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

Collocated optical backscatterance suspended solids sensors and electr omagnetic flowmeters were deployed at Stanhope Lane Beach, P.E.I., Can ada, 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 (magni tude and direction) was highly variable both spatially and temporally. With respect to the relative magnitudes of the suspended sediment tra nsport: (1) that induced by ''quasi-steady'' mean currents and oscilla tory currents at both incident and infragravity frequencies were almos t equal in magnitude, when the total surf zone was averaged over the s torm event; (2) waves in the near-infragravity band were quantitativel y more important during the waning of the storm, especially where (whe n) surf persisted well after the initial decay of the offshore wave fi eld; (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 re spect to the direction of net sediment transport: (1) that attributabl e to mean currents was directed offshore in the inner surf zone, proba bly reflecting the presence of an undertow, while further seaward, an onshore transport occurred, probably resulting from mass transport ind uced by surf bores, or drift velocities induced by the infragravity wa ve boundary layer; (2) that induced by oscillatory currents at inciden t wave frequencies was generally onshore at any one location throughou t the storm event. In contrast, oscillatory transport at low frequenci es changed direction (and also varied in magnitude) at any given locat ion; in part these changes reflect the existence of significant oscill atory sediment transport at discrete infragravity frequencies. The sta nding infragravity waves consistently transported suspended sediment t owards antinodes in the water surface elevation.