b. Swell characteristics also affect water drift, in that long-period waves 

 have onshore pulses which prevail temporarily, and thus cause net 

 onshore transport of sediment. 



c. Tidal surge asymmetry includes components of both onshore and 

 offshore sediment transport across the inner shelf. 



d. Tidal flux does not have a significant effect on surge asymmetry. 

 However, May (1979) found that 35 percent of the kinetic energy of 

 currents above the 30-m isobath in the Northern Middle Atlantic 

 Bight was at a tidal frequency, thus indicating the importance of 

 tidal currents in affecting sediment transport on the shelf. In 

 macrotidal environments tidal currents probably dominate the inner 

 shelf transport (Wright 1981). 



e. Wind affects the ratio for durations of current flow and bottom drift, 

 thus resulting in upwelling and downwelling flow. 



Cook and Gorsline (1972) and Trowbridge and Madsen (1984) discuss 

 the importance of sediment transport under asymmetric waves and related 

 orbital asymmetry in generating both onshore and offshore components of 

 cross-shore sediment transport. Also, time and space variations in bed 

 roughness when considering orbital asymmetry can affect both magnitude 

 and direction of sediment transport. Oscillatory currents over rippled 

 beds can cause a significant phase angle between instantaneous suspended 

 sediment concentration and instantaneous velocity, resulting in sediment 

 flux in a direction opposite to the net current or wave-induced mass trans- 

 port (e.g. Nielsen (1979)). 



Larsen (1982) also found that the net offshore transport of sediment on 

 the inner shelf is a function of the net offshore orbital asymmetry of 

 waves. Currents forced by the radiation stress of variable amplitude swell 

 (the higher waves suspending the sediments) are an important mechanism 

 in suspending sediments resulting in the cross-shore transport of sediment 

 on mid-continental shelves. 



Smith and Hopkins (1972) found that orbital asymmetry-created cur- 

 rents during wave shoaling are the dominant control of net onshore trans- 

 port of sediment, primarily of coarse material, on the inner shelf. 



Wave-current interaction. Grant and Madsen (1979a, 1986) theoreti- 

 cally discussed combined wave-current bottom fluid shear stress and 

 stated that the actual transport across the inner shelf is, in most cases, the 

 result of wave-current interaction. Effects of wave-current interaction on 

 the boundary layer include the following: 



a. Increases in rate of frictional dissipation of waves. 



b. Reduction in mean current speed near the bed. 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



27 



