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Monterey Bay Studies


Monterey Bay Studies

 

Monterey Bay National Marine Sanctuary Project, 1999

Retreat of a Weakly Lithified Coastal Cliff during a High-Rainfall Year, San Mateo County, California

by Monty A. Hampton, John R. Dingler, Thomas E. Chase, and Henry Chezar

Abstract

Landward retreat of a weakly lithified, sandy to muddy coastal cliff at Vallejo Beach, San Mateo County, CA during the high-rainfall season of 1994-1995 was primarily the result of landsliding, accompanied by debris flow, spring sapping, and gullying. The processes of retreat were influenced by physical properties of the cliff-forming sediment , surface runoff, and groundwater flow. Storm waves attacked the cliff occasionally, and their principal effect was to remove landslide debris from the base of the cliff rather than to undercut or oversteepen it and thereby induce slope instability, as reported for many coastal cliffs elsewhere. Landslides occurred in a variety of situations, mostly during or shortly after rainstorms. All had a steeply inclined failure surface, which maintains a nearly vertical cliff face, and were from a few inches to more than a meter deep. Some slides extended from the top to the bottom of the cliff (from 2 to 5 m), others first affected the lower meter or so and progressed upward, and yet others originated at the top of the cliff. Groundwater seepage near the base of a 1-m-thick surficial soil layer caused sapping and oversteepening that led to failure. Limited groundwater seepage from the base of the cliff caused slaking and small slides. Surface runoff, confined within shallow swales on the adjacent uplifted terrace, eroded large gullies into a limited section of the cliff and generated some debris flows. Between storms, some sections of the cliff sediment shrank slightly, which formed small prismatic blocks that tumbled from the cliff face. The cliff retreated up to a few meters during the rainy season, and despite areal variability of the processes and physical properties, the cliff edge remains relatively straight over its 500-m length.

This abstract (WPG# M96-0532) is from Monterey Bay National Marine Sanctuary Symposium, Sanctuary Currents '96, Building Community Connections in Science, Education and Conservation, Poster Session, 1996.


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