UC San Diego

Repeated aerial and ground-based observations that measured seasonal beach sand level changes along 195 km of coastline for up to 16 years were used to characterize and model Southern California beach sand level changes. A wave buoy network initialized a spectral refraction wave model that provided nearshore (~10 m depth) hourly wave estimates every ~100 m alongshore throughout the entire study region. Localized and regional biannual ground-based surveys captured seasonal subaerial beach profile changes across San Diego County, as well as the associated bathymetry changes not detected by aerial methods. Volume changes along the surveyed bathymetry transects varied seasonally owing to fluctuations in the incident wave energy. Comparative analysis of the volume changes along the beach profiles demonstrated estimating the fraction of subaqueous volume change based exclusively on subaerial observations is highly variable with a reduction in the total volume change captured by the subaerial observations particularly during periods of excessive erosion. Notably, the link between near-shore the displaced beach face sand volume (e.g. both subaerial and bathymetric profile components) and that of the subaerial beach is obscure.

Observations at five focus sites in San Diego County characterized short-term and long-term shoreline changes and waves, including the impacts of two significant El Niño winters, from 1997 through 2013. An existing wave-driven shoreline model accurately predicted shoreline fluctuations during years of normative wave and beach conditions, but over-predicted erosion during remarkable erosion events such as El Niño. Modifications to the model formulation account for erosion resistant features such as non-erodible seawall back beaches or durable strata (e.g. cobbles) exposed during severe erosion. This improved the model skill (i.e. data to model correlation R2) during highly erosive wave events.

High spatial resolution biannual aerial lidar surveys captured regional beach sand level changes spanning the entire 195-km study region. Alongshore variations in both waves and beach sand levels were notable with distinct large-scale differences occurring between the northern and southern halves of the study site. Additional ground-based data were used conjointly with the aerial observations to tune a regional shoreline model spanning 90 km of San Diego County coast.