Beach Nourishment and the Sediment Budget in Southern California

Bathymetry at four San Diego County beaches was monitored for between 7-15 years, with monthly surveys of the subaerial beach and quarterly profiles from the backbeach to 8m depth. During observations, each site was nourished with a subaerial pad extending roughly 50m cross-shore and as much as 1km alongshore (Figure 1a). During times unaffected by nourishment, on sandy reaches away from local perturbations (e.g. canyon, lagoon and relic shoal), elevation changes are highly correlated spatially (alongshore and cross-shore). Exploiting these patterns, elevation data is interpolated using objective mapping, and observational and interpolation error is quantified. Volume changes over time, the natural variability in the local sediment budget, of unknown origin, are sometimes larger than typical nourishment volumes.

Although wave heights were similar, the evolution of nourished beaches differed dramatically between sites. At a site with similar nourishment and ambient sand diameters, the initially subaerial nourishment was primarily transported offshore during the first winter storms, 6 months after placement. In contrast, the coarser-than-ambient nourishments were transported primarily alongshore (Figure 1), and a significant fraction of the larger grained nourishment sand remains on the subaerial beach 3 years later (Figure 1d). Seasonal changes in the sign of alongshore coast translation of the nourishment (Figure 1) are consistent with modeled alongshore currents, and the performance of various alongshore sediment transport models is being tested.