Dynamics of mud banks and sandy urban beaches in French Guiana, South America

Abstract

Beach rotation is a widely described process characterized by periodic alternations in sediment transport involving erosion at one end of the beach and accretion at the other. The 1500-km-long coast of the Guianas, South America, is a unique system dominated by large migrating mud banks, muddy, mangrove-rich shorelines, and rare sandy beaches. Interactions between waves and the rare beaches on this coast are affected by the mud banks which are separated by ‘inter-bank’ areas. Kourou beach is situated near the site of the European Space Agency’s satellite-launching pad in French Guiana. The beach has maintained multi-decadal stability, but its interaction with mud banks has led to phases of severe erosion. To understand these changes, which constitute a risk for the urban front of Kourou, we combined a mesoscale temporal (1950–2017) analysis of shoreline fluctuations with a short-term approach based on photogrammetric monitoring of beach change conducted in 2017–2018 and on bathymetric surveys of the nearshore zone. The results show that Kourou beach evolves in a context of ‘rotation’, a process involving periodic alternations in beach erosion and recovery. Rotation is characterized during inter-bank phases by ‘normal’ sand transport to the northwest generated by the prevailing NE waves, and during mud-bank phases by drift reversal to the southeast generated by refraction of these waves at the leading front of a bank. Due to the aperiodic nature of these bank and inter-bank phases, erosion and accretion involved in beach rotation may prevail over variable periods of time (several years to decades). The large mud banks migrating from east to west first protect the southeastern sector of the beach, blocking the ‘normal’ northwestward longshore sand transport, but generating, through differential refraction, southeastward counter-drift. These processes and the irregular timescale of beach rotation they entail have not been compatible with the recent urbanization of the beach front in the southeastern sector, resulting in erosion and a sense of threat to beachfront property. Insight gained from an understanding of the rotation process and its irregular timescales should contribute to better beach-front management.