|Title||Recent deep-seated coastal landsliding at San Onofre State Beach, California|
|Publication Type||Journal Article|
|Year of Publication||2015|
|Keywords||california; Coastal cliff; coastal erosion; Deep-seated; Landslide|
Airborne LiDAR collected during the period 1998–2010 and differential GPS surveys conducted over 2008–2013 show recent reactivation and movement of a large deep-seated coastal landslide at San Onofre State Beach, San Diego County, California. The overall slide complex extends about 700 m alongshore, 150 m inland, and an unknown distance offshore. Differencing digital elevation models and tracking field monuments (benchmarks) provide time series of quantitative topographic landslide changes and new insight in to the slide motion sequences and mechanics. The slide contains several distinct primary and secondary regions moving and deforming at different rates. Primary slide motion includes slow seaward translational motion, rotational slipping, and upward offshore movement. Secondary processes of basal wave erosion and new inland cliffline failures contribute to primary landslide destabilization. The landslide exhibits lithologic and structural controls, is driven by a combination of marine and subaerial processes, influences local beach morphology, and deviates from typical southern California coastal cliff processes which mostly involve shallow landslides and topples. Large-scale, cross-shore slide rotation has recently created new nearshore reefs. Eroded cliff sediments provide a local beach sand source and probably influence local nearshore ecosystems. All known time periods of major historical landslide activity were preceded by elevated seasonal rainfall and analysis suggests elevated rainfall generated primary slide motion as opposed to wave action. As of spring 2013, landslide activity has slowed, but continued positive feedbacks including toe removal by wave activity suggest that future landsliding will probably threaten coastal infrastructure.
"The landslide exhibits complex motion, periods of large-scale primary landslide activity triggered by elevated groundwater conditions, and contains several distinct regions moving and deforming at different rates. Secondary processes of wave toe erosion, head scarp (cliff-line) toppling, and localized slumping are positive feedbacks and possibly contribute to primary slide instability but probably did not directly trigger primary landsliding. Large-scale cross-shore landslide rotation produces transient reefs in the nearshore and possibly influences beach elevations by upthrust. Modeled subsurface baseflow provides a possible proxy for future primary landslide motion, but marine processes are important in destabilizing the overall slide mass."