POST-FIRE DEBRIS-FLOW MONITORING IN SOUTHERN CALIFORNIA: OBSERVATIONS FROM IN-SITU MEASUREMENTS OF 28 DEBRIS FLOW EVENTS

Steep, recently burned watersheds are extremely vulnerable to debris flows. In southern California, 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. The basins ranged in size from 0.01 to 1 km². 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, soil water-content, flow stage, pore pressure, and, at one site, video. Here, we present initial observations and comparisons of 28 debris-flow events recorded at four sites in the Station burn area (San Gabriel Mountains) and one site in the Jesusita burn area (Santa Ynez Mountains). The measurements show that the flows are tremendously complex and have characteristics that vary considerably with basin scale, geology, and time. Measured surge heights ranged from 0.2 m at the smallest site to 4 m at a large site. High surges were not always associated with high volumes of transported debris. Debris flows at the San Gabriel sites were triggered by rain intensities 5 times lower than at the Santa Ynez site, despite similar slope and burn characteristics. Halfway through the storm season at the smallest site, the flow response changed from primarily debris flow to primarily water-dominated flow lead by brief debris surges. Despite such variability, the debris flows also had several things in common. All of the debris flows were generated by entrainment of sediment from hillslope rilling and channel erosion. All of the events were strongly correlated with local peaks in the 10-minute rain intensity. And, all occurred within a few minutes of this peak. Debris flow occurrence was not strongly related to the total rain prior to the event or pre-storm soil-moisture levels. Continued post-fire debris-flow monitoring in Southern California should provide detailed documentation that will help better understand the processes that control basin response, and this, in turn, will help develop and constrain modeling approaches for post-fire debris flows.