Abstract

During early May of 1941 the southern California mountain resort of Wrightwood was partly inundated by surging flows of muddy debris which occurred daily for more than a week. The immediate cause of flowage was rapid melting of deep winter snow. Intense shattering and rapid weathering of the bedrock (Pelona schist) within the San Andreas rift zone at this location contributed materially to preparing debris for flowage.

Field studies, eyewitness accounts, and movie films provide the following information. Debris was transported 15 miles by the mudflow, and on a gradient as low as 75 feet per mile at the outer extremity. The flow advanced in successive waves or surges of “slimy gray cement-like” mud containing abundant stones. Velocities of the surge fronts ranged from a few to nearly 15 feet per second and averaged close to 10 feet per second during the more fluid phases. The fronts of the more fluid surges slithered and splashed along about like a rapidly advancing tongue of water. No breaker-like motion was evident, although the top of the front tended to shoot ahead of the base. In more viscous surges a bouldery embankment developed at the front and was shoved along by the material following behind.

One sample of the fluid debris had a density of 2.4, indicating a water content of 25–30 per cent by weight. Calculated viscosities range from 2 × 103 to 6 × 103 poises. An average sorting coefficient of 3.94 indicates poor sorting, although somewhat better than in many glacial tills which the flow deposits strongly resemble. Earlier mudflows have occurred here, and others will undoubtedly take place in the future.

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