Evolution of submerged large transversal bedforms in a shallow nearshore area along a macrotidal sandy coast

The nearshore area of many sandy beaches exhibits morphological patterns which are rhythmic along the coast. Most studies on submerged sandy bedforms have focused on their morphodynamics in the deeper part of the nearshore due to operational limitations when performing bathymetric measurements. Along part of the macrotidal Belgian west coast, a remarkable field of coast-normal submerged dunes is found in a very shallow area that partly emerges at low tide.

For the first time, the morphology, volumetric budgets and evolutionary trend of these large transverse submerged dunes have been investigated using high resolution multibeam bathymetric surveys performed over an observation period of 9 years and returned at intervals from daily to a few years.

The morphological analysis indicates the presence of seven large transverse sharp-crested sandy dunes with a height of 1–2 m and a spacing up to 200 m, extending from the beach to 2.7 km offshore. Their morphodynamic patterns show a remarkable rhythmicity along the coast with a persistence over time. The large dunes are an integral part of the coastal system contributing to the long-term natural beach accretion. They migrate at a rate of 36 m yr⁻¹ in the direction of the longshore sediment transport towards NE mostly driven by flood currents. The morphology of the large dunes is also controlled by storm-wave action depending on their depth below sea level. Storm-waves do not influence the dune mobility. Their morphology is forced by the bathymetric context as well as the presence of coastal engineering structures such as groynes.

The morphodynamics and evolution of the large transverse dunes suggest complex morphosedimentary interactions of a shallow nearshore area, where a constant large natural sand supply interferes with strongly fluctuating coastal hydrodynamics.