1         Project description

1.1         Background

The Netherlands is a low-lying country, which is adjoining the North Sea and the main part of this land is located below mean sea level. Therefore, there is a constant threat of inundation, unless the coastal protection is sufficient. A natural ‘soft’ protection is present in the form of sand dunes, which developed during the past millennia. However, due to relative sea level rise and land subsidence, the safety level of the coastal protection should be adjusted to these changes.

To protect the Netherlands from the water, the Dutch government chose to ‘preserve’ the coastal system out of four possible options. The other options were coastline retreat, partly preserving the coastline or seaward extension of the coastline. Because of the fact that sand nourishments could be used, the decision was made to ‘’dynamically preserve’’ the coastal system. The Dutch government decided that the coastal system should be preserved as it was in 1990. ‘’Dynamically preserve’’ entails the concurrent rise of the coastal profile along with, for instance, relative sea level rise.

1.2         Coastal foundation

The coastal system consists of three parts, namely the coastal foundation, the Wadden Sea and the Westerschelde estuary. The coastal foundation is a body of sand along the Dutch coast, stretching from the Belgium border up to the German border. The inner boundary is located at the inner side of the dunes and extends to a depth of -20m NAP (outer boundary). The coastal foundation can, subsequently, be divided in three parts; the delta area in the South, the Holland coast and the Wadden Sea area. The Holland coast consists of an uninterrupted coastline, whereas the Wadden Sea area consists of tidal inlets. The southern deltaic area consists of closed-off estuaries. Figure 2.1 illustrates the coastal foundation area in the form of the yellow dashed area.

  
Figure 2.1

1.3         Research

The -20m NAP seaward border of the coastal foundation has been discussed a lot. The basic assumption is that beyond this border, no significant sediment transport on time scales of 50-200 years occurs, although a physical proof is not given yet. This seaward boundary has great influence on the maintenance strategy. With a reduction of this depth by only one or two meters, the volume of sand and therefore the costs to maintain the coast will decrease significantly. Additional, dredging costs will decrease as well, due to the fact that the distance to transport sand to the coast will decrease also.

In this research, the seaward extend of the active zone of sediment transport will be investigated. Longer time scales will be regarded to include very slow processes, which contribute to long-term, large-scale sediment transport. These processes can cause erosion  or sedimentation over long time scales, say decades to centuries. With this investigation, the validity of the -20m NAP will be investigated, which might have effect on the maintenance strategy. This will be done by investigating the response time-scale of certain depth-contours, starting close to the shoreline and extending further offshore.