Bart Schultz
The 117,500 hectare management area currently consists of 72,000 hectares of polders that discharge their excess water into the collection system of Rijnland, which covers an area of 4,500 hectares of canals and lakes. There is also direct discharge and seepage from other areas into the system.
Originally, excess water from the management area was discharged through discharge sluices at Spaarndam. Later, sluices were added at Halfweg, Katwijk, and Gouda. In conjunction with the reclamation of the Haarlemmermeer Polder (1848-1852), which reduced the surface area of the collection system by approximately 18,000 hectares, steampower pumping stations were built at these four locations from the mid nineteenth century onwards. These pumping stations were, in turn, replaced by diesel and electric pumping stations at the beginning of the twentieth century.
Together, the pumping stations now have a capacity of 2,400 cubic metres per minute. This is equal to a daily discharge of a water layer of 15 millimetres counted over the total area of Rijnland. In this way, Rijnland can, in principle, keep the water level in the collection system well under control.
Over the centuries, the requirements for the water levels in the polders have become increasingly stringent. In the era of the windmills, the objective in the polders was to ensure that conditions between April and October were suitable for growing one crop. Higher water levels occurred in winter. Winter drainage actually only began in the second half of the nineteenth century, and was made possible by the introduction of steampower pumping.
During the draining of the Haarlemmermeer, the water level in the collection system of Rijnland fluctuated predominantly between 0.61 and 0.26 metres below Normaal Amsterdams Peil or NAP (Amsterdam Ordnance Datum). This led to many complaints. Therefore, since 1863, the pumping stations were set at a level of 0.28 metres below NAP from December to January, and 0.33 metres below NAP in the spring. During that period, water level exceedances in the collection system were common.
In 1940, a preferred water level of 0.60 metres below NAP was used, with a margin of 10-15 centimetres on either side. Currently, the preferred water level in the collection system of Rijnland is 0.61 metres below NAP in summer and 0.64 metres below NAP in winter. Current water levels deviate from this by only a few centimetres.
Due to increased urbanization in the management area of Rijnland, which causes excess rainwater to be discharged more quickly than from the original rural area, and the more extreme rainfall that occurs, the polder pumping stations discharge more water into the collection system, especially under extremely wet conditions. This places a heavier load on it. Therefore, there was a growing need for increased storage and/or pumping capacity for the collection system. For the time being, two so-called peak storage areas have been created, to which the water in the collection system can be discharged during extremely wet periods, and then pumped out after such periods. These storage areas are located in the Haarlemmermeer Polder and in the Nieuwe Driemans Polder.
The peak storage area in the Haarlemmermeer Polder consists of meadows on the southwest side of the polder and covers 67 hectares. It can store one million cubic metres of water, which equates to a water layer within the area of approximately 1.5 metres. The water is admitted from the canal on the south side and, when possible, discharged back into the main canal of the Haarlemmermeer Polder. From there, the water is pumped out by the polder pumping stations into the collection system, from where it is ultimately discharged into the North Sea.
The Nieuwe Driemans Polder is a nature and recreation area of over 300 hectares located between the municipalities of Zoetermeer, Leidschendam-Voorburg, and The Hague. The peak storage capacity in one section of the polder can store two million cubic metres of water, also based on a water layer of 1.5 metres. This brings the total storage capacity to over 2.5 millimetres counted over the management area of Rijnland, representing an increase of over 15%.
It's expected that Rijnland will be able to manage this for the time being. The pace of urbanization and climate change will determine when the next step will be necessary.
