Bart Schultz
Well-known collection systems include those of Delfland, Friesland, the North Sea Canal, Rijnland, Schieland, and the Schermer. The IJsselmeer also functions as a collection system for the IJsselmeer polders, before the water is discharged into the Wadden Sea through the discharge sluices in the Enclosing Dam or through the Oranje discharge sluices to the North Sea Canal.
The watercourses in the collection systems often flow at a high level between the dikes along the adjacent polders. Due to the land subsidence that is still occurring, especially in the peat polders, these dikes are increasingly rising above the adjacent land. This is particularly evident in the areas of the Water Authorities of Schieland and Friesland, where most of the polders with peat soil are located.
The Netherlands has a total of 14,000 kilometres of regional dikes, many of which are dikes along the collection systems. Due to their extensive total length and lower safety standards than for primary dikes, these dikes pose the greatest relative risk of failure. The risk of fatalities is relatively low, but flood damage can be considerable, especially if there are urban areas in the polder in question. Some examples of failures of dikes along collection systems include Tuindorp Oostzaan in 1960, Hazerswoude in 1964, Dubbele Wiericke and Lisserpoelpolder in 1967, Aarlanderveen in 1995, Wilnis and Terbregge in 2003, and Reeuwijk in 2021.
In these systems, the preferred water level of about half a metre below mean sea level has often remained more or less the same for centuries, although this level can now be maintained better than before thanks to supplementary pumping. For example, in IJmuiden, there is a large pumping station for the discharge of excess water to the North Sea, and in Gouda, Katwijk, Halfweg, and Spaarndam, there are pumping stations for the collection system of Rijnland. Near Lemmer, there is a pumping station for pumping excess water from the collection system of Friesland.
A new pumping station – the largest in the Netherlands – is currently being built in the Enclosing Dam to pump water from the IJsselmeer to the Wadden Sea. However, there are also many other pumping stations for pumping water from the collection systems. Examples include Abraham Kroes for Schieland and C. Mantel for the Schermer.
To be able to supply water to the polders during dry summer periods, there is now also a system whereby water can be transported from the IJsselmeer through the inland watercourses to the west and northeast of the Netherlands and from there it can be admitted to the polders.
These often centuries-old systems allow excess water from the polders to be discharged to the open water within the prescribed limits and can now be supplied during dry periods. Responsibility for this lies primarily with the Water Authorities and, in addition, with Rijkswaterstaat. If these agencies continue to adequately anticipate changes in land use in the polders and the impact of climate change, sound management of the collection systems will certainly be possible for the centuries to come.
