Debris-Flow Hazard Assessment of the Area Burned by the 2013 Beaver Creek Fire

Science Center Objects

In August 2013, the Beaver Creek wildfire burned more than 100,000 acres of public and private land northwest of Hailey, Idaho. According to the U.S. Forest Service, about 57 percent of the area is considered moderately burned, and the risk of post-fire soil erosion is high on more than 8,400 acres.

In cooperation with Blaine County, we have developed a model to estimate the probability of occurrence, volume, and combined hazard ranking of debris flows in the burned area. This model uses topographic, soil, burn severity, and storm (rainfall) intensity variables to estimate the probability and volume of debris flows. The probability and volume estimates are ranked and combined to estimate a relative hazard ranking for debris flows. The models evaluate three storms scenarios:

  1. 2-year-recurrence, 1-hour duration rainfall of 13 millimeters (mm), referred to as a 2-year storm (a 50-percent chance of occurrence in any given year)
  2. 10-year-recurrence, 1-hour-duration rainfall of 19 mm, referred to as a 10-year storm (a 10-percent chance of occurrence in any given year)
  3. 25-year-recurrence, 1-hour-duration rainfall of 22 mm, referred to as a 25-year storm (a 4-percent chance of occurrence in any given year)

Probability

The probability of debris flow occurrence is highest in the eastern part of the Willow Creek watershed, the southern part of the Greenhorn Creek watershed, most of the Deer Creek watershed, the Democrat Gulch subwatershed, and some drainages in the Baker Creek watershed.

Probabilities of debris flow occurrence were low for most of the Warm Springs Creek watershed except for a basin in the Thompson Creek subwatershed. The probability of debris-flow occurrence increased with each increase in storm intensity; the highest probability is 78 percent for a basin in the Willow Creek watershed.

Volume

Estimates of debris-flow volume were highest in two basins in the Willow Creek watershed, the headwaters basin in the Baker Creek watershed, and the Thompson Creek subwatershed in the Warm Springs Creek watershed. These basins are not coincident with the high probability basins.

Hazard (combined probability and volume)

Because many of the high probability and volume basins do not overlap, no combined hazard rankings had a high value of 5. Hazard rankings primarily are greater in the eastern part of the Willow Creek watershed, the downstream part of the Deer Creek watershed, some basins in the Baker Creek watershed, and the Thompson Creek subwatershed within the Warm Springs Creek watershed.