Empirical models for post-fire hazard assessments: Analysis of terrain, burn severity, rainfall, and soil influences on post-fire erosion by debris flow

Post-fire debris flows pose severe hazards to communities in southern California and the western U.S. Timely and accurate assessments of post-fire debris-flow hazards are needed to protect these communities against potentially devastating impacts. Current post-fire hazard assessments provide discrete drainage-basin scale estimates for the probability and magnitude of debris flows within two years of a fire. These assessments depend on manually defined locations of drainage-basin outlets and only provide predictions of debris-flow probability and magnitude for these locations. In this study, newly acquired data for post-fire debris-flow volumes, mapped locations of erosion and deposition within drainage basins, and measurements of the magnitude of erosion within drainage basins are examined to develop more precise models for assessing post-fire debris-flow hazards throughout recently burned areas. In the process of developing these models, the critical influences of terrain, burn severity, rainfall, and soil characteristics on post-fire erosion by debris flow were assessed.;New empirical models for predicting volumes of post-fire debris-flow and sediment-laden floods at various times since a fire were developed from expanded databases. The models provide new tools for emergency assessments of post-fire debris-flows that are possible within two years of a fire and for ongoing assessments of smaller debris flows and sediment-laden floods that are possible after two years. A new approach for implementing the models along drainage networks, rather than for individual drainage-basin outlets, was developed to enable continuous predictions along the drainage network. In combination with a new model for identifying the portion of the drainage network dominated by erosion, continuous post-fire debris-flow volume estimates along drainage networks are possible that do not require manually defined drainage-basin outlets. Finer-scale measurements of post-fire volumes of material eroded per one-meter channel length (termed the "yield rate") were measured to characterize the magnitude of erosion within burned drainage basins. These data were analyzed for correlations with measures of terrain, burn severity, rainfall, and soil characteristics and to develop an empirical model for predicting yield rates normalized by the burned contributing area as a function of slope.;The models and methods for implementation developed by this dissertation provide more precise, efficient, and consistent assessments for post-fire debris-flow magnitudes at drainage-basin outlets and along drainage networks. In addition, new information is provided throughout a recently burned area for the range of possible debris-flow volumes and the location and magnitude of debris-flow erosion within drainage basins. The analyses of post-fire debris-flow volumes, yield rates and measures of terrain, burn severity, rainfall, and soil characteristics identify the influence of these characteristics on the initiation and magnitude of debris flows in recently burned drainage basins.