Nearshore Suspended Sediment Load During Storm and Post-Storm Conditions

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Abstract
As part of the DUCK-X experiment at the CERC field research facility at Duck, North Carolina in September, 1978, suspended sediment measurements were made along the CERC pier. In situ bulk water samples were collected during a moderate northeast storm and two days later during post-storm wave conditions. Concentrations varied from approximately 0.01 g/l to over 10.0 g/l. Vertical arrays of suspended sediment samples indicated that concentration decreases rapidly up to two meters above the bed, then remains relatively constant, reflecting the nature of the suspension; intermittent suspension of sand near the bed, and continuous washload higher in the water column. Concentrations were at a maximum during storm conditions when measured values were 3 to 5 times higher than during non-storm conditions. The total load of sediment in a pier cross section during sampling periods in storm and post-storm conditions was calculated from arrays of 49 samples each. With H1/3 exceeding 2.3 m and the surf zone width over 300 m during the storm, the total load of sediment in suspension was approximately 10 times higher than during post-storm conditions (H1/3 = 1.2 m and surf zone width approximately 100 m). Estimates of the longshore flux of suspended sediment indicate that as much as 60 times more sediment was transported during storm than during post-storm conditions. Longshore transport of sediment measured from 5 cm above the bed to the surface reached the equivalent of 22,330 m3/day. This value corresponds very closely to longshore transport predicted from wave energy flux. During post-storm conditions, on the other hand, transport of suspended sediment accounts for less than one-third of the transport predicted from wave energy flux.

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