Probability and volume of potential postwildfire debris flows in the 2011 Wallow burn area, eastern Arizona

Abstract

This report presents a preliminary emergency assessment of the debris-flow hazards from drainage basins burned in 2011 by the Wallow wildfire in eastern Arizona. Empirical models derived from statistical evaluation of data collected from recently burned drainage basins throughout the intermountain western United States were used to estimate the probability of debris-flow occurrence and debris-flow volumes for selected drainage basins. Input for the models include measures of burn severity, topographic characteristics, soil properties, and rainfall total and intensity for a (1) 10-yearrecurrence, 1-hour-duration rainfall and (2) 25-year-recurrence, 1-hour-duration rainfall. Estimated debris-flow probabilities in the drainage basins of interest ranged from less than 1 percent in response to both the 10-year-recurrence, 1-hour-duration rainfall and the 25-year-recurrence, 1-hour-duration rainfall to a high of 41 percent in response to the 25-year-recurrence, 1-hour-duration rainfall. The low probabilities in all modeled drainage basins are likely due to extensive low-gradient hillslopes, burned at low severities, and large drainage-basin areas (greater than 25 square kilometers). Estimated debris-flow volumes ranged from a low of 24 cubic meters to a high of greater than 100,000 cubic meters, indicating a considerable hazard should debris flows occur. Introduction Debris flows, fast-moving slurries of sediment and water, have been documented after many wildfires in the western United States (Cannon and others, 2010) and can threaten lives, property, infrastructure, aquatic habitats, and water supplies. Wildfires can denude hillslopes of vegetation and can change soil properties that affect watershed hydrology and sediment-transport processes. Even small rainstorms after a fire can increase overland runoff that erodes soil, rock, ash, and vegetative debris from hillslopes (Cannon and others, 2008). This increased runoff concentrates in drainage networks and entrains additional sediment that may lead to the generation of destructive debris flows. Debris-flow hazards are most significant 1 to 3 years following wildfires (Susan Cannon, U.S. Geological Survey, written commun., 2010). Debris flows have not been documented from the area burned by Wallow fire in the past, yet potential debris flows in such a large burned area remain a concern for emergency managers. This report presents a preliminary emergency assessment of the debris-flow hazards from drainage basins burned in 2011 by the Wallow wildfire in eastern Arizona (figs. 1, 2, 3, and 4, table 1). This assessment was done by the U. S. Geological Survey (USGS) in collaboration with the State of Arizona, U. S. Department of Agriculture Forest Service, and the Federal Emergency Management Agency Region IX. Estimates are provided of the predicted probability of debris-flow occurrence and volume of debris that could flow from 42 drainage-basin outlets in response to two design storms: (1) 10-year-recurrence, 1-hour-duration rainfall of 39–49 millimeters (mm) (a 10 percent chance of occurrence in any given year) and (2) 25-year-recurrence, 1-hour-duration rainfall 47–58 mm (a 4 percent chance of occurrence in any given year). The methods used for this assessment are based on the work by Cannon and others (2007; 2010) and Ruddy and others (2010).