POSTFIRE DEBRIS FLOW MODELING FROM A PRE-FIRE PLANNING PERSPECTIVE, SOUTHERN CALIFORNIA

Research into the factors that lead to the development of debris laden flows and debris flows has refined our understanding of how they are generated from recently burned watersheds. It is recognized that most debris flows from burned watersheds result from a runoff-dominated process, where rainfall on the burned slopes leads to progressive bulking of storm runoff with the greatest contribution from the erosion of sediment stored within the channels (Wells, 1987; Santi et al., 2007). At some point within the drainage sufficient sediment and debris become incorporated into storm runoff so that it is transformed into a debris flow. This change is important because a debris flow moves as a viscous mass with internal shear strength sufficient to transport large objects as suspended load. The occurrence of debris flows is documented by the sedimentological properties of deposits following postfire events (Meyer and Wells, 1997; Cannon et al., 2003). These deposits are differentiated from those of fluvial origin, and the factors present that correspond to the occurrence of debris flows are used in empirical equations.

The California Geological Survey (CGS), working with the California Department of Water Resources and the California Office of Emergency Services, is using the empirical models developed by the USGS (Cannon et al., 2009; Gartner et al., 2008) as well as southern California debris yield models (Gatwood et al., 2001; Gartner et al., 2009), to assess postfire debris yields from a pre-fire planning perspective. Modeling is being conducted on fifteen representative watersheds within five southern California counties. A range of burn percentage and rainfall scenarios are being modeled in each watershed to provide information on the potential for debris laden flows and debris flows to occur, 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.