Charles Parrett

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

Empirical models to estimate the probability of occurrence and volume of postwildfire debris flows can be quickly implemented in a geographic information system (GIS) to generate debris-flow hazard maps either before or immediately following wildfires. Models that can be used to calculate the probability of debris-flow 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-flow probability as a function of readily obtained measures of areal burned extent, soil properties, basin morphology, and rainfall from short-duration and low-recurrence-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 fires. This model describes debris-flow 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 fire in central Idaho. The predictive strength of the approach in this setting is evaluated using information on the response of this fire to a localized thunderstorm in August 2003. The mapping approach presented here identifies those basins that are most prone to the largest debris-flow events and thus provides information necessary to prioritize areas for postfire erosion mitigation, warnings, and prefire management efforts throughout the Intermountain West.

Flood hydrology for Dry Creek, Lake County, Northwestern Montana

Dry Creek drains about 22.6 square kilometers of rugged mountainous terrain upstream from Tabor Dam in the Mission Range near St. Ignatius, Montana. Because of uncertainty about plausible peak discharges and concerns regarding the ability of the Tabor Dam spillway to safely convey these discharges, the flood hydrology for Dry Creek was evaluated on the basis of three hydrologic and geologic methods. The first method involved determining an envelope line relating flood discharge to drainage area on the basis of regional historical data and calculating a 500-year flood for Dry Creek using a regression equation. The second method involved paleoflood methods to estimate the maximum plausible discharge for 35 sites in the study area. The third method involved rainfall-runoff modeling for the Dry Creek basin in conjunction with regional precipitation information to determine plausible peak discharges. All of these methods resulted in estimates of plausible peak discharges that are substantially less than those predicted by the more generally applied probable maximum flood technique.

Potential effects of urbanization on peak flows in Rattlesnake Creek, Missoula County, Montana

The potential effects of urbanization on the 1-percent-chance flood in Rattlesnake Creek near Missoula, Montana, were assessed using a rainfallrunoff hydrograph model. The model, TR-20, developed by the U.S. Soil Conservation Service, was used to generate a 1-percent-chance flood hydrograph for existing conditions, natural (no urban development) conditions, and three different levels of potential urbanization provided by the Missoula Planning Office. The model showed no significant change (1 percent or less) in the 1percent-chance flood magnitude at the mouth of Rattlesnake Creek, which drains an area of 79.7 square miles. The model did show, however, a marked increase of as much as 124 percent in the magnitude of the 1-percent-chance flood peak from the 6.7 square miles comprising the southern part of Rattlesnake Creek basin that is subject to urban development. Model results are applicable only to rain-caused floods and not to the more common snowmeltcaused floods on Rattlesnake Creek.