Geomorphological control of sandy beaches by a mixed-energy tidal inlet

Abstract

Coastal areas adjacent to tidal inlets are highly valuable areas for marine ecosystems where a wide range of social and economic activities can be found. These areas can be differentiated from open coast due to their specific morphological behaviour caused by a complex nearshore bathymetry and interactions between wave and tidal forcing. The understanding of these interactions is usually limited by the lack of a systemic approach and the scarce availability of comprehensive datasets covering subtidal, intertidal and supratidal areas at large spatial and temporal scales. Here, the complementary use of different bathymetric and topographic datasets and the development of innovative methods using satellite imagery, offers the opportunity to develop a comprehensive understanding of the timescales and the key processes involved in the dynamics of tidal inlets and its control on the downdrift coast. The use of satellite-derived bathymetric maps, computed over the last two decades, showed two sequences of 8 and 9 years where large sandy shoals migrated along the tidal inlet and welded onto the downdrift coast. Although the study period covers one of the most energetic winter over the last decades, the most significant sediment volume changes (±300 m3/m) observed along the three kilometres of beach located south to the inlet were mostly attributed to the migration and welding of these sandy shoals, whereas the offshore wave forcing was of secondary importance. It was also demonstrated that these migrating and welding events had an impact on the pattern of the nearshore bathymetry and sandbars down to 10 km south to the inlet. Primary welding events near to the inlet are associated to the formation of kilometre-long and alongshore uniform nearshore sandbars that subsequently migrate further down the coast causing secondary welding events. The ability to understand and define the spatial and temporal boundaries at which beach behaviour is controlled by a local tidal inlet gives the opportunity to develop sediment compartment approach in order to make accurate predictions of future beach behaviour.