Effects of climate change and wave direction on longshore sediment transport patterns in Southern California

Changes in deep-water wave climate drive coastal morphologic change according to unique shoaling transformation patterns of waves over local shelf bathymetry. The Southern California Bight has a particularly complex shelf configuration, of tectonic origin, which poses a challenge to predictions of wave driven, morphologic coastal change. Northward shifts in cyclonic activity in the central Pacific Ocean, which may arise due to global climate change, will significantly alter the heights, periods, and directions of waves approaching the California coasts. In this paper, we present the results of a series of numerical experiments that explore the sensitivity of longshore sediment transport patterns to changes in deep water wave direction, for several wave height and period scenarios. We outline a numerical modeling procedure, which links a spectral wave transformation model (SWAN) with a calculation of gradients in potential longshore sediment transport rate (CGEM), to project magnitudes of potential coastal erosion and accretion, under proscribed deep water wave conditions. The sediment transport model employs two significant assumptions: (1) quantity of sediment movement is calculated for the transport-limited case, as opposed to supply-limited case, and (2) nearshore wave conditions used to evaluate transport are calculated at the 5-meter isobath, as opposed to the wave break point. To illustrate the sensitivity of the sedimentary system to changes in deep-water wave direction, we apply this modeling procedure to two sites that represent two different coastal exposures and bathymetric configurations. The Santa Barbara site, oriented with a roughly west-to-east trending coastline, provides an example where the behavior of the coastal erosional/accretional character is exacerbated by deep-water wave climate intensification. Where sheltered, an increase in wave height enhances accretion, and where exposed, increases in wave height and period enhance erosion. In contrast, all simulations run for the Torrey Pines site, oriented with a north-to-south trending coastline, resulted in erosion, the magnitude of which was strongly influenced by wave height and less so by wave period. At both sites, the absolute value of coastal accretion or erosion strongly increases with a shift from northwesterly to westerly waves. These results provide some examples of the potential outcomes, which may result from increases in cyclonic activity, El Nio frequency, or other changes in ocean storminess that may accompany global climate change.