SUSPENDED SEDIMENT TRANSPORT AND THE ROLE OF INFRAGRAVITY WAVES IN A BARRED SURF ZONE

Collocated optical backscatterance suspended solids sensors and electromagnetic flowmeters were deployed at Stanhope Lane Beach, P.E.I., Canada, to assess the contributions of wind waves, infragravity waves and mean currents to the cross-shore flux of suspended sediment across a barred surf zone. The local net transport of suspended sediment (magnitude and direction) was highly variable both spatially and temporally. With respect to the relative magnitudes of the suspended sediment transport: (1) that induced by ''quasi-steady'' mean currents and oscillatory currents at both incident and infragravity frequencies were almost equal in magnitude, when the total surf zone was averaged over the storm event; (2) waves in the near-infragravity band were quantitatively more important during the waning of the storm, especially where (when) surf persisted well after the initial decay of the offshore wave field; (3) incident waves were important where (when) the waves were not saturated i.e. during the early stages of the storm at all locations and persistently throughout the storm on the seaward slope of the bar, where ''wave saturation'' was limited to a very short period. With respect to the direction of net sediment transport: (1) that attributable to mean currents was directed offshore in the inner surf zone, probably reflecting the presence of an undertow, while further seaward, an onshore transport occurred, probably resulting from mass transport induced by surf bores, or drift velocities induced by the infragravity wave boundary layer; (2) that induced by oscillatory currents at incident wave frequencies was generally onshore at any one location throughout the storm event. In contrast, oscillatory transport at low frequencies changed direction (and also varied in magnitude) at any given location; in part these changes reflect the existence of significant oscillatory sediment transport at discrete infragravity frequencies. The standing infragravity waves consistently transported suspended sediment towards antinodes in the water surface elevation.