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Abstract

The identification of sand sources and the volumetric contribution from each during a given time interval is essential to our understanding of coastal processes and the computation of meaningful sediment budgets for the littoral zone. Fourier grain-shape analysis (FGSA) of detrital quartz from the medium-sand fraction of 83 fluvial, seacliff, beach and inner continental-shelf samples was performed to identify and quantify the contributions from local sand sources within the Oceanside Littoral Cell, which occupies the coastal reach between Dana Point and Point La Jolla in southern Orange and San Diego Counties.

Differences in shoreline orientation, physiography and the occurrence of Oceanside Harbor may be used to divide the Oceanside Littoral Cell informally into five segments. These are, from northwest to southeast, the Capistrano subcell (Dana Point to San Mateo Point), the San Onofre subcell (San Mateo Point to the north breakwater at Oceanside Harbor), the Oceanside-Carlsbad subcell (south jetty at Oceanside Harbor to the onshore projection of Carlsbad Submarine Canyon), the Encinitas subcell (Carlsbad Submarine Canyon to La Jolla Submarine Canyon), and the La Jolla Cove subcell (La Jolla Submarine Canyon to Point La Jolla). FGSA demonstrates that each of these subcells is compositionally distinct with regard to grain-shape populations, which, in part, reflects a characteristic set of local sources supplying each subcell.

Although shape-compositional differences occur between and among the five subcells, the shape composition within each cell is rather uniform with the possible exception of the Santa Margarita River sand in the San Onofre subcell. This homogenization may reflect sediment mixing as a result of seasonally related, bidirectional, longshore and onshore transport during a relatively recent but undefined time interval.

With the probable exception between the San Onofre and Oceanside-Carlsbad subcells since 1942, an appreciable amount of sand has been exchanged naturally between each pair of adjacent subcells. Although the reason(s) is (are) not clear, the northernmost two subcells are dominated by downcoast and southern shelf sources associated with each subcell, the central subcell appears somewhat transitional with regard to upcoast and downcoast sources, and the southernmost two subcells are dominated by upcoast and northern shelf sources. Continued monitoring is required to estimate the volumes involved and the rates of exchange, which will determine the effectiveness of the subcell boundaries as barriers to littoral-sand transport.

An average of approximately 36 percent of the quartz in the medium-sand fraction of the foreshore samples was derived from the inner continental shelf. Although it seems likely that storm activity, perhaps coupled with shoreward transport through ridge and runnel systems, may be involved, fundamental questions concerning the mechanism, volumetric aspects, rates of transport and transport pathways remain unanswered.

Weighted local source estimates supplied by FGSA, grain-size and mineralogic analyses must be integrated with mass-balance computations to improve sediment budget analysis and to constrain sand-transport pathways to and within the littoral zone.

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