SUSPENDED SAND TRANSPORT ON A DISSIPATIVE BEACH

John P. Downing

Abstract


Field studies were conducted in November 1979 to measure suspended sand transport on Twin Harbor Beach, Washington U.S.A. This beach has an average slope of 0.02 and is composed of well-sorted sand with a mean diameter of 0.21mm. The significant height of breaking swells approaching the beach at small angles can be estimated from variance spectra of water level by 4.36 0^. In the surf zone significant wave heights are linearly related to the local mean water depth by Y = H /h » 0.45. Variance, a \, in the gravity wave band (0.048 to 1.0 Hz) is proportional to water depth squared. Low-frequency variance (less than 0.048 Hz) although not systematically related to water depth, is usually largest in the inner surf zone where breakers are small. Sand suspension is correlated with strong offshore flows that recur at about one-fifth the incident wave frequency. Vertical mixing of sand in the water column by these water motions rather than turbulence generated by shear at the bed associated with individual wave oscillations is a key mechanism in sand transport on dissipative beaches. The largest sediment loads occur in the inner surf zone where lowfrequency motions dominate the breakers. Maximum longshore transport rates, however, were measured in the middle of the surf zone because of the higher longshore current speeds there. The suspended load probably accounts for as much as 45 percent of the litoral drift on a dissipative beach exposed to moderately-high swells approaching the shore at small angles.

Keywords


suspended sand; sediment transport; dissipative beach

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