HR: 1340h
AN: H43F-1692    [Abstracts]
TI: Storm Rainfall Conditions for Floods and Debris Flows from Recently Burned Basins in Southwestern Colorado and Southern California
AU: * Cannon, S H
EM: cannon@usgs.gov
AF: U.S. Geological Survey, Box 25046, DFC, MS 966, Denver, CO 80225, United States
AU: Gartner, J E
EM: jegartner@usgs.gov
AF: U.S. Geological Survey, Box 25046, DFC, MS 966, Denver, CO 80225, United States
AU: Wilson, R C
EM: rwilson@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Bowers, J C
EM: jcbowers@usgs.gov
AF: U.S. Geological Survey, P.O. Box 865, Apple Valley, CA 92307,
AU: Laber, J L
EM: jayme.laber@noaa.gov
AF: National Weather Service, 520 North Elevar Street, Oxnard, CA 93030, United States
AB: Debris flows generated during rainstorms on recently burned hillslopes have destroyed lives and property throughout the Western U.S. Field evidence indicates that, unlike debris flows that start as landslides, these events have no discrete initiation source and can occur with little or no antecedent moisture. Using rain gage data and information on basin runoff response from five burned areas in Colorado and southern California, we documented the rainfall conditions that triggered post-fire debris flows and developed empirical rainfall intensity- duration thresholds for the occurrence of debris flows and floods following wildfires in these areas. This information can provide guidance for warning systems and planning for emergency response under similar conditions. Debris flows were produced from recently burned basins in Colorado in response to convective storms. Debris flows were triggered after as little as six minutes of storm rainfall. Most of the storms that generated debris flows lasted less than one hour, ranged in average intensity between 1.0 and 32.0 mm/hr, and had recurrence intervals of two years or less. Threshold rainfall conditions for floods and debris flows sufficiently large to pose threats to life and property from recently burned areas in south-central Colorado are defined by I = 6.5D^-0.7, and for southwestern Colorado by I = 9.5D^-0.7, where I = rainfall intensity (in mm/hr) and D = duration (in hours). Debris flows were generated from recently burned areas in southern California in response to frontal storms. The flows occurred after as little as two hours, and up to 16 hours, of low intensity (1-20 mm/hr) rainfall. The storms had recurrence intervals of two years or less. Threshold rainfall conditions for life- and property- threatening floods and debris flows during the first winter season following fires in Ventura County are defined by I = 12.5D^-0.4, and for the San Bernardino, San Gabriel, and San Jacinto Mountains as I = 7.2D^-0.4, respectively. The thresholds defined here are lower than most identified for unburned settings, perhaps because of the difference between extremely rapid, runoff-dominated processes acting in burned areas and longer-term, infiltration-dominated processes on many unburned hillslopes.
DE: 1810 Debris flow and landslides
DE: 1821 Floods
DE: 1854 Precipitation (3354)
SC: Hydrology [H]
MN: 2007 Fall Meeting