GEOTECHNICAL OBSERVATIONS AND ANALYSES OF RECENT COASTAL BLUFF FAILURES, PACIFICA CALIFORNIA

Geotechnical observations have been made of recent coastal bluff failures near Pacifica, California caused by wave swash erosion from typical high winter tides. The study site consists of an area of very steep, unconsolidated, marine terrace deposits up to 25 meters in height that have been subjected to cyclical toe erosion from waves reaching the base of the bluffs. The site was visited ten times over a period of one month in order to make detailed observations of the failure modes and processes active at this site. Geodetic analysis tools were used periodically to characterize bluff change in a more quantitative way. A general discussion of the modes and processes that govern bluff retreat will be presented and the potential for detailed geotechnical analyses will be discussed. The use of geotechnical analysis tools for predicting the one-time likely maximum retreat event will also be suggested and discussed and put in context with more typical models that calculate the average rate of retreat for predicting future coastline erosion. Support from the U.S.G.S. Coastal and Marine Geology Program is gratefully acknowledged.