VOLUMETRIC-ANALYSIS AND HYDROLOGIC CHARACTERIZATION OF A MODERN DEBRIS FLOW NEAR YUCCA MOUNTAIN, NEVADA

On July 21 or 22, 1984, debris flows triggered by rainfall occurred on the southern hillslope of Jake Ridge, about 6 km east of the crest of Yucca Mountain, Nevada. Rain gages near Jake Ridge recorded 65 mm and 69 mm on July 21, and 20 mm and 17 mm on July 22. Rates of rainfall i ntensity ranged up to 73 mm/h on the twenty-first, and 15 mm/h on the twenty-second. Digital elevation models with 2.0 m grid-node spacing, measured from pre-storm and post-storm aerial stereo-photographs, were used to map hillslope erosion and the downslope distribution of debri s. Volumetric calculations indicate that about 7040 m(3) of debris was redistributed on the 49,132 m(2) hillslope study area during the two- day storm period. About 4580 m(3) (65%) of the eroded sediment was dep osited within the study area and the remaining 35% was deposited outsi de the study area in a short tributary to Fortymile Wash and in the wa sh itself The maximum and mean depths of erosion in the study area wer e about 1.8 m and 5 cm, respectively. The mean depths of erosion on th e upper and middle hillslope were 27 cm and 4 cm, respectively. The me an depth of deposition on the lower hillslope was 16 cm. Analysis of t he values of cumulative precipitation in the context of the precipitat ion-frequency atlas of the National Oceanic and Atmospheric Administra tion indicates that precipitation from the main storm on July 21 was m ore than double that expected, on average, once during a 100-year peri od. The relations of precipitation intensity/duration, developed from data recorded at a nearby precipitation gage, indicate a storm interva l of 500 years or greater. The amount of erosion caused by such a stor m is primarily dependent on three variables: storm intensity, developm ent of the drainage network on the hillslope, and the amount of availa ble colluvium. Additionally, the erosive ability of successive storms of equal intensity will decrease because such storms would tend to pro gressively isolate and reduce the amount of colluvium available. The p reservation of Pleistocene deposits on hillslopes of Yucca Mountain, i n general, indicates that erosional events that strip 5% of the availa ble hillslope colluvium must be quite rare. We conclude that the recur rence interval of an erosional event comparable to the July, 1984 even t is probably much longer than 500 years.