In the pre-feasibility phase, a literature study supported the idea that an extraction site can be designed and operated in such a way that a good ecological status (species assemblage, high biomass and species richness) can be achieved after extraction. By taking the potential ecological response into account both during the early stages of the design phase and when determining the operational methods, enriched ecological conditions of the sand extraction site could be reached. This is especially the case if the seabed in the sand extraction site is landscaped, e.g. by creating bedforms such as large-scale sand waves or tidal ridges.
The literature study demonstrated that habitats will vary with seabed morphology, hydrodynamic conditions, water depth and sediment composition. In more detail, ecological surveys on tidal ridges and sand waves, as do occur in the subject area, showed differences in benthic community composition on the trough, slope and crest of the ridge (Baptist et al. 2006, Van Dijk et al. 2012).
Fig. 1. Relation between numbers of species and the location on tidal ridges (tops/crests, swales or troughs or the slopes (van Dijk et at. 2012).
A typical species for the crests of sand waves is the crustacean Urothoe elegans and for troughs the polychaete Spiophanes bombyx . A typical species for the shoreface-connected ridge area is the polychaete Nephtys cirrosa. The swale of the shoreface-connected ridge is typified by the polychaete Eteone longa (Baptist et al. 2006). The study of van Dijk et al. (2012) shows that marine habitats over two tidal ridges in the North Sea vary from low-density/low-diversity communities on the well-sorted sandy crests of the ridges to high-density/high-diversity communities in the poorly sorted muddy, gravelly sediments in the adjacent troughs (see adjacent figure Van Dijk et al. 2012).
Fig. 2. Hypothetical benthic fauna distribution at sand waves.
The troughs of sand waves which are characterised by fine sediment and higher mud content, biodiversity often appeared to harbour more species and a higher biomass (Fig. 3). In turn, the macrozoobenthic communities are an important food source to other species such as flatfish. Clearly, natural physical gradients due to the presence of bedforms have a positive effect on the overall biodiversity and biomass. Landscaping an extraction site to increase the variation in physical parameters such as morphology and hydrodynamic conditions is therefore expected to have a similar effect on the ecosystem in the area.
In consultation with marine ecologists, it was decided to test this hypothesis by incorporating this type of morphological features in an extraction site. Two pilot locations were chosen in the sand extraction site for the Maasvlakte 2 expansion. It was decided to make the gradients as large as possible to maximise any potential developments and to assess effects at the highest possible level of discrimination (accuracy and significance).