Bart Schultz
Water management in the Netherlands is carried out on two levels. The waters of national importance are managed by the State and the waters of regional importance by the (currently 21) Water Authorities. For centuries, the State and the Water Authorities have managed the water based on a preferred level and provisions for extreme dry and wet periods. There are a number of differences in the management in the low Netherlands and the high Netherlands, where "high" is a relative term, because we do not really have "high" in the Netherlands. As far as I know, the highest point in the country is 327.5 metres above mean sea level, namely on the Vaalserberg at the three-country point, which we share with Belgium and Germany.
If we first look at the history, then the houses in the lowlands of the Netherlands were generally built on higher ground and it was especially important for water management that crops could be grown in the period from April to October. When the land in the lowlands increasingly flooded due to subsidence and sea level rise, people initially started living on artificial mounds and around the end of the tenth century they started reclaiming land.
Another problem was to counteract the salt intrusion from the North Sea. That also led to a futuristic plan by Henric Stevin in 1667 to close off the sea inlets between the Wadden islands. In 1667 he wrote:
"First the North Sea will be separated from the Zuider Sea, by damming all the gaps of Staelduynen over Texel, Eyerland, Vlieland, Derchellingen and Ameland, and closing this off to Friesland. And install sluices in as many gaps as are necessary to discharge excess water into the North Sea at low tide. And with this the Zuider Sea will be kept at the lowest low tide, and (because salt water will never be let in and fresh river water will continually flow in) will be freshened in time: Yes, undoubtedly much ground suitable for reclamation. Secondly, before or after, the Y oort will also be stopped, making, to promote the navigation of the largest city Amsterdam, through Wykermeer and Beverwyc to the North Sea, harbours with shiplocks according to the New way of strong construction by means of shiplocks of our Father, and of the 11th book of this year. So that one can indeed sail out to the sea with ships according to the method of the 9th book, with all winds, even in a common storm."
That water management was focused on the summer period is also reflected in the first series of water management maps that were published between 1865 and 1890. These maps only mention the preferred summer levels and no winter levels. Of course, people tried to keep the water as low as possible in the winter, but with the pumping by windmills, higher water levels often occurred. Winter water levels were only really pursued when steampower pumping made this possible during about the end of the nineteenth century.
Throughout the centuries, people have always aimed for a higher water level in the peat polders than in the clay polders. Our ancestors also recognised the problem of subsidence in the peat polders quite well. This subsidence has been about one centimetre per year for centuries. The clay polders experience subsidence mainly in the first decades after their reclamation. In the Flevopolders, for example, this subsidence amounts to 1 to 1.25 metres. However, this subsidence process is finite and more or less stops about thirty years after the start of the reclamation. In the city of Lelystad, for example, the subsidence has almost stopped.
As far as water management at national level is concerned, after the dry summer of 1976 we developed a system to discharge water from the IJsselmeer, mainly to the west of the Netherlands, in order to keep the water in the polders at the desired level. In connection with this, a higher level is aimed for in the IJsselmeer in spring or at the beginning of the summer, in order to have a larger buffer available. This system has been improved over the years and has also functioned well in the past dry summers. Incidentally, under normal conditions the IJsselmeer has a preferred summer water level of 0.20 m-MSL (metres below mean sea level) and a winter water level of 0.40 m-MSL.
At regional and local scale, especially in the peat polders, there is often a summer water level of about 20 centimetres higher than the winter water level. This is used to aim for a more or less optimal level in both summer and winter. In the clay polders, the same level is generally preferred all year round. However, pre-drainage is increasingly applied if there is a expectation that a wet period is coming and the summer water level is set somewhat higher if there is a very dry period.
As far as the high Netherlands is concerned, there is indeed a problem in dry periods. There are weirs in the various streams and rivers to be able to raise the water, but where this is insufficient, it will probably be a costly matter to improve this.
