UC Santa Barbara

ABSTRACT

Landslides: Geomorphology and Sea Cliff Hazard Potential, Santa Barbara – Isla Vista, California

by

Julia Klath

Coastal areas are often characterized by high population densities and variable geologic formations. Over 80% of the world’s coastal regions are dominated by steeply sloping surfaces (sea cliffs) that are subjected to various erosional and geological processes. Due to the ever changing nature of these areas, a deeper understanding of how these surfaces have changed in the past may enable populations to anticipate future behavior and discover more effective ways to mitigate future coastal hazards. In this study, mapping and analysis of local bedrock morphology and petrology focuses on further understanding the relationship between bedrock lithology and landslide frequency and volume. Using field mapping techniques in conjunction with digital maps and non-parametric comparative statistical methods, a series of landslide characteristics, including landslide volumes and areas, compressive rock strength surrounding landslides, average cliff heights, and bedding dips around landslides, have be collected and analyzed. Four geologically distinct areas exist along the coastal reach between Santa Barbara and Isla Vista, dominated by the Sisquoc shale and subunits of the Monterey Shale. Each unit displays varying lithology, and as a result, each area experiences weathering and failure in different ways.

The underlying lithology and structure of the region influence the nature and extent of landslide activity >100m3. Generally it has been found that in sections with beds dipping south, towards the ocean, at >30º there are more landslides per unit area. Based upon sea cliff profile analysis it has also been found that sections with greatest cliff height, >35m, also have distinctive profiles where weathering of rocks above is prevalent, with cliff bases maintaining some bedding structure. The orientation of bedding as well as the dominance of either Si or Ca rich interbeds has an impact on landslide activity. Generally, Ca rich interbeds are more susceptible to weathering than Si rich beds. Lastly it has been noted that when measuring compressive rock strength with a Schmidt hammer, return values may be more indicative of the degree of weathering of the bedrock around a landslide rather than a measure of actual rock compressive strength. Significant variations in return values occur between the cliff base and as little as 0.5m higher up the cliff face. Average strength of cliff bases is recorded for each section and show the lowest values for quaternary units and areas where the bedrock is highly weathered, with higher values occurring where wave action has removed talus materials from the area and has direct access to the cliff bases.