SUSPENDED SEDIMENT TRANSPORT AND MORPHOLOGICAL RESPONSE ON A DISSIPATIVE BEACH

This paper reports on some results from a field experiment conducted under highly dissipative surf zone conditions, occurring during a storm in Lake Huron, Ontario, Canada. Measurements of sediment resuspension and sediment flux were conducted at a number of stations across a nearshore bar. This bar moved approximately 25 m offshore during the storm, at a rate of almost-equal-to 2.5 m h-1. The maximum erosion depth over the former bar crest was 0.92 m, with simultaneous accretion on the lakeward slope of the bar reaching 0.63 m. During the early hours of the storm, the accretion rate over the lakeward slope was approximately 0.05 m h-1. Mean sediment concentrations were up to 6.4 g l-1, time series of sediment concentration were characterized by low-frequency fluctuations and the sediment resuspension was strongly constrained by infragravity waves. The major mechanism responsible for the offshore sediment flux was an offshore directed mean current, which reached a velocity of -0.34 m s-1. However, the reason for the erosion and the offshore migration of the bar was the strong spatial gradient in the cross-shore suspended sediment flux. This gradient was induced mainly by the infragravity waves, which transported sediment onshore at the former bar crest where the oscillatory flux balanced the mean flux, and offshore over the lakeward slope, reinforcing the mean flux at this location.