OBSERVATIONS DEMONSTRATING THE RUNOFF INITIATION OF THE AUGUST 26, 2010 POSTFIRE DEBRIS FLOWS, HAIWEE CREEK, INYO COUNTY, CALIFORNIA

On August 26, 2010 in southern Inyo County, CA, at about 2:30pm, a thick slurry of boulders, trees, ash, soil, and water issued out of Haiwee Creek after an intense, short duration cloud burst on a drainage basin that was partially burned by the Clover Fire in July 2008. The wildfire burned 766 acres (3.1 km²), or 21.6 % of the South Fork Haiwee drainage basin, with <2 %of the watershed burned at low severity, and 19.7 percent at moderate to high severity. From the topographic apex of the Haiwee alluvial fan, the debris flow traveled in surges for over 4.8 kilometers before carrying a semi-tractor and trailer off US Highway 395 at approximate Post Mile 23.7. Flows overwhelmed highway drainage structures and overtopped both the south and northbound lanes with 0.9 to 1.2 meters of debris. The flows proceeded east past the highway continuing another 1.6 kilometers to the valley floor.

Rainfall affecting the Haiwee Creek watershed resulted from a convective system traveling from the southwest, inundating the burn area first and unburned slopes second. The debris flows originated from the burn area in the south fork of Haiwee Creek, developing as concentration of runoff caused progressive increase in flow depth (and channel shear stress) leading to channel scour, bank collapse and the entrainment of sediment and debris. Limited channel scour and debris generation occurred in the unburned portion of the watershed. Within the rugged upland area along Haiwee Creek, east of the confluence of the south and north forks, the debris flows deposited a 1.4 meter thick marginal levee composed of matrix supported cobble and boulder clasts. In contrast, where the flow became unconfined and more broadly distributed near US 395, deposits lack internal stratification, are relatively thin (up to 0.60 meter) and consist of clayey silt with sand and gravel, including scattered cobbles and boulders with numerous 1-2 millimeter size voids. These contrasting sedimentological characteristics underscore the spatial and temporal variation in flow behavior commonly observed in other debris flow studies. The deposit distribution and textural characteristics coupled with radar rainfall distribution suggest a rainfall runoff sequence that initially generated viscous granular debris flows, followed by a recessional stage dominated by fluidized debris flows.