, and networks of tipping-bucket rain gages are used to develop estimates of the peak discharges of the debris flows and to define the conditions that resulted in the debris flows. Debris flows were produced from basins underlain by interbedded sandstones, siltstones and conglomerates, and from basins underlain by gneissic quartz monzonite and quartzite. Debrisflow producing basins ranged in size from 0.01 to 8.24 mi 2 , had average gradients between 26 and 94 percent, and relief ratios between 16 and 73 percent. Basins burned at moderate and high severities over more than 50 percent of their areas were susceptible to debris-flow activity. Nearly 70 percent of the debris-flow generating storms were of durations equal to or less than two hours, and 93 percent of the storms had recurrence intervals of less than or equal to 2 years. The average intensities of the debris-flow triggering storms ranged between 0.04 and 0.67 in/hr, with 10-minute peak intensities up to 2.46 in/hr. Estimates of debris-flow peak discharges between 315 and 5581 ft 3 /s were obtained using indirect methods, and values of peak discharge