Pebble tracing experiment at the Promenade des Anglais (Nice, France): A contribution towards beach management efforts

In this paper, the results of a short-term tracing experiment carried out at a beach compartment along the Bay of Nice (southern France) are presented. Nice urban beach is characterized by persistent offshore sediment loss issues that force the local administration to operate frequent artificial replenishments to maintain the current configuration, which also protects the well-renown Promenade des Anglais from high-energy events. As beach refills are quite expensive, the aim of the paper is to provide novel insights about the transport processes of pebble-sized tracers, which might support the Municipality to better adjust future interventions. Pebbles were tracked by means of the Radio Frequency Identification technology, largely used in such settings because of its reliability and efficiency. In addition, the morphology of the beach was monitored during the three-days-long experiment by airborne and ground topographic surveys, as well as the shape and the size of the tracers. Finally, a wave model was produced to simulate wave propagation in the nearshore, which validates the observed transport patterns. The results documented a low recovery rate (56%) 4 h after tracer injection, which is uncharacteristic considering that it jumped to 91% after the second survey, 24 h after the injection. At the end of the experiment (48 h), the recovery rate sank to 14%. These data were adequate to identify a few trends in pebble transport: tracer recovery rate in the swash zone was very low, while many marked pebbles were found at the step crest. Although this transport pattern may corroborate the offshore movement of the sediments, the topographic surveys revealed the destruction and re-formation of the fair-weather berm overnight, which would imply the presence of a shoreward transport under low energy wave conditions. While size did not single out any tendency, shape did: spheres rolled down the beachface earlier than disks; by contrast, disk-shaped pebbles moved for longer distances than spheres. These findings will be useful for local coastal managers because next beach fills will be planned and optimized based on the observed transport patterns. Though the selection of disk-shaped pebbles implies increasing costs, replenishments would be more efficient and fine-tuned for this sector of coast, which would ultimately save resources reducing negative impacts on the environment along the way. These considerations are valid for the Bay of Nice, but they might be useful wherever coarse-clastic beaches need recurring replenishments.