HR: 0830h
AN: H51D-1094    [PDF]
TI: Process-Modulated Contrasts in Slope Exposure of Landslides in the San Francisco Bay Area
AU: * Pike, R J
EM: rpike@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road MS 973, Menlo Park, CA 94025 United States
AU: Sobieszczyk, S
EM: ssobie@usgs.gov
AF: U.S. Geological Survey, 10615 S.E. Cherry Blossom Drive, MS 975, Portland, OR 97216 United States
AB: {\it Beaty (1956)} first quantified the relation between slope exposure (aspect), the compass direction faced by a hillside normal to its contours, and landsliding in the Bay Area; he found that 78 of 112 recent small landslides east of the city of Oakland occupied NW to NE slopes. Frequency distributions of DEM-calculated aspect revisit this relation for two large sets of landslides in seven 1:24,000 quadrangles centered on Oakland: 30-m source areas of the 1,943 debris flows that formed during the severe 3-5 January 1982 storm {\it (Wieczorek et al., 1988)}, and 116,360 30-m grid cells on 6714 pre-1970 deep-seated landslides of various types exclusive of debris flows {\it (Nilsen, 1975; Pike et al., 2001)}. Normalizing each distribution of landslide aspect (in percent, in 10$^{\circ}$ intervals) by the aspect of all slopes in the study area above 75 m in elevation and steeper than 3$^{\circ}$ corrects for the excess of NE and SW slopes imparted by the NW-SE grain of local topography. Both resulting distributions are skewed, but in directions that reflect contrasting processes of landsliding. The differences suggest that any incorporation of slope aspect into landslide susceptibility in the Bay area must reflect specific types of failure. The deep-seated landslides favor ENE slopes and are deficient on southerly slopes. The prevailing explanation for this observation was first proposed in the study area by {\it Beaty (1956)}. Compared to steep S-SW slopes, exposed to afternoon sun at near-normal angles of incidence (northern hemisphere), N-NE slopes are more shaded, sustain less evapo-transpiration, and thus accumulate more and deeper moisture over extended periods-conditions favoring deep-seated failure triggered by prolonged rainfall or seismic shaking. Such conditions are unlikely to have controlled location of the 1982 debris flows, which are conspicuously deficient on NE slopes but strongly favor S-SSE slopes. One explanation is wind-driven rainfall, which may have delivered moisture preferentially to steep southerly slopes. This hypothesis is consistent with the 155$^{\circ}$ median of 33 hourly measurements of wind direction during the January 1982 rainstorm at nearby Oakland International Airport, where recorded wind speed averaged 19 kts. Alternative explanations and other Bay Area locations remain to be explored.
DE: 1824 Geomorphology (1625)
DE: 1854 Precipitation (3354)
DE: 1866 Soil moisture
DE: 6309 Decision making under uncertainty
SC: Hydrology [H]
MN: 2003 Fall Meeting