POSTFIRE RUNOFF MODELING FOR PRE-FIRE HAZARD MITIGATION PLANNING, SOUTHERN CALIFORNIA

Assessment of post-fire runoff in areas recently burned by wildfire is an essential element of hazard recognition and mitigation. Methods range from regional-scale studies to site-specific investigation. Ideally, assessments are completed prior to seasonal rainfall. Regional assessments provide valuable information for local and state officials developing hazard mitigation strategies and preparing emergency response and recovery plans. Site-specific investigations identify local conditions with the intent of addressing potential impacts to specific values at risk. However, accurate post-fire assessment is difficult to accomplish because of the many factors that affect runoff response in a watershed. Significant research developments have occurred in the past decade that address regional-scale post-fire runoff hazards with the intent of providing information to agencies responsible for floodplain management and public safety decisions.

In June 2015, the California Geological Survey (CGS) completed a project with the California Department of Water Resources and the California Office of Emergency Services. This hazard mitigation planning study uses empirical models developed by the USGS (Cannon et al., 2009; Gartner et al., 2014) as well as southern California debris yield models (Gatwood et al., 2001; Gartner et al., 2009), to assess post fire debris yields from a pre-fire planning perspective. Modeling was completed on twenty representative watersheds within five southern California counties. A range of burn percentage and rainfall scenarios were modeled in each watershed to provide information on the potential for debris laden flows and debris flows to occur under a range of scenarios, should a watershed burn. The results from the three different models are compared with debris basin cleanout records, where available, and are provided in order to present a range of possible postfire debris yields at debris basin locations situated above populated areas. The modeling results are intended to be used by planners and floodplain managers in the development of emergency response and hazard mitigation plans.