Bookmark and Share

Will the transport of sand and silt in the sea change?

Morphology studies the interaction among the movement of the water, the transport of sand and silt in the sea and the erosion and sanding and silting up that takes place along the Dutch coast and in the North Sea.

Silt transport mainly occurs in a strip about ten to twenty kilometres wide along the coast. The greatest sand transport takes place in the surf zone. This is the shallow foreshore where waves break on the coast. In deep water, sand transport only happens under the influence of tidal currents. Sand and silt transport in the sea occur cyclically and mainly in a northward direction. The direction of the sand transport in the surf zone depends on the orientation of the coast in relation to the incoming waves. Due to this sand transport, the coastline and the shallow foreshore change continuously. The coast may erode or sand up.

Effects of Maasvlakte 2 on morphology

Maasvlakte 2's construction affects morphological processes. The land reclamation forms a physical barrier to waves and to the tidal currents along the coast. This affects the transport of sand and silt in the sea. The position of the seabed can thus change in the longer term. Possible consequences of Maasvlakte 2 are:

  • More or less maintenance dredging work in the waterways from and to the port of Rotterdam (Euro-Maas channel and Maas Entrance). An accurate prediction of the future extent of this work is important because of the costs associated with it;
  • Changes in the necessary coastal maintenance in the area immediately north and south of Maasvlakte 2 (coast of Delfland, Voorne and Goeree);
  • A changed structure of the surface-intertidal area with channels, flats and banks to the south of Maasvlakte 2 (Haringvliet mouth), with possible consequences for the nature present. If the position of the seabed in the Haringvliet mouth alters, this will have consequences for the waves there. This could lead to reduced salt mist on the dune coast of Voorne and Goeree, which could affect the nature in the area.

Maasvlakte 2's seawall

Due to its more seaward location, Maasvlakte 2's coastline will be confronted with more severe wave and current effects than the existing Maasvlakte. Account has been taken of this in the design of the seawall that must protect the land reclamation against the sea.

For the most part, Maasvlakte 2 is getting a soft seawall. Just as along most of the Dutch coast, this consists of dunes with beach in front. This sand does not just stay there. The beach erodes due to waves and currents. Where this happens, filling up with extra sand is needed, what is known as sand suppletion. This method has been used along the entire Dutch coast since 1990. The timing and quantities of sand suppletion needed at Maasvlakte 2 are the subject of extensive study. The results are of great importance for the maintenance budget for the seawall.

The expected erosion of Maasvlakte 2's beach will be influenced by many factors. The final shape and layout of the soft seawall play a role in this. The sand used for the construction of the seawall and the suppletion is also of importance. The smaller the sand grains, the more easily they are washed away. So the larger the grain, the less maintenance. Relatively coarse sand is therefore being used for the soft seawall.

Erosion gully
Severe erosion of the seabed is expected off Maasvlakte 2's west coast due to the increased tidal currents. Due to this, what is known as an erosion gully will arise. Something similar happened after the construction of the port dams at IJmuiden. In theory, erosion gullies may become several tens of metres deep. The depth is currently between fifteen and twenty metres.

Whether and to what extent an erosion gully develops will need to be observed through study. If it becomes deeper than twenty metres, then, according to agreements, extra compensation will be needed in the seabed protection area being instituted to the south of Maasvlakte 2. The formation of an erosion gully may possibly be restricted by taking additional precautions, such as the local protection of the seabed through rock deposition.

Silt transport
Maasvlakte 2's construction also affects the large-scale transport of silt in the North Sea. In order to predict the ecological impact of this on the Dutch coastal zone, it is important to map out these changes in the silt economy carefully. The effects occur around the future land reclamation, and also off the coast near Voorne, Goeree, Delfland and further northward. It has emerged from previous studies that Maasvlakte 2 and the sand extraction for it may have an influence on the North Sea environment as far as the entrance to the Waddenzee. Rock-solid predictions of the exact effects are not possible given the current state of science. However, the Council of State stipulated that more clarity on the matter is needed. For this reason, additional research of Maasvlakte 2's effects on large-scale silt transport in the North Sea took place in 2005.

During the flood tide, part of this silt transport along the Dutch coast flows into the mouth of the port and the port basins behind it, where it then sinks to the bottom. To keep the port of Rotterdam at the correct depth, around eight million m³ of silt must presently be dredged annually from the Maas Entrance. It is apparent from existing research that the Maas Entrance and port basins will silt up less due to Maasvlakte 2's construction, and will therefore need less maintenance. The current pattern off the port will change due to the land reclamation. The silt concentrations immediately off the port entrance will thus change particularly during the flood tide.

Effect of sand extraction
Some hundreds of millions of m³ of sea sand are needed for Maasvlakte 2's construction. A separate study is concentrating on the possible effects of this sand extraction on life in the North Sea. When extracting sand with trailing suction hopper dredgers, a small percentage of silt is released, which is distributed around the sand extraction locations by the current. From an ecological point of view, it is important to know what silt concentrations are involved. What about the transparency and cloudiness, and where will the silt finally be deposited, covering the seabed life?

The sand extraction pits will also disturb the seabed for a longer period. Therefore it has been investigated what the best shape (position, depth, dimensions) of these pits is, from an ecological point of view. Usually, only two metres are excavated from the seabed, but for Maasvlakte 2 the pit will be excavated up to 20 metres deep. This will have the effect that less seabed will be disturbed.

Advanced computer models
Right from the initial design activities for Maasvlakte 2, many renowned institutions and consultancy bureaus have been working on the development and validation of a comprehensive set of model-based instruments to address morphological issues. The techniques, software and hardware used have evolved markedly during the last seven years. As a result, realistic predictions of the morphological developments in the longer term are now possible. The Netherlands is presently the world leader in modelling morphology and silt transport. Current research into silt transport in the sea because of the monitoring means further enrichment for the study field. This research is also of great importance when considering any even more detailed calculations in the future.