Abstract

The shoreline along the southern portion of Monterey Bay California is undergoing severe erosion. Industrial and architectural sand has been dredged from this portion of the bay for the last 70 years. This withdrawal of sand coupled with weak to absent longshore contributions of new sand has been blamed for modern beach erosion. To determine the relative importance of potential sources to the sediment budget, 30 samples of 130-300 grains each were compared using Fourier grain shape analysis. Potential sources (including Flandrian and preFlandrian dunes, Salinas River, and offshore sands) can be discriminated on the basis of grain shape. Comparison of data from the beach samples and samples from the mining operations shows that most beach sand, offshore (shallow water) sands, and the mined sand are primarily (70%-90%) derived from Flandrian and preFlandrian dunes. North of the Salinas River, the beach is dominated by fluvial sediments. This study does not prove that sand mining is the major cause of erosion along the southern half of the bay. This analysis does demonstrate however, that recharge of sand onto the southern beaches from the Salinas River is not significant and so cannot be invoked to provide a mechanism to support the concept that the mined sand is a renewable resource.

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