Anatomy of a debris flow, Pacifica, California
A major debris flow occurred on January 4,1982, in the Oddstad Boulevard area of Pacifica, California. The flow emanated from a previously unrecognized colluvium-filled swale (one of many making up first-order drainages in the region), moved down a 21°, 172-m-long slope, and extended into an urban area. The failure involved the upper 4.5 m of a 6.1-m-thick colluvial section in the upper of two bedrock basins underlying the swale. Soil-stratigraphic measurements show that upper-basin colluvium accreted slowly to form a cumulic soil profile, characterized by thick surface (mollic epipedon) and subsoil (argillic) horizons. An approximately 500-yr-old mean residence time (MRT) radiocarbon date from the prefailure mollic epipedon indicates that the average sedimentation rate was about 0.6 m/1,000 yr and, accordingly, that colluviation began at least 8,000 to 10,000 yr ago. In contrast, the lower basin is characterized by at least four pre-1982 slide deposits. These deposits emanated almost wholly from within the lower basin, and are distinguished by clast lithology and angularity, and by the local presence of capping buried paleosols.
Radiocarbon MRT dates of approximately 2 to 3 ka for the upper, older debris flows, and the presence of a moderately developed argillic horizon on an underlying flow, suggest that lower basin failure recurrence is on the order of 1,000 to 4,000 yr.
A simple, three-stage evolutionary model for the Oddstad swale is postulated for engineering-geologic comparisons with swales elsewhere: (1) initial basin incision by fluvial processes in late Pleistocene time; (2) change of climatic regime and resultant colluvial filling of the upper basin in Holocene time; and (3) exhumation and renewed fluvial incision of the upper basin following the 1982 debris flow.