Rainfall intensity–duration thresholds for postfire debris-flow emergency-response planning

Abstract

Following wildfires, emergency-response and public-safety agencies can be faced with evacuation and resource-deployment decisions well in advance of coming winter storms and during storms themselves. Information critical to these decisions is provided for recently burned areas in the San Gabriel Mountains of southern California. A compilation of information on the hydrologic response to winter storms from recently burned areas in southern California steeplands is used to develop a system for classifying magnitudes of hydrologic response. The four-class system describes combinations of reported volumes of individual debris flows, consequences of debris flows and floods in an urban setting, and spatial extents of the hydrologic response. The range of rainfall conditions associated with different magnitude classes is defined by integrating local rainfall data with the response magnitude information. Magnitude I events can be expected when within-storm rainfall accumulations (A) of given durations (D) fall above the threshold A = 0.4D ^0.5 and below A = 0.5D ^0.6 for durations greater than 1 h. Magnitude II events will be generated in response to rainfall accumulations and durations between A = 0.4D ^0.5 and A = 0.9D ^0.5 for durations less than 1 h, and between A = 0.5D ^0.6 and A = 0.9D ^0.5 or durations greater than 1 h. Magnitude III events can be expected in response to rainfall conditions above the threshold A = 0.9D ^0.5. Rainfall threshold-magnitude relations are linked with potential emergency-response actions as an emergency-response decision chart, which leads a user through steps to determine potential event magnitudes and identify possible evacuation and resource-deployment levels. Use of this information in planning and response decision-making process could result in increased safety for both the public and emergency responders.