Article

Mega-scale Sand Extraction from the North Sea

Predicting Impacts with Idealised and Engineering Models

By Pieter Roos, University of Twente and Jebbe van der Werf, Deltares, the Netherlands • November 2, 2010

A realistic future scenario or just science fiction? Imagine a ‘mega-scale’ sand extraction trench in front of the Dutch coast: 200 kilometres long, 10 kilometres wide and several metres deep. In their long-term sand extraction strategy, Dutch government coastal managers take this concept seriously. What is the impact of such immense human intervention on the dynamics of the North Sea? At a recent workshop at the University of Twente, experts gathered to discuss model results and share their views.

Extracting an annual amount of about 25 million m³, the Netherlands is leading in sand mining from offshore parts of the North Sea. The sand is needed for large-scale infrastructural projects, land reclamation and shore nourishments. These shore nourishments, performed either underwater in the nearshore zone or directly on the beach, are part of the Netherlands' coastal defence policy. To cope with sea level rise, the nourished volume is likely to increase considerably over the next decades to centuries: from an annual amount of 12 million m3 to about 80-100 million m3 (for an extreme sea level rise scenario). Long-term considerations of this repeated and growing sand extraction make the scenario of a ‘mega-scale' trench, gradually arising in front of the Dutch coast, a possible one (Figure 1). Rijkswaterstaat Noordzee (part of the Dutch Ministry of Transport, Public Works and Water Management), who adopted this concept, want to find out about the impacts of such an intervention on hydrodynamics, morphodynamics and ecology of the North Sea.

Knowledge Gap
The term ‘mega-scale' emphasises the unprecedented spatial extent of the extraction trench. Indeed, the much smaller pits currently being created e.g. for the extension to Mainport Rotterdam, with horizontal dimensions of several kilometres, are already referred to as ‘large-scale'. Most of the existing knowledge on the impacts of sand extraction concerns these large-scale pits (Roos et al. 2008). The effects for these pits, such as flow contraction and a gradual deformation of the pit itself, are restricted to the extraction area itself and its immediate surroundings. The mega-scale extraction scenario sketched above, however, may imply an unknown feedback on the tidal system of the southern North Sea as a whole. Below we describe two process-based model approaches to fill this knowledge gap.

Process-based Modelling
Process-based models are useful tools to assess the impact of the mega-scale trench on, for example, hydrodynamics and morphodynamics. Here, ‘process-based' means that the physical conservation laws of water and sediment motion are expressed in differential equations and then solved using mathematical techniques. Two approaches exist: idealised and engineering models. Idealised models, commonly used in the scientific community, are specifically designed to unravel physical mechanisms. To this end, the model domain and physical mechanisms are represented in a schematised way, while retaining the essential elements. Emphasis is laid on analytical and semi-analytical solution techniques, which generally lead to a quick tool. In turn, this allows for an extensive sensitivity analysis by systematically varying the model parameters. Alternatively, engineering models specifically aim at answering practical engineering questions. Unlike the idealised models, there is no need for schematisations of geometry and physics. Local geometric details and state-of-the-art formulations of the physical processes can be readily implemented. As a consequence, relatively complicated numerical solution techniques are required, making this approach more time-consuming and less attractive for extensive sensitivity analyses.

Idealised Model Results
De Boer et al. (2010) studied the impact of a mega-scale trench in the North Sea using an idealised model approach, focusing on the impact on tidal dynamics, a key aspect in hydrodynamics and morphodynamics. Here, the southern North Sea is represented as a rectangular basin, with the trench as a local deepening (Figure 2). It was found that a 20 billion m3 reference trench (224km long, 15km wide and with a mean depth of 6m), affects the tidal range by several centimetres, showing zones of increase and decrease, also well beyond the trench itself. Tidal flow generally displays acceleration in the trench (due to the reduced effect of bottom friction) and flow contraction at the southern edge. Changes of the order of cm/s and are felt roughly throughout the whole tidal basin (Figure 3). A sensitivity analysis with respect to trench geometry shows that the response is more sensitive to trench width and depth than to length. Apart from the obtained orders of magnitude, the main conclusion is that the mega-scale trench indeed feeds back onto the tidal system as a whole.

Engineering Model Results
In a separate study, Van der Werf et al. (2010) examined mega-scale sand extraction in the North Sea using the Delft3D simulation package, well-known in the engineering community. Their tidal flow results display the same physical mechanisms as the idealised study mentioned above; the larger changes are due to their reference trench being larger (Figure 4, below). In addition, they investigated the effects of wind waves, the implications for sediment transport and the local aspects of several extraction alternatives. For example, wave height increases over the trench (Figure 5, left). Finally, a local narrowing halfway the trench turns out to act as a sand attractor, which affects the coastal sand balance by extracting sand from the coastal regions south of this narrowing.

Workshop
Scientists, engineers and coastal managers gathered recently for a workshop at the University of Twente to discuss the idealised and engineering models presented above. Based on the agreement between the two types of results, it was concluded that idealised models help to explain the physics behind the more complex results obtained with engineering models. In addition, idealised models may serve as a quick tool for a pre-study of a wide variety of design alternatives. The most promising options can then be studied in more detail using an engineering model. Nevertheless, a more systematic effort is needed to bring these two approaches closer together. This can be done by performing engineering model simulations with the idealised model geometry, or by extending the geometry and physics of the idealised models. Finally, as noted by the participants of the workshop, the ecological impact of mega-scale sand extraction is a topic requiring further attention.

Acknowledgements
The authors would like to thank Wiebe de Boer for providing figures 1-3 and Leendert Dorst, Suzanne Hulscher, Thaiënne van Dijk and Ad Stolk for their comments.