Until the end of 2020, the extraction was not allowed to go deeper than 5 meters below a bathymetric reference model. The Continental Shelf Service defined a new reference surface, based on scientific principles. Since 2021 this surface is the new limit for extraction. A number of criteria were taken into account when defining this surface:
- limited to the top sediment layer, to best preserve the integrity of the seabed;
- preservation of the structure of the sandbanks, based on their role in the protection of the Belgian coast;
- maximum use of the available sand in mobile structures such as sand waves;
- limited impact on hydrodynamic conditions.
Based on the available scientific data, reference surfaces were defined for each control zone. From these reference surfaces, maps of the available volume of sand are drawn up. These volumes are defined as the difference between the reference surface and the actual sea bed surface (bathymetry). The volume maps form the basis for setting the boundaries, within the various sectors, of the areas where the limit is reached and exploitation is not permitted. The definition of these areas is reevaluated annually, taking into account the most recent data.
The five control zones and the exploration zone with the subareas closed for extraction in 2022.