Abstract

This report presents an emergency assessment of potential debris-flow hazards from basins burned by the 2009 Station fire in Los Angeles County, southern California. Statistical-empirical models developed for postfire debris flows are used to estimate the probability and volume of debris-flow production from 678 drainage basins within the burned area and to generate maps of areas that may be inundated by the estimated volume of material. Debris-flow probabilities and volumes are estimated as combined functions of different measures of basin burned extent, gradient, and material properties in response to both a 3-hour-duration, 1-year-recurrence thunderstorm and to a widespread 12-hour-duration, 2-year-recurrence storm. Debris-flow inundation areas along the northern margin of the fire are mapped using the volumes of material estimated for both storms. Inundation areas along the San Gabriel mountain front are also mapped using the volumes estimated for both storms, and consider situations when the total capacity of each sediment-retention basin is available for storage, and when basins experience drain, spillway, or outflow channel blockages or failures. This assessment provides critical information for issuing warnings, locating and designing mitigation measures, and planning evacuation timing and routes within the first two winters following the fire.

Tributary basins that drain into Pacoima Canyon, Big Tujunga Canyon, Arroyo Seco, West Fork of the San Gabriel River, and Devils Canyon were identified as having probabilities of debris-flow occurrence greater than 80 percent, the potential to produce debris flows with volumes greater than 100,000 cubic meters, and the highest Combined Relative Debris-Flow Hazard Ranking in response to both storms. The predicted high probability and large magnitude of the response to such short-recurrence storms indicates the potential for significant debris-flow impacts to any buildings, roads, bridges, culverts, and reservoirs located both within these drainages and immediately downstream from the burned area.

Conditions in all but the smallest basins along the San Gabriel mountain front between Big Tujunga Canyon and Arroyo Seco resulted in probabilities of debris-flow occurrence greater than 80 percent, debris-flow volumes between 10,000 and 100,000 cubic meters, and high Combined Relative Debris-Flow Hazard Rankings in response to both storms. The combination of high probabilities and large magnitudes determined for these basins indicates a significant potential for debris-flow impacts to neighborhoods and infrastructure along the San Gabriel mountain front.

Debris-flow inundation simulations along the San Gabriel mountain front when the storage capacity of all of the sediment-retention basins is available indicate that debris-flow material may be deposited in neighborhoods immediately below unprotected drainages and along the Angeles Crest Highway. When sediment-retention basins are overtopped, or experience drain, spillway, or outflow channel blockages or failures, debris flows may be deposited in neighborhoods and streets, and impact infrastructure, between the mountain front and beyond Foothill Boulevard.

High debris-flow probabilities and large volumes estimated for the largest basins on the north edge of the burned area in response to both storms also indicate the potential for debris-flow impacts to infrastructure on this side of the fire. Simulations of potential debris-flow inundation areas indicate that in response to the 3-hour-duration, 1-year-recurrence storm, debris flows may travel between 2.0 and 3.0 km from the mouths of some canyons, while in response to the 12-hour-duration, 2-year-recurrence storm, debris flows may travel up to 5 km from canyon mouths.

Revised April 12, 2010

First posted October 6, 2009