27 Engineering Geology (Posters)

Sunday, 18 October 2009
9:00 AM - 6:00 PM
Hall A

THE REMARKABLE OCCURRENCE OF LARGE RAINFALL-INDUCED DEBRIS FLOWS AT TWO DIFFERENT LOCATIONS IN THE SOUTHERN SIERRA NEVADA, CA ON JULY 12, 2008

Paper number: 27-3
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

On July 12, 2008, two convective cells about 96 miles apart produced a brief period of intense rainfall triggering large debris flows in the southern Sierra Nevada. Cell formation was due to a high pressure ridge causing monsoonal moisture to be drawn in from the southeast. The northern cell was centered over Oak Creek, an east-flowing drainage, and its tributaries near Independence, CA. About 5:00 PM, debris flows passed down the south fork and main Oak Creek channels to merge into a large single feature. The timing is based on the recollection of an observer at Oak Creek Campground and the transmission times of a nearby weather transmitter. Based on run up, its velocity immediately downstream from Oak Creek Campground was at least 12 mph. The higher elevation watershed, where rilling generated material contributing to the debris flow, was burned in a wildfire the previous August. The Independence debris flow damaged the historic Mt. Whitney State Fish Hatchery, damaged and destroyed homes near the channel and blocked California Highway 395 just north of the town.

At about the same time, the southern cell was largely centered over Erskine Creek, a main tributary of the west-flowing Kern River. Debris flows issued from the headwater channels of Erskine, Thompson and Clear creeks. Estimated velocities along one headwater channel of Erskine Creek were between 8.5 and 10.5 mph. In Erskine Creek, the largest of the three drainages, tributary debris flows coalesced into a single flow that passed down the main channel, through the town of Lake Isabella, CA and into the Kern River. A helicopter observed it reaching Lake Isabella about 5:30 PM. Extensive rilling was observed in the upper watershed burned over by the Piute wildfire during the previous 14 days. While some damage occurred along the channel through Lake Isabella, the more significant impacts were the trapping of engines and fire crews within the Erskine Creek Canyon through the night and adverse effects to the water supply system for Bakersfield, CA.

In addition to their similar triggering mechanism and time of occurrence, the July 12th debris flows at both locations: 1)involved widespread upland rilling, 2) eroded and incorporated significant material from within channels, 3) were very fluid, and 4) had a long runout distance from their source areas.