Direct measurements of the hydrologic conditions leading up to and during post-fire debris flow in southern california, USA

Abstract

Steep, recently burned watersheds can be vulnerable to debris flows. In southern California, USA, the combination of mountainous terrain, dense population, and high fire frequency put new areas at risk to debris flows each year. In an effort to improve predictions of the timing and magnitude of post-fire debris flows, the U.S. Geological Survey (USGS) established five debris-flow monitoring sites in different southern California watersheds burned in 2009. These sites recorded, for the first time, detailed measurements of the hydrologic conditions leading up to and during post-fire debris flows. Measurements included precipitation, hillslope soil-water content, flow stage, and pore pressure. Here, we present initial observations and comparisons of debris flows measured during four storms at our smallest study site (0.01 km2) located in the Station Fire burn area in the San Gabriel Mountains. The monitored debris flows were generated by progressive entrainment of sediment from hillslope rilling and channel erosion, which occurred in response to short-duration bursts of intense rainfall. The measurements show a distinct change in the flow response over the course of the winter storm season, beginning with a debris-flow dominated response in the first part of the season and followed by more watery flows in the latter part of the season. The change in flow response is presumably related to a decrease in sediment availability.