Understanding long-term beach width change in the Oceanside Littoral Cell, California

The Oceanside Littoral Cell spans southern Orange and northern San Diego counties of southern California, extending from Dana Point in the north to the Scripps-La Jolla submarine canyons in the south. The Oceanside Cell contains some of the most intensively used beaches in southern California. Beaches in this area drive the tourism economy as well as provide valuable habitat to coastal wildlife and protection to coastal development.; The main sources of sediment to the Oceanside Cell beaches are: fluvial sediment from coastal rivers and streams, seacliff erosion, and beach nourishment. Long-term sediment supply has changed as a result of dam construction, coastal armoring, and beach nourishment. The combined result of these actions, however, is that sediment supply, on average, is actually higher today than it was under pre-existing natural conditions.; Inter-annual variability in sediment supply to the Oceanside Cell occurs in response to climate fluctuations as well as beach nourishment projects. The volume of sediment supply to the Oceanside Cell from rivers and streams and seacliff erosion, in general, is much greater during wet years or years during El Nino dominated climate conditions.; To assess the impacts of changes in sediment supply, beach width was measured for the years of 1946, 1955, 1963, 1975, 1980, 1986, 1996, and 2001 in the southern half of the littoral cell. Measurements were made using georeferenced historical aerial photographs and the Digital Shoreline Analysis System (DSAS) extension in ArcView. The littoral cell was divided into 15 sections and beach width was calculated as the distance between the wet/dry line and the back beach. Beach width was measured at 50 m intervals for 47 km of coastline over 8 years yielding over 7500 measurements of beach width alongshore for the period of 1946 through 2001.; Beach width has responded to changes in sediment supply by oscillating, but little net long-term change was seen. Most of the significant changes in beach width resulted from direct human activity on the beach such as beach nourishment, harbor construction, or placement losses due to houses or other buildings built directly on the beach.; A relationship was not seen between climate regimes, such as ENSO and the Pacific Decadal Oscillation. Relationships were found between factors associated with these climate patters, such as beaches narrowing in response to increased wave height and shifting direction of approach and beaches widening in response to increased precipitation yielding increased sediment supply from rivers and cliffs.; Three oscillations in beach width were seen that were not directly related to human activity. First, beaches were wider than normal in 1986. This was probably the result of increased sediment supply from the San Luis Rey River and the San Dieguito River during the large flood events and large episodes of cliff retreat of 1979-1980. The narrowest beaches of the period were during 1980 and 1996, probably both as a result of increased wave activity immediately preceding those periods.; Overall, in the absence of large inputs of sediment (> 1 million m 3) to the beach, beach width remained relatively stable. Narrow cliff-backed beaches fluctuated within a range of +/- 10 m of their long-term mean beach width while wider lagoon-fronting beaches fluctuated within a range of +/- 15 m of their long-term mean beach width indicating that beaches tend to stay within an equilibrium range of beach width.