Paper No. 2
Presentation Time: 8:00 AM-6:00 PM
WITHIN-STORM EVOLUTION OF RAINFALL INTENSITY-DURATION THRESHOLDS FOR POST-FIRE DEBRIS FLOW INITIATION IN SOUTHERN CALIFORNIA, USA
HANSEN, Matthew1, STALEY, Dennis M.
2, KEAN, Jason W.
2 and GARTNER, Joseph E.
3, (1)Geography and Environmental Science, University of Colorado - Denver, Campus Box 172, P.O. Box 173364, Denver, CO 80217, (2)U.S. Geological Survey, Denver Federal Center, P.O. Box 25046, MS 966, Denver, CO 80225, (3)U.S. Geological Survey, Box 25046, MS 966, DFC, Denver, CO 80225, hmatkay@msn.com
Rainfall thresholds are commonly used to identify precipitation characteristics that trigger landslides and debris flows in burned and unburned watersheds throughout the world. In southern California, the National Weather Service (NWS) and United States Geological Survey (USGS) use rainfall intensity-duration thresholds as the basis of an early warning system for post-fire flash floods and debris flows. While threshold conditions are known to vary spatially as the result of different geologic and topographic characteristics, very little is understood regarding temporal variations of threshold conditions within a storm event. We analyze these temporal dynamics using a database of the timing of debris flows and the rainfall characteristics leading up to the events. The database for debris-flow timing consists of eyewitness accounts and data collected from debris-flow monitoring stations. Rainfall data were recorded at rain gages within 5-km of the basins where debris flows were documented.
First, we investigate the durations of rainfall intensity that most closely correspond with the initiation of debris flows. Rainfall intensities were calculated for durations ranging from 2 to 1080 minutes. Preliminary results suggest intensities measured over shorter durations (15 minutes or less) are better correlated with the timing of debris flows. Much lower correlation coefficients are identified for intensities measured over time periods in excess of one hour. We also assess the influence of length of the storm and total accumulation (i.e. antecedent moisture) on threshold conditions. Results indicate that there is no clear relationship between the total storm duration and total rainfall accumulation with rainfall intensities preceding debris flows. Therefore, a single set of short duration thresholds are adequate for predicting the onset of debris flow from burned areas. These findings will allow NWS forecasters to reduce the number of rainfall characteristics that need to be monitored during a rainstorm, thereby improving our ability to provide warnings based upon real-time observations of the conditions that trigger destructive debris-flow events.
