Field Experiments on Longshore Sand Transport in the Surf Zone

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Abstract
The short term longshore sand transport rate and its on-offshore distribution were measured in multicolor fluorescent sand tracer experiments performed in energetic surf zones. Agreement is found between the measured total longshore sand transport rate and a predictive expression due to Bagnold involving the incident breaking wave height and average longshore current velocity. This agreement includes one case where the current direction was opposite to that expected from the direction of the incident waves. The existence of low frequency motion of sand tracer alongshore, into the sea bed and other time dependent transport effects were clearly demonstrated in these experiments. This included significant pulsations in tracer migration alongshore over a line crossing the surf zone. The depth of tracer mixing was not constant across the surf zone, but exhibited a bimodal distribution with maxima in the swash zone and breaker zone in two experiments, and a distribution with a maxima in the swash zone in a third experiment. The on-offshore distribution of the longshore transport rate was also found to be bimodal in one particularly detailed experiment. A minimally model dependent definition of the depth of mixing is formulated based on the data obtained from a large number of core samples. Various aspects of fluorescent tracer experiment methodology, analysis and sampling equipment are also discussed.

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