Morphological trends of beach, dune foot, shoreface and adjoining seabed

The volumetric and geographical changes of dune attachment, beach, shoreface and adjoining seabed along the Flemish coast were determined and updated until 2019 on the basis of the regular topographical measurement flights of the part of the coastal barrier that comes dry at low tide, and the regular bathymetric recordings of the adjoining part below low tide up to approx. 1.5 km outside the dune base or sea dike. The observed changes are described using time graphs, trend figures, elevation maps and differences in altitude. The sand from embankments, nourishments and dredging discharges in the various coastal areas are accounted. In this way the morphological evolution and "efficiency" of the operations can be evaluated. The observed changes lead to hypotheses regarding the natural morphological processes and some recommendations for further monitoring and study work.

The beach and the dunes, taken as a whole of the Flemish coast, show a strong increase in volume. The observed increase since 1985 can be explained by sand nourishment works. Due to the sand supply, the volume is on average about 310 m³/m large on the entire coast than in 1985, which means a substantial improvement of the overall coastal safety. Virtually all sand supply has remained above low tide. At the current rate of sea level rise over the past 36 years, 22.5 m³/m increase would be needed to achieve growth in line with sea level, but the observed growth is more than 10 times larger. The shoreface experienced strong growth between 1992 and 2000, followed by a stable period. Most of the growth is natural and is located west and east of the outer port of Zeebrugge. The seabed loses sediment, increasingly from 2010 onwards.

The system Potje -Broers Bank gradually migrates to the east at an average speed of 15 to 20 m/year. Over the 30 km long strip between the IJzer estuary up to and including Wenduine, and further locally in Blankenberge and in Knokke-Zoute, all growth since 1983 is entirely due to repeated beach nourishments. Following the construction of the new port dams of Oostende in 2009-2010, the shoreface grows locally, both to the west and east of the groins. Due to the construction and maintenance dredging of the new channel to Oostende, lager volumes of sand leaves the nearshore system. Large growth zones are located on the shoreface to the west and east ("Plaat van Heist") of the Zeebrugge. However, the growth rate there has decreased since 2009. Deepen all tidal channels off the coast. Compared to 2000, the land subsidence is often 0.5 to 1 m, locally up to 2 m. The deepening has strongly accelerated from the year 2010.

The study shows the large interaction between the beach and the coastal marine domain. In the part under low tide, strong sediment transport occurs, as evidenced by the morphological changes that followed the expansion of the ports of Zeebrugge and Ostend. All observations point to sediment transport and morphological changes to the east. A hypothesis for the recently accelerated deepening of the tidal channels is related to the confining the marine currents into narrower channels. The sandbanks seaward of the tidal channels near the coast may slowly but systematically move towards the coast. The channels cannot move sideways towards land because the coastline is held by the beach nourishments. The current sections are therefore reduced in size, which causes the channels to deepen. This evolution will eventually lead to the undermining of the shoreface, which will be followed by beach erosion. This hypothesis needs further study. If confirmed, more investigations are necessary in order to intervene in the coastal system.

Principeschets van een sectie

The sections used to vertically delimitate parts of the active beach at the Belgian coast.