Nourishing an unstable coast

Abstract:

Storms, sea level rise, and diminishing sand supplies threaten beaches worldwide. Nourishment, the mechanical delivery of off-site sand to the beach, is used around the globe, to protect coastal infrastructure from flooding and erosion, and to provide oceanfront recreational space for locals and tourists. After construction, wind, waves and currents redistribute the nourishment sand. Predicting the evolution of beach sand levels after nourishment remains challenging, especially for locations with complex coastal features and offshore bathymetry. Here we show that the subaerial alongshore evolution patterns of a nourished beach (Figure 1b), on an unstable complex coastline, could be predicted by observed multi-year subaerial sand volume trends (Ludka et al., 2019; Figure 1a) and time-mean alongshore gradients in the modeled Sxy radiation stress (O’Reilly et al., 2016; Figure 1c). Much of the relatively coarse-grained sand placed in front of a flood-prone erosion hotspot was transported to an accretion hotspot at the edge of a river mouth. Eventually this build up of sand contributed to the clogging of an estuary, causing deadly hypoxic conditions upstream (Ludka et al., 2018). Using wave model output (O’Reilly et al., 2016), we find that the hotspots are controlled by the orientation of the coast relative to a relic ebb shoal. This result suggests that assuming similar wave conditions, the hotspots will persist unless the coastal orientation is significantly altered, or hard structures are added. Placing more sand on the erosion hotspot, without addressing these underlying conditions, will likely increase the occurrence of river mouth closure. Globally, nourishments are often placed to mitigate erosion hotspots. This study provides a framework for characterizing wave-driven alongshore sand transport patterns on unstable coasts, to inform resilient solutions.

B.C. Ludka et al. (2018) Nourishment evolution and impacts at four southern California beaches: a sand volume analysis. Coast. Eng., 136, 96-105

B.C. Ludka et al. (2019) Sixteen years of bathymetry and waves at San Diego beaches. Sci. Data, 6, 161

O’Reilly et al. (2016) The California coastal wave monitoring and prediction system. Coast. Eng. 116, 118-132