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Sediment transport processes on reflective beaches: field experiments in the Algarve
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This thesis presents a detailed description of new developments in the use of fluorescent sand tracers, a technique that offers good reproducibility, low cost and low enviromnental impact. A series of field studies were carried out on three Portuguese beaches (Garrão, Faro and Culatra, all in Algarve) to study longshore sediment transport processes, to assess sediment mixing depths and to compare numerical model predictions with field measurements. Tracer dispersal was studied collecting shallow beach cores and applying the Spatial Integration Method. All the
studied beaches had a reflective profile with a steep upper foreshore (tanfi of O. 10-0. 14) and a more gentle low tide terrace. In all experiments, plunging and collapsing breakers were observed on the beach face.
The longest field experiment was carried out at Culatra Beach over one and a half tidal cycles in October 1993 using sand tracers in conjtmction with wave and current monitoring. Variations of mean significant wave height during the experiment were limited (0.34-0.37 m), with mean zero-up crossing periods of 5.1-5.8 sec. Mean longshore current velocities in the breaker zone reached a peak in the second tide (0.28 m sec'1), while they were one order of magnitude smaller during the frrst (0.02 m sec'1) and third tide (0.04 m sec'l). The increase in current speed was due to a moderate wind that was blowing alongshore during the second tide. The average advection velocity of the tracer cloud and longshore currents showed a good correlation, leading to calculations of a much larger transport rate for the second tide (l.38x10'2 m3 sec'1)
than for the other two (0.23x10'2 m3 sec'1). The average depth of sand mixing was 10.6 cm on the beach face. It was 29% of breaking wave height and showed a marked uni-modal distribution. with a maximum of 15 cm in the breaker zone.