50 Theory and Practice: Engineering Geology in the Cordillera (Posters)

Saturday, 29 May 2010
8:30 AM - 12:00 PM
Platinum 5-6

GRAIN SIZE ANALYSIS AND MECHANICS OF THE 2008 MOUNT WHITNEY FISH HATCHERY MUDFLOW ON THE EASTERN SLOPE OF THE SIERRA NEVADA, INDEPENDENCE, CALIFORNIA

Paper number: 50-7
Program: Theme Sessions
Time: 8:30 AM – 12:00 PM

Authors

Abstract

On July 5th 2007 a wild fire flared in the hills west of the Mt. Whitney Fish Hatchery grounds located on the eastern slope of the Sierra Nevada Mountains in Independence, California. In the summer of 2008 intense rainfall subsequently mobilized the regolith on those hills into mudflows that raced down Oak Creek. The north fork mudflow coursed down to California Highway 395 leaving an unbroken layer of mud four miles long with a drop in elevation of more than 2,000 feet.

On site written and photographic records were made of the flow and six representative samples spaced along the length of the mudflow were taken. Samples were collected for sediment lab grain size analysis at CSULA.

Results of the sample size analyses follow: mud (clay and silt size) ranged from 5-22% with an avg. of 15%; sand size ranged from 53-74 % with an avg. of 64.2%; gravel size ranged from 8-35% with an avg. 20.7%.

Each sample studied exhibited clear grain size grading from gravel to mud size. Small air pockets measuring 1-2 mm in diameter were noted to make up 10 % of the flow by volume.

A thin film (1-2mm thick) of clay formed as a top layer giving the flow a lustrous sheen. Prominent shrinkage cracks developed all over the surface of the mudflow as it hardened further indicating a high clay content. On top of the flow various human and animal foot prints have been preserved illustrating that the flow hardened over time.

Laboratory density measurements were made on the mudflow samples and compared to the density of a representative set of various rocks. The mudflow specimens were significantly less dense.