36 



processes, mean cross-shore flows were of greatest importance. There ex- 

 isted no relation between bed stress by instantaneous cross-shore velocity 

 and suspended sediment concentration. The mean cross-shore flow re- 

 versed with the tide. During high tide, weak offshore flows occurred, 

 while during low tides stronger onshore flows resulted. Bed load and sus- 

 pended load quantities were nearly equivalent. 



In fair-weather conditions, Wright et al. (1991) found that cross-shore 

 flows differed according to depth. Overall, flows at the -8-m depth tended 

 to be more energetic and had greater sediment transport rates by an order 

 of magnitude. At a depth of -8 m, suspended sediment transport, which 

 was dominated by mean cross-shore flows, was predominantly offshore. 

 However, these flows reversed direction more often than those at a depth 

 of -17 m. Conversely, at a depth of -17 m, a slight landward flow from 

 mean flow and oscillatory currents resulted. 



Larsen (1982) stated that offshore sediment transport on the shelf is a 

 slow but steady seaward motion of resuspended sediments. This contra- 

 dicted the conclusions of other researchers (e.g. Wright et al. 1991) who 

 stated that offshore sediment transport on the shelf occurred during a few 

 events with a strong offshore component. The time required to establish 

 steady flow conditions is approximately a tidal cycle offshore, but de- 

 creases to several hours at shallower depth at the inner shelf due to 

 friction. 



Moderate energy sediment transport 



Moderate energy processes, and related sediment transport, as studied 

 at Sandbridge, VA, in 1988, were dominated primarily by mean flows, inci- 

 dent wave orbitals, and tidal currents (Wright et al. 1991). The dominant 

 flow was oriented onshore (which may be a function of tidal currents and 

 upwelling from west winds during the study period). As in fair-weather 

 processes, there was little relationship between suspended sediment trans- 

 port and bed stress during moderate energy conditions. Suspended sedi- 

 ment concentration, which at times equaled 1.5 kg/m^, varied considerably 

 over the period. Tidal variation also occurred, as it did during fair- 

 weather processes. However, in deference to fair-weather process peri- 

 ods, weak onshore currents occurred during higher tides. 



Swell-dominated processes 



Swell-dominated processes, as measured at Duck, North Carolina, in 

 1988 (during wave conditions of H^ of 0.85-1.4 m and periods of 

 10-14 sec), resulted in overall onshore flow (Wright et al. 1991). How- 

 ever, many flow reversals occurred due to constant weak offshore-directed 

 cross-shore mean flows, which opposed high-frequency landward-directed 

 wave-induced oscillatory flows. These wave orbital velocities (maximum 

 of 0.5 m/sec) were the main source of bed shear stress. 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



