FIELD-MEASUREMENTS OF LONGSHORE SAND TRANSPORT AND CONTROL PROCESSES ON A STEEP MESO-TIDAL BEACH IN PORTUGAL

A field experiment was carried at Culatra Beach in Algarve (Southern P ortugal) to determine longshore transport rates and sand mixing depth on a steep (slope 0.11) meso-tidal beach. The experiment was undertake n over one and a half tidal cycles using sand tracers in conjunction w ith wave and current monitoring. Variation of mean significant wave he ight during the experiment was 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 first (0.02 m sec(-1)) and third tide (0.04 m sec(-1)). The increase in current sp eed was due to a moderate wind that was blowing along shore during the second tide. Average advection velocity of the tracer cloud and longs hore currents showed a good correlation, leading to calculation of muc h larger transport rates for the second tide (1.38X10(-2) m(3) sec(-1) ) than for the other two (0.23X10(-2) m(3) sec(-1)). Average depth of sand mixing of 10.6 cm in the beach face was 29% of breaking wave heig ht and showed a marked uni-modal distribution, with maximum of 15 cm i n the breaker zone. Previously published empirical formulae do not pre dict satisfactorily this behavior in depth of sand mixing that seems t o be peculiar of steep beaches under plunging waves. Empirical formula e were used to compute theoretical longshore transport and compare it with field observations. They all underestimated measured transport ra tes of about one order of magnitude, thus confirming that the morphody namics of steep beaches are characterized by relatively high sediment transport even in relatively low energy regimes.