Predicting the probability and volume of postwildfire debris flows in the intermountain western United States

Abstract

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.

A probabilistic approach to post-wildfire debris-flow volume modeling

“…Complete model output for each of the basins examined in this study can be found in Appendix A. States: Arizona (Wohl and Pearthree, 1991), California (Doehring, 1968), Colorado (Cannon et al, 1998), Idaho (Cannon et al, 2010;Meyer et al, 2001) and Utah (Giraud and McDonald, 2007;, as shown in Figure 6a. Measured volumes of these debris flows range from 1700 m 3 to around 100,000 m 3 , and the proportion of the basins burned at moderate and high severity ranged from 0.3469 to 0.…”

“…Cannon et al (2010) compiled a list of the most common of these variables and assessed their impact on the debris flows in the same database that is used in this study. These variables describe a range of factors, including basin morphology, basin areas burned at different severities, soil material properties, rock type, and rainfall amounts and intensities.…”

“…Soil properties have been examined, and while debris flows have been shown to be more likely to occur in loose, cohesionless soils (Major et al, 1997) such as dry ravel deposits , these soils have not been shown to directly affect debris-flow volumes. Cannon et al (2010) showed that basin gradient, aerial burned extent, and storm rainfall were useful variables for empirically predicting volumes in burned areas. In the eastern Italian Alps, Marchi and D'Agostino (2004) proposed a modified geological index that attempts to quantify the erodibility of the underlying bedrock; Schimming (2012) related this geological index to debris-flow yield rates in the western United States.…”

“…Many models have been developed to predict debris-flow volumes by utilizing a number of different parameters related to rainfall, basin geomorphology and material properties (e.g., Gartner, 2005;Bovis and Jakob, 1999;Cannon et al, 2010;Gartner et al, 2008;Hungr et al, 1984). To this point, these models have provided their user with a single value expected to represent the volume of the debris flow most likely predicted by their input variables.…”

“…These results compared favorably with those generated by an existing volume model. This As a result of these and similar events, a concerted effort has been undertaken among geoscientists to detail the physical behavior and impact of debris flows: from identification of triggering mechanisms and the probability of their occurrence (Cannon et al, 2010), to mitigation measures (Prochaska et al, 2008), and to their effect on local populations (Santi et al, 2011). Debris flows pose an especially significant hazard in burned areas, as the loss of vegetation and development of hydrophobic soils increase the potential for catastrophic events.…”