30 



b. Mean current flows (in the Nova Scotia non-barred example, these 

 flows were offshore-directed undertows). 



Additional causes of non-barred inner shelf sediment transport include 

 swell, while additional causes of barred inner shelf sediment transport in- 

 clude high-frequency wind wave oscillatory currents. 



Osborne and Greenwood (1992) also differentiate between sediment 

 transport at different locations on the bar. On the lakeward slope of the 

 bar, a net offshore sediment transport component of mean currents results 

 from the offshore flow of undertow and group-forced bound long waves, 

 and the landward flow mechanism of wind wave oscillatory currents. This 

 is in contrast to studies on Padre Island, Texas, by Hill and Hunter (1976) 

 who show that net onshore bottom currents are dominant on the seaward 

 side of the bars and the bar crests under normal breaking wave conditions 

 of 0.3 to 1.0 m. On the bar crest, Osborne and Greenwood (1992) state 

 that there was no net transport of sediment due to a balance between off- 

 shore mean transport (undertow) and onshore net oscillatory transport (in- 

 teraction between both high- and low-frequency waves). Landward of the 

 bar crest and in the trough, although the wind waves decrease due to dissi- 

 pation of wave energy, suspended sediment transport by low-frequency 

 waves is most important, thus transporting sediment in a predominantly 

 onshore direction (Osborne and Greenwood 1992). 



Sediment trends 



Wright (in press), in a study at the Field Research Facility at Duck, 

 North Carolina, documented that the grain size of the inner shelf over the 

 upper 18 m exhibits a slight tendency to fine seaward (Figure 8). Fine to 

 very fine sand (D^^ = 0.09-0.13 mm) prevails, while silts and clays com- 

 prise 10-15 percent of the surficial sediment. This seaward-fining se- 

 quence is a result of decreases in energy in an offshore direction. 



Different magnitudes and properties of offshore versus onshore flow 

 across the inner shelf have resulted in the differential transport of fine ver- 

 sus coarse sediments. Smith and Hopkins (1972) state that during storm 

 events fine material is transported offshore, while coarse material is trans- 

 ported onshore. They documented that fine sand moves as suspended load 

 from the nearshore and is transported offshore during severe storms. Dur- 

 ing non-storm periods, both fine and coarse sand move onshore by wave- 

 driven bottom currents, which have a net onshore component. 



Basically, coarse material moves onshore due to the greater energy ex- 

 erted by the onshore-directed wave orbitals which are shorter, and exert 

 great velocities on the bed. Fine material moves offshore as suspended 

 load by the offshore-oriented orbitals, which are longer and of less energy. 

 Thus, the coarse material is moved onshore while the fine material moves 

 offshore (Wright et al. 1991). 



Chapter 3 Evidence of Cross-Shore Sediment Transport 



