Abstract

Uncertainty and debate have existed as to whether suspension or bed-load transport is most important in the longshore movement of sands on beaches. The evidence pertaining to this fundamental question is examined. A model based on measured concentrations of suspended sediments in the surf zone indicates that the suspended load comprises some 25 percent or less of the total drift, the bed-load forming the remaining 75 percent. Such estimates are obtained even when all approximations and assumptions made in developing the model are made in favor of the estimate of the suspension load. An examination of the relative rates of longshore movements of sand tracer grains as compared to the longshore currents indicates that the sand grains lag far behind the water flow. This indicates not only that suspension transport must be small, but also that our 25 percent upper estimate of suspension transport in the model must be revised downward, possibly to less than 10 percent. Other evidence is also presented that indicates the suspension load is much less significant than the bed-load transport on beaches.

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