Quantitative regional observations of gravel and bedrock influence on beach morphologies

Geological features such as underlying bedrock and coarse sediment layers/deposits are known to influence sandy beach morphology, however existing observations are often spatially limited, qualitative, and/or focused on limited geological feature types. Using mobile terrestrial LiDAR data collected twice yearly from 2017 to 2021, this study quantitatively examines the influence of intermittently and/or semi-permanently exposed gravel deposits and bedrock on seasonal subaerial beach elevations along ~40 km of coast in northern San Diego County, California. Both seasonal beach elevations, surface gravel/bedrock exposure, and modeled nearshore wave conditions varied considerably in space and time. Sandy beaches with limited bedrock/gravel exposure generally had relatively large typical winter-erosion and summer-accretion elevation changes, while beaches with frequent bedrock exposure often had relatively small elevation changes and occasionally exhibited localized seasonal elevation change reversals (e.g. winter-accretion). Beaches with surface gravel often had relatively large foreshore elevation changes compared to the upper beach, and exhibited upper beach elevation change reversals (e.g. winter-accretion). The results also suggest gravel and bedrock influences can be interrelated. Localized seasonal elevation change reversals were also observed on beaches with large alongshore variation in radiation stress near submarine canyons. This study highlights the utility of spatially large-scale and concurrent quantitative mapping of beach elevation and multiple geologic feature types.