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[1] Debris flows often occur in burned steeplands of southern California, sometimes causing property damage and loss of life. In an effort to better understand the hydrologic controls on post-fire debris-flow initiation, timing and magnitude, we measured the flow stage, rainfall, channel bed pore fluid pressure and hillslope soil-moisture accompanying 24 debris flows recorded in five different watersheds burned in the 2009 Station and Jesusita Fires (San Gabriel and Santa Ynez Mountains). The measurements show substantial differences in debris-flow dynamics between sites and between sequential events at the same site. Despite these differences, the timing and magnitude of all events were consistently associated with local peaks in short duration (< = 30 min) rainfall intensity. Overall, debris-flow stage was best cross-correlated with time series of 5-min rainfall intensity, and lagged the rainfall by an average of just 5 min. An index of debris-flow volume was also best correlated with short-duration rainfall intensity, but found to be poorly correlated with storm cumulative rainfall and hillslope soil water content. Post-event observations of erosion and slope stability modeling suggest that the debris flows initiated primarily by processes related to surface water runoff, rather than shallow landslides. By identifying the storm characteristics most closely associated with post-fire debris flows, these measurements provide valuable guidance for warning operations and important constraints for developing and testing models of post-fire debris flows.

Empirical models to estimate the probability of occurrence and volume of postwildfi re debris fl ows can be quickly implemented in a geographic information system (GIS) to generate debris-fl ow hazard maps either before or immediately following wildfi res. Models that can be used to calculate the probability of debris-fl ow production from individual drainage basins in response to a given storm were developed using logistic regression analyses of a database from 388 basins located in 15 burned areas located throughout the U.S. Intermountain West. The models describe debris-fl ow probability as a function of readily obtained measures of areal burned extent, soil properties, basin morphology, and rainfall from short-duration and lowrecurrence-interval convective rainstorms. A model for estimating the volume of material that may issue from a basin mouth in response to a given storm was developed using multiple linear regression analysis of a database from 56 basins burned by eight fi res. This model describes debris-fl ow volume as a function of the basin gradient, aerial burned extent, and storm rainfall. Applications of a probability model and the volume model for hazard assessments are illustrated using information from the 2003 Hot Creek fi re in central Idaho. The predictive strength of the approach in this setting is evaluated using information on the response of this fi re to a localized thunderstorm in August 2003. The mapping approach presented here identifi es those basins that are most prone to the largest debris-fl ow events and thus provides information necessary to prioritize areas for postfi re erosion mitigation, warnings, and prefi re management efforts throughout the Intermountain West.

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