Study of the long term morphology | CREST project

Study of the long term morphology

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Study of the long term morphology

Added on 2018-08-07
Present-day measurements and morphological observations in the CREST project areas Oostende-Mariakerke and Oostduinkerke-Groenendijk must be evaluated against longer term morphological trends and processes.


Picture 1. Pilot area Oostduinkerke-Groenendijk (author: Evelien Brand)


The Flemish Government – Coastal Division conducts regular morphological surveys since 1983 of the dune foot and intertidal beach area, and since 1992 of the corresponding nearshore. They consist of annual, sometimes semi-annual records of the elevation covering the same area completely and therefore allow the establishment of a reliable, relatively long-term time series of maps and related volume figures in horizontal elevation slices. Analogous maps and volume trends were also determined for De Haan, where soft coastal protection works similar to the scheme recently adopted at Oostende-Mariakerke have been carried out some 15 years earlier. De Haan therefore acts as a reference location.


Pilootgebied Oostende-Mariakerke

Picture 2. Pilot area Oostende-Mariakerke (author: Evelien Brand)


The coast at Oostende-Mariakerke and at De Haan suffers long-term structural erosion. In both cases, the tidal flow channel found just off the beach tends to deepen and to shift landward. This makes the shoreface steeper and narrower, and is followed by intenser storm erosion of the beach. At De Haan, the 1992-1997 large-scale beach and nearshore nourishments effectively repaired the coast. The erosion rates of the upper shoreface and beach have significantly declined since then and no significant storm erosion occurred. At and near Oostende-Mariakerke, the successive nourishments appear to obtain the same effect. The evolution since the beach and subtidal nourishment in 2014 seems to indicate erosion rates have dropped here too.




Graph. Hillshade view of the seawall (bottom of image), beach, shoreface and seabed of sections 102 to 107 (boundaries in pale pink) of 1 x 1 m DEM grid of autumn 2014 multibeam survey (upper part of figure, with contour lines of 0, -2, -4, -6 and -8 m TAW) and of 2 x 2 m DEM grid of LiDaR survey (lower part). Immediately seaward of the groin tips, a shallow trough with dunes separates them from the bar, created by the 2014 underwater nourishment. Then follows the shoreface slope (brighter) and the flat part of the seabed. This shows 0.3 – 0.4 m deep potholes in sections 104 and 108 and a circular 0.6 m high mound at the boundary of sections 102 and 103. Some areas of the seabed are completely flat, other areas show low-relief bedforms. Author: Rik Houthuys


The shore at Oostduinkerke-Groenendijk shows pluri-decadal growth. This is possible because the shoreface is wide and shallow. The seaward side of the shoreface is affected by mild, long-term erosion. This might eventually affect the beach at Oostduinkerke-Groenendijk. However, nourishments are taking place at the resorts of De Panne and Koksijde, located updrift of Oostduinkerke-Groenendijk. A part of the sand supplied there is taken offshore and may be returned by the coastal processes to contribute to temporary accretion of the shoreface at Oostduinkerke-Groenendijk.

The analysis documents these main results with exact figures and graphs.


For further information: Rik Houthuys

 


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