SAN FRANCISCO BAY AREA DEBRIS FLOWS LOCALIZED BY WIND-DRIVEN PRECIPITATION

The azimuthal distribution of slopes hosting debris flows in major San Francisco Bay area rainstorms differs dramatically from that of the region's topography. Analysis of slope aspect for source areas of 1,942 debris flows mobilized in the Oakland-Berkeley hills during the catastrophic 3–5 January 1982 storm suggests that wind direction, together with lineated topography and two other factors, explains this difference.

An aspect-frequency plot calculated from a 30-m digital elevation model reveals that over half the 1982 debris flows face SW but few face N. The region's conspicuous NW-striking terrain, which yields an elongate frequency plot dominated by both NE and SW slopes, accounts for many of the SW-facing debris flows. This topographic effect masks other causal factors. To reduce the effect, we divided debris-flow aspect (in percent, by 10 deg. bins) by terrain aspect (54,082 sample grid cells). The terrain-normalized plot retains the dearth of N-facing debris flows and merely shifts modal aspect from SW to SE. These two persistent asymmetries suggest that additional, azimuthally skewed, conditions controlled location of the 1982 debris flows.

Both asymmetries are consistent with slanting wind-driven rain, which delivers more moisture to steep windward slopes than to level terrain or leeward slopes: Southerly slopes in the Bay area are exposed to rain driven by winds from the south (characteristic of the region's winter storms), whereas northerly (leeward) slopes are partially sheltered from the inclined rainfall. Hillslopes that face SE were most directly exposed to the 1982 storm winds out of the SE, recorded at nearby Oakland Airport (6.3 m/s mean wind speed) and six other Bay area stations. Accordingly, rainfall thresholds for debris-flow initiation were exceeded preferentially on SW-SE slopes.

Azimuthal variations in digital-map data for vegetation type and geology also correlate well with the predominance of debris flows on SW-SE slopes. (Correlation with debris-flow aspect is less certain for terrain elevation and slope gradient and indeterminate for available data on slope curvature and soil expansivity.) Incorporating terrain aspect, wind direction, vegetation, or geology in susceptibility models could reduce uncertainty in predicting the location of future Bay area debris flows.