Overall, during swell-dominated conditions, the bed was strongly agi- 

 tated at all times (suspended sediment concentration exceeded 1.0 kg/m^). 

 Findings indicated that the suspended sediment load is dominant over bed 

 load, and was directed onshore due to the landward-oriented incident 

 wave orbital motion. 



Storm-dominated processes 



Storm-dominated processes were measured during a 'northeaster' storm 

 at Duck, North Carolina, in 1985 (storm surge of 0.6 m; wave heights of 

 1-1.4 m, wave periods averaging 8 sec)(Wright et al. 1991). Sediment 

 transport prior to the storm was bidirectional but was net offshore during 

 the storm and was greater than that of fair-weather and moderate proc- 

 esses by an order of one to two magnitudes. This net offshore transport of 

 sediment occurred due to onshore winds, the resulting 0.6-m rise in mean 

 water level, and associated downwelling and offshore-directed bottom 

 mean flows. However, this offshore sediment transport is much less than 

 alongshore transport of sediment. 



During storm-dominated processes, suspended sediment concentrations 

 averaged above 1.0 kg/m-' throughout the study and were up to 4.0 kg/m^ 

 associated with wave orbital velocities up to 1.0 m/sec (Wright et al. 

 1991). During the height of the storm, suspended sediment concentrations 

 were 4,000 mg/1 at 14 cm above the bed; 1,400 mg/1 at 34 cm above the 

 bed; and 200mg/l at 106 cm above the bed. Although there was a relation- 

 ship between suspended sediment concentration and wave orbital velocity, 

 there was no relationship between suspended sediment concentration and 

 bed shear stress. The effect of the bed shear stresses on the bed (in order 

 of occurrence) included: 



a. Negligible changes in bed level response to the initial impulses of the 

 storm including wind, mean and oscillatory currents, and suspended 

 sediment concentration maxima. 



b. Gradual, but significant, scour of the bed of 5 cm during the storm 

 phase that followed the initial impulse. 



c. Initiation of accretion of the bed during the second and stronger peak 

 of the storm. 



d. Rapid accretion of the bed (15 cm) during the waning phases of the 

 storm (this accretion, the authors note, may be a migrating bed form 

 or offshore pulse-like migration of sediment). 



These bed level changes are believed to be associated with high-energy 

 wind waves, which cause mixing and mobility of the upper sediment col- 

 umn thus causing offshore-oriented sediment exchanges (Wright et al. 

 1991). 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



37 



