ESTIMATION OF INUNDATION AREAS OF POST-WILDFIRE DEBRIS FLOWS

Predicting the area that can be inundated by debris flows is important for hazard assessment and the location of mitigation structures. Previous work has shown that a set of physically-based equations can be used to reasonably forecast areas likely to be inundated by debris flows, rock avalanches and lahars. There are currently several methods used to estimate debris flow volume (V) and the relations A=a₁V^2/3 and and B=a₂V^2/3 are used to estimate the cross-sectional area (A) and total valley planimetric area (B) likely to be inundated by a flow. Calibration of these equations, using empirical data provide different values for the constants a₁ anda₂for different types of flows. Geographic Information System (GIS) programs, such as LAHARZ, provide a platform where these equations can be applied to delineate the areas at risk from these flows and to create hazard maps. Debris flows that occur shortly after fires initiate through different processes than debris flows in unburned areas, rock avalanches and lahars, therefore they need different models. Detailed mapping of 22 debris flows triggered by the December 25, 2003 storm in the areas burned by the Old and Grand Prix fires in San Bernardino County, California was used to calibrate the equations to post-wildfire debris flows. Regression analysis and analysis of variance of the datasets suggest the equations A=0.26V^0.40and B=7.4V^0.81can be used to effectively calculate inundation areas of post-wildfire debris flows. These empirical models fit the data better than the physically based models. The differences in these models can be explained by the differing characteristics between flow types.