Morphological expressions of coastal cliff erosion processes in San Diego county

High-resolution, Terrestrial Laser Scanning (TLS) data have been acquired seasonally since 2006 to define the style and magnitude of cliff erosion along the southern 20 km of coastline within the Oceanside Littoral Cell (OLC). In particular, twelve sites with cliff collapses were mapped repeatedly to examine how these collapses propagate along the cliffs and to identify feedback mechanisms between the liberated material and subsequent cliff failures. Grain size analyses of the failed material (retention cutoff) were performed to estimate the contribution to the beach sand inventory. Despite a relatively short time series (only six years) on a geologic scale, the high spatial and temporal resolution of the study has provided important insights into the fine details of processes controlling cliff erosion in the OLC. In addition, the seasonal TLS established a quantitative baseline from which future change may be assessed. Both lithological and environmental conditions are known to play a major role in governing the rate and style of cliff erosion; however, other factors such as beach width, elevation, and precipitation also exert control on rates and styles of cliff failures. The findings of this study reveal that cliff erosion is subaerially dominated where the beaches are wider and elevation is higher. Alternatively, erosion is marine dominated where the beaches are narrow and have lower average elevation. A direct relationship exists between beach elevation and undercutting and erosion along the failure edges and thus might provide a mechanism to create the observed linear retreat of the cliffs in the OLC rather than the formation of promontories and embayments. Other morphological expressions on the cliff face, such as honeycomb patterns and sawtooth-style frontage, indicate mechanisms that control predominant styles of erosion in particular locations. This time series documents seasonal and short-term erosional patterns and rates as well as establishes a baseline to understand cliff erosion in response to rapid sea level rise (>3 mm/yr)