The transportation of sediment by passing waves in the offshore zone has been investigated using SCUBA equipment in the sea off southern California. It has been found that the total grain-size population exposed on a sandy bottom, and not selective size grades, is moved by wave-generated oscillatory currents. Threshold velocities for the transport of sand by individual surges are largely a function of ripple mark relief. The relation between the quantity of sediment placed in motion and surge energy is arithmetic. Even under turbulent conditions almost all sand transportation is within a few inches of the sea floor. The concentration of sand decreases logarithmically above the bottom. The suspension of sandy sediment depends on current strength and ripple-mark dimensions. Although the sediments may be moved preferentially in certain directions, ripple marks are stationary and do not migrate along the sea floor.

The oscillating currents caused by a wave regime are not regular but have a spectrum of velocities. Where the seaward pulses are longer in duration, the highest velocities may be associated with either onshore or

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offshore flow. In the absence of strong coastal currents, the net water drift along the sea floor may be either toward or away from the beach, and it is influenced by the wave climate, tidal flux, wind, and internal waves. Research currently is in progress to determine the relation of hydrodynamic parameters to directions of sediment migration.

Rip currents also are important agents of sediment transport on the inner continental shelf and they have been studied in the Los Angeles area. Their development is influenced by wave climate, beach morphology, tidal level, and wind. Large rip currents carry significant quantities of sediment seaward. Rips of all sizes winnow fine-grained, low-density sand from the surf zone and thereby concentrate coarse grains and heavy minerals on the beach. The sand transported by rip currents settles to the sea floor beyond the breakers where it is influenced by wave-generated surge.

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