California Sea Grant College Program

Seacliff erosion and retreat in California threatens public and private property, coastal infrastructure, transportation corridors, and public safety. Seacliffs also contribute sediment to California beaches, which drive the coastal tourism economy. Thus, understanding the processes which govern seacliff erosion is paramount in protecting the coast and mitigating erosional issues. This research effort, focused on the Oceanside Littoral Cell, builds upon past projects and provides new insight into seacliff morphological processes.

This study utilized airborne and terrestrial LIDAR (Light Detection And Ranging) and GIS (Geographic Information System) analysis to quantify short-term seacliff morphology and the effectiveness of erosion control methods. Additionally, the stability of cantilevered seacliffs was also explored using finite element analysis and elastic beam theory. Numerous field investigations and photographic surveys also provided indispensable information pertaining to erosional mechanisms.

It is difficult to make any long-term conclusions from the data given the relatively short study period. However, the results of the airborne LIDAR analysis indicate that seacliff sediment contributions were a significant sediment source of beach-sand in the Oceanside Littoral Cell for the study period, and that the relative percentage of annual seacliff sediment contribution was higher than found in previous studies. It was also found that converting volumetric erosion rates into linear retreat rates averages the erosion over the entire cliff face, thus reducing the episodic nature of cliff retreat measurements, and proving the efficacy of LIDAR in short-term studies. A comparison of cliff face retreat rates between unprotected and protected seacliffs indicated that erosion control methods that provided both lower and upper cliff protection performed better than methods which only provided partial protection. Additionally, the findings of this study suggested that properly controlling groundwater and surface runoff could reduce cliff erosion caused by subaerial processes.

Terrestrial LIDAR analysis demonstrated valuable applications at both a regional and site-specific level. The regional results illustrated that small erosional events, which may have previously gone unnoticed, may constitute a significant fraction of eroded material, underscoring the importance of evaluating short-term cliff morphology. The site-specific analysis of erosional hot spots, yielding individual failure volumes and profiles, allowed identification of landslide mechanisms, and permitted the back-calculation of soil properties from failure criteria. Recent advances in LIDAR technology and use are not yet fully exploited and will provide further understanding of seacliff morphology in the future.