Bart Schultz
The water management systems in our polders are based on target water levels in the watercourses, which the water boards strive to maintain as much as possible. In some polders, this level remains the same year-round, while in others, a different target level can be maintained during summer and winter, with the transition typically occurring around April 1st and October 1st. With improved weather forecasting and available options, a more flexible level can be pursued where possible, taking into account, for example, expected dry or wet periods, or day and night electricity rates.
Within the polders, the target water levels are aligned with land use, with a broad distinction made between agriculture, urban and/or industrial areas, and nature and recreation. Originally, the polders were primarily used for agriculture. Urban development has been taking place in the polders, particularly since World War II. In recent decades, there has also been considerable development of nature and recreation areas. Therefore, in some polders, it has become increasingly complex to align the target water levels with land use as effectively as possible.
The functioning of water management systems is determined by the combination of excess water storage in the waterways and in the ground, along with pumping station capacity. When drainage was still managed by sluices and later by windmills, a relatively large storage area was required because, on the one hand, high outside water levels could obstruct drainage, or, on the other hand, too little or too much wind prevented the windmills from turning. During that period, roughly until the end of the nineteenth century, the land was often marshy or flooded during the winter months. Therefore, the first generation of water management maps, drawn up around 1860, only show summer water levels in the polders. With the introduction of steam pumping from the mid-nineteenth century, this gradually improved, and winter pumping was also introduced, which also included the winter water level. The storage area in the waterways, which often amounted to at least five percent of the polder area with wind pumping, could be reduced somewhat.
The introduction of diesel and electric pumping from the early twentieth century further increased the certainty of excess water drainage, allowing the surface area of waterways, especially in the clay polders, to be reduced to approximately three percent with a pumping capacity of approximately 8-12 millimeters per day over the polder surface. In the IJpolders and Flevoland polder, there is the added advantage of the cracked, heavy clay soils, where the cracks provide significant additional storage capacity, even exceeding that of the waterways.
Urbanization after World War II, in particular, resulting in accelerated drainage of paved surfaces, often leads to a renewed desire for larger storage areas and/or higher pumping station capacity. For example, in Almere-Stad, the Weerwater – 150 hectares – and the Leeghwaterplas – 23 hectares – have been incorporated into the watercourse system, resulting in relatively large storage capacities.
Thanks to the ongoing automation of polder drainage, improved control options tailored to fluctuating electricity prices, and improved weather forecasting, water management effectively tailored to land use is very well possible. Publicity often highlights the changing circumstances due to the impact of climate change. What, in my opinion, is often overlooked are the changes resulting from land subsidence and rapid urbanization in our polders. If these aspects are properly taken into account, sound water management in our low-lying country will be possible for many years to come.
