c. Increases in bottom shear stress due to a combination of components. 



The importance of wave-current interaction in determining the magni- 

 tude and direction of sediment transport is also considered by Vincent, 

 Young, and Swift (1983). They found that when wave orbital velocities 

 and slowly varying bottom boundary layer velocities are combined, 

 stronger onshore combined flow results. Moreover, depending on bed 

 roughness and the horizontal angle between wave incidence and the mean 

 current, the vector resultant of the sediment flux may be opposite that of 

 the mean current. 



Wave groupiness. Wave groupiness is also an important factor of net 

 offshore transport of sediment across the inner shelf. Wave groupiness 

 causes space and time variations in wave amplitude and in radiation stress 

 (5 ). Thus, momentum balance requires that slowly time-varying mean 

 water level (T],) be depressed and elevated under high and low waves, re- 

 spectively (where S is greater and less, respectively). Variances in/ 

 cause a long-period infragravity wave. This infragravity wave has peaks 

 at low primary waves which result in onshore sediment transport (i.e. 

 shoreward values of/ (or the cross-shore long wave flow constituent)) and 

 troughs at high primary waves, which result in offshore sediment transport 

 (i.e. seaward values of/). Since the large primary waves in the trough of 

 the long wave suspend more sand (offshore-directed) than the small pri- 

 mary waves of the long wave crest, there is a net seaward transport 

 (Wright etal. 1991). 



Gravity-induced currents. Gravity-induced inner shelf offshore- 

 directed sediment transport (as stated by early references considering the 

 equilibrium profile concept (e.g. Cornaglia 1889, Ippen and Eagleson 

 1955, Bruun 1962, Inman and Bagnold 1963) occurs due to the slope of 

 the inner shelf being oriented in an offshore direction. This gravity- 

 induced offshore transport of sediment is accentuated where fine-grained 

 sediments are present, since these types of sediment can be easily sus- 

 pended, especially during storm events. 



Seymour (1986), in studying different models of turbidity currents and 

 their relation to inner shelf transport, confirms that these currents trans- 

 port nearshore sand in an offshore direction during storms. 



Wright et al. (1991) noted that gravity plays a significant role during 

 high-energy events when bed shear stress and suspended sediment concen- 

 tration were greatest. If a density current develops, and the sediment is 

 suspended at a greater rate than it is deposited, an autosuspending 

 offshore-directed turbidity current can form. Kobayashi (1982), who de- 

 veloped a model for net downslope sediment transport by oscillatory 

 flows acting on a gentle slope, found that gravity-induced offshore- 

 directed transport of sediment is significant. 



28 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



