THE OAK CREEK MUDFLOWS AND DEBRIS FLOWS OF JULY 12, 2008, OWENS VALLEY, INYO COUNTY CALIFONRIA

On July 12, 2008 the remains of hurricane Bertha moved from the Gulf of Mexico across the southwestern United States bringing tropical moisture to the eastern Sierra Nevada. Over 32 mm/hr (1.25 in/hr) of precipitation fell on the Oak Creek drainage north of Independence, in Inyo County, California, which had been burned during the Inyo Complex fire of July 6, 2007, generating debris and mudflows that ran out 6 to 7 km from the mountain front, destroying 25 homes, damaging another 25, 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 1.53 million cubic meters of sediment was scoured from channels and deposited over an area of more than 3 km², mostly on younger alluvial fans. Mudflow surges moved down the north fork of Oak Creek at estimated speeds of 2 m/sec (~6 m/hr) to 5.4 m/sec (~16 m/hr) and were one to three meters high. Sand-rich, hyperconcentrated mudflows followed the active channel of the north fork of Oak Creek and abandoned channels on the fan filling them, and spread laterally across the interfluves. On the south fork of Oak Creek, boulder-rich debris flows clogged the active channel, leaving a boulder field of at least 1500 m long and 75 m wide, blocking the channel and forcing the south fork to a new course to the west. The largest boulders moved during the storm ranged from 3300 kg (~1300 lbs) to 17,000 kg (11,800 lbs). Nearly all the damage to private property and infrastructure was long the active stream channels where the hazard is the highest. Older uplifted alluvial fans were unaffected and are the least hazardous areas, while younger fans were greatly affected and should be considered, for the most part, highly hazardous. Mitigation strategy should emphasize avoidance and education to promote public awareness.