Cross-shore and longshore transport of tracer pebbles on a macrotidal mixed sediment beach, Somme Estuary, France

Data on the transport of material on mixed sand and shingle beaches using newly designed synthetic pebbles has been. collected on a macrotidal beach at Cayeux-sur-Mer in northern France. Deployment occurred on three separate days at three locations: on the upper, the middle and the lower part of the beach, down to depths of ~ 30 cm. The recovery rates varied from 26% after one tide to 95% at the end of the experiments. The pattern of pebble behaviour after one tide on the lower, middle and upper parts of the beach seemed to be consistent. Pebbles injected on the lower and middle beach showed a clear longshore displacement whereas pebbles injected at the top of the beach were mostly displaced cross-shore (seaward). The distance pebbles travelled varied with the location in which they were placed, with pebbles from the upper beach tending to travel further than pebbles placed in the middle and lower parts of the beach. Wave conditions and water level also influenced pebble movement.