DEBRIS FLOWS INITIATED BY RUNOFF IN COTTONWOOD CANYON, SAN ISABEL NATIONAL FOREST, CENTRAL COLORADO

About 6:30 pm on July 22, 2002, an intense thunderstorm caused debris flows in about 30 small (generally < 0.5 sq. km), south-facing tributary basins of Cottonwood Creek, a steep-walled, formerly-glaciated drainage about 9 km west of Buena Vista, Colorado. The debris flows crossed Chaffee County Rd. 306 in 11 places, trapping several motorists. The thunderstorm occurred during a mature phase of the North American Monsoon, but was within a several year period of drought. A weather station 2 km north of Cottonwood Creek recorded 22 mm of antecedent rainfall between June 1 and July 22, and 22 mm of rain between 6 and 7 pm on July 22, 2002, but there was little debris-flow activity near the station, suggesting that rainfall may have been greater in the Cottonwood Creek drainage. Field observations after the event indicated the debris flows initiated on slopes that were mantled by grus derived from underlying, hydrothermally altered, quartz monzonite. The grus was dominated by sand (48-59%), followed by gravel (20-49%), and silt/clay (2-21%). About 95% of debris flows initiated from erosion and entrainment of grus by rilling that initiated where slope wash became concentrated on 26 to 35º slopes. Rills initially progressed downslope by forming peripheral to vegetation (shrubs and grasses), but displaced vegetation as they continued to enlarge. Plunge pools within rills, and matrix-supported levees, were first observed about 30 and 100 m downslope from rill initiation zones, respectively. Rills began to decrease in size, and widespread deposition began, near the heads of debris fans where slopes decreased to less than about 20º. Less than 5% of debris flows initiated from landslides or from a firehose effect where concentrated runoff from bedrock mobilized downslope grus. Average saturated hydraulic conductivities ranged from 145 to 175 mm/hr at landslide headscarps, and from 30 to 115 mm/hr in rill initiation zones. Assuming these values are minima for infiltration capacities in the debris-flow source areas, the rainfall rate that initiated rilling must have exceeded 30 mm/hr. This study suggests that high antecedent moisture conditions are not required for runoff-initiated debris flows, and that runoff-initiated debris flows increase in volume as they progress downslope and therefore pose a serious hazard.