Sand-mud model could predict effects of opening sluices

September 05, 2003

Dutch Ph.D. student Mathijs van Ledden has demonstrated that the often used rule of thumb 'the calmer the water, the finer the bottom sediment' is not always true. In addition to the flow rate, a range of other factors determine whether the bed consists of sand or is potentially muddy. This knowledge is relevant, for example, to the discussion about the opening of the Haringvliet sluices.

Researchers at the Delft University of Technology have developed a model that can simultaneously calculate the movements of both sand and mud in tidal areas such as the Wadden Sea and the Westerschelde. Existing models could only consider either sand or mud.

The researchers discovered that the frequently used rule of thumb 'the calmer the water, the finer the bottom sediment' is not always true. The sand-mud model revealed that there are locations in tidal areas where little sedimentation of mud occurs because the water flows too quickly. In addition to this, 'mud-poor' areas can arise because the water no longer contains any mud. Other reasons are the strong mixing of the bed due to physical or biological factors or developments in the preceding period.

One of the areas the researchers chose to test the new sand-mud model, was the tidal area between the river Waal and the Haringvliet sluices. The calculations from the model were found to closely agree with the measured sedimentation of sand and mud in the period following the construction of the sluices. These sluices were constructed in 1970, with the primary aim of protecting the hinterland against storm floods. As a consequence of this, the tide in the hinterland largely disappeared. A lot of sand as well as polluted and non-polluted mud has ended up there.

The researchers expect that in future the model can be used to predict the effect of human intervention in tidal waters. The recent discussion about the future management of the Haringvliet sluices is a good example of this. The Dutch Department of Public Works is considering whether to reopen the sluices in order to partially restore the pre-1970 situation. The new sand-mud model can provide answers to the various questions that are emerging in this debate. Will polluted mud layers, which are now safely under a thick layer of clean mud and sand, be released and damage the environment? If this is the case, where will the polluted sediment end up? And over what timescale will changes in the ecosystem take place?
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Further information can be obtained from Mathijs van Ledden (Delft University of Technology, now at Royal Haskoning, Nijmegen), tel. 31-24-328-4227, fax 31-24-360-5483, e-mail: m.vanledden@royalhaskoning.com. The doctoral thesis will be defended on 16 September 2003. Mr Van Ledden's supervisor is Prof. H.J. de Vriend.

An image can be obtained from www.nwo.nl/news. An animation of the sand and mud movements can be found at www.hydraulicengineering.tudelft.nl/public/ledden

The research was funded by the Technology Foundation STW.

Netherlands Organization for Scientific Research

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