MONSOON-GENERATED DEBRIS FLOWS, FOREST FALLS, CALIFORNIA

The San Bernardino Mountains community of Forest Falls is located on forested alluvial fans composed almost entirely of debris flow deposits. At least 13 monsoon rainfall debris flow events caused destruction to property from 1955 to 1999. Smaller, non-destructive debris flows occurred more frequently.

Forest Falls is located at the base of the north side of Yucaipa Ridge which rises abruptly to over 900 m. and is composed of fragmented basement rocks. Runoff from high-intensity, short-duration, summer and autumn monsoon rains erode and mobilize surface material to form slurries that entrain rocky debris forming debris flows with no evidence of landsliding. Clast size of a meter is common; the largest clast about 5 m. The surface of the fans is dominated by debris flow remnants, abandoned channels, levees, and debris flow snouts. Based on dendrochronology the oldest fan surface is about 300 years. Most flows are relatively viscous, moving slowly (<20 km/hr) in a laminar fashion and have a characteristic blunt snout. Higher velocity flows (80-100 km/hr) travel in a turbulent fashion, lack a blunt snout and leave relatively clean deposits. On July 11, 1999, 6.3 cm of rain fell in a 30 minute period at Forest Falls and generated debris flows in three adjacent canyons. Slow moving flows occurred in two canyons and a debris avalanche, in the third. Eyewitnesses described the debris avalanche as three separate ‘waves' with heights up to 7 m over a period of 45 seconds. Minimum superelevation measured after the 13th was 3.3 m. This superelevation is a minimum because slow moving debris flows on the 13th covered the low side of the faster flows of the 11th; measurements were made after the 13th. At a sharp bend in the channel the three debris flow waves overshot the channel wall at slightly different points. The waves of debris removed trees above the channel wall and degraded the fan surface leaving a clean swept appearance. After overshooting the channel walls the waves ‘spread-out' down the fan destroying houses and came to rest as clean-appearing rocky debris lacking a characteristic rocky snout of the more slowly moving flows.