Mixed sand and gravel beaches of Buenos Aires, Argentina. Morphodynamics and stability

摘要

Coastal erosion is a critical problem that affects several countries around the World that would increase concerning the anthropogenic sea-level rise. Gravel beaches have proved to be more stable facing coastal erosion. The mixture of sand and gravel has been used to replenish beaches as a soft mechanism that increases beach stability. Most of the Buenos Aires coastline is composed of sand, although some beaches are composed of mixed sand and gravel. These beaches vary spatially and temporally in the sand-gravel proportion; the gravel fraction is mainly composed of volcanic lithics derived from Patagonia, with caliche and bioclasts of local contribution. Six mixed sand and gravel beaches (MSGB) with diverse grain size and sand-gravel rates were monitored to evaluate the morphodynamic changes and grain-size variability in cross-shore profiles throughout a year (May 2016-April 2017). Also, the wave conditions were analyzed to identify high-energy wave events using ERA5 dataset. A general behaviour of the six beaches was discerned, where the gravel fraction increases significantly in the seasons with more high-energy wave events (autumn and spring), especially in the upper-foreshore. In these periods, the gravel-runs formation and the sediment armouring were described. In summer times, an increment in the accretion of the cross-shore profiles and sand fraction proportion were recorded. The increase of gravel proportion, gravel runs development and armouring processes are some responses of MSGB to high energy conditions that cause greater stability.