Continuous measurements of suspended sand concentration in a wave dominated nearshore environment

A miniature optical backscatter sensor (MOBS) and an electro-magnetic flowmeter were deployed seaward of the surf zone at Pte Sapin, New Brunswick during the first Canadian Coastal Sediment Study (C²S²), autumn 1983. The MOBS had sensing elements at five vertical locations above the sea bed, and was sampled at 10 Hz. The suspension of sand is well correlated with the passage of individual waves and also with wave groups, with the influence of wave groups progressively more dominant at higher elevations above the bed. Suspension appears to be stronger during the onshore directed phase of the wave motion than during the offshore motion and there is evidence that initiation of suspension may be determined by fluid acceleration more than velocity. Time lags between suspension events at different heights suggest that vertical gradients in sediment flux are more important than horizontal gradients.