The Oak Creek post fire debris and hyperconcentrated flows of July 12, 2008 Inyo County, California: a geologic investigation

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Organization: 

Publication Year: 

2012

Abstract: 

On July 12, 2008 remnant moisture from hurricane Bertha moved from the Gulf of Mexico across the southwestern United States bringing tropical moisture to the Eastern Sierra Nevada. Rainfall intensities reportedly as high as 97 mm/hr (3.8 in/hr) occurred for a period of 39 minutes on the Oak Creek drainage north of Independence, in Inyo County, California. This area had been burned during the Inyo Complex fire of July 6, 2007. The storm generated debris and hyperconcentrated flows ran out 6 to 7 km (~3.8 to 4.4 mi) from the mountain front, reportedly damaging or destroying 50 residential structures, severely damaging the historic Mt. Whitney Fish Hatchery, and disrupting traffic on State Highway 395 for nearly a week. Although slopes were extensively rilled, most of the estimated 1.5 million cubic meters (~2.0 million yd3) of transported sediment was scoured from channels and deposited over an area of more than 3 km2, mostly on younger alluvial fans. Surges moved down the North Fork of Oak Creek at estimated velocities of 2 m/sec (~4.5 mi/hr) to 5.4 m/sec (~12 mi/hr) and were one to three meters high. Sand-rich, hyperconcentrated flows followed the active channel of the North Fork of Oak Creek and filled the channel where it debouched on the alluvial fan surface, spreading sediment and debris laterally across the distributary fan interfluves. Several avulsions occurred as the result of either channel plugging and flow redirection, or as channel overflow. On the South Fork of Oak Creek, boulderrich debris flows clogged the active channel, and created a boulder field of at least 1,500 m (4,600 ft) long and 75 m (230 ft) wide on the upper portion of the young alluvial fan surface. The channel of the South Fork was forced to a new course to the west. Boulders ranging from less than one meter to over three meters across, weighing up to ~26,000 kg (~57,000 lbs) were moved by the flows. Flooding and debris deposition occurred within the active stream channels and on significant portions of the young alluvial fan surfaces; older alluvial fan surfaces were unaffected, supporting the hypothesis that younger fan surfaces are the ones most likely to be affected by post-fire debris and hyperconcentrated flows.

Document Type: 

  • Report

Tags: 

  • critical issues, database publication, hazards, landslide, california