Coastal Dispersal in the Southern California Bight

A good understanding of the transport and dispersal of material in the coastal environment is important for several biological processes such as larval transport and the transport of nutrients to kelp ecosystems. The Regional Ocean Modeling System (ROMS) is able to simulate the coastal circulation with very high horizontal resolution (250m) allowing a robust characterization of the transport and dispersal of material close to the shoreline.

In this study, a statistical analysis of the dispersal of particles near shore was carried out in the Southern California Bight. The particle dispersal was investigated with respect to the distance from the shore, coastal geometry (bays versus headlands), and direction relative to the coastline.

Several synthetic particles (~1000) were released every 12 hours near the ocean surface over several release sites and tracked for about five days. The particle releases were repeated continuously for about three months, yielding robust statistics of the separation or dispersal between pairs of particles with a small initial separation. The results give mean dispersal rates in the along- and cross-shelf direction, respectively, generally increasing with increasing the distance from the shore and more strongly for the cross-shelf component. Headlands consistently exhibit larger dispersal rates than bays regardless of direction. Close to the shore, both bays and headlands show dispersal rates along-shelf on average 10 times larger than those in cross-shelf direction.

The reduced dispersal rates in bays versus headlands and the preferred dispersal along the shelf have important implications for the transport of nutrients from deeper ocean to nearshore waters as well as the retention of storm water runoff close to the shore. This was also investigated numerically with experiments of storm water runoff within the Santa Barbara Channel. The simulations have shown that the fresh water plumes are generally retained nearshore and spread on average 10 times faster along the shelf than in the cross-shelf direction, which is qualitatively consistent with the two-particle statistics described above.

Future work will investigate the effects of surface waves on the dispersal rate of particles and fresh water runoff within the nearshore environment.