Bart Schultz
Just think, for example, of the many new residential areas in the west of the Netherlands, in and around other major cities, and not least in the IJsselmeerpolders. Many new residential areas will also be built in polders in the coming decades.
Originally, when developing new urban quarters in polders, the land was usually raised to a certain level above the water level in the adjacent collection system. Consider, for example, the Bijlmermeer and several other new urban quarters in and around Amsterdam.
An exception was the drained lake Watergraafsmeer on the east side of Amsterdam, where urbanization took place both before and after World War II. I once attended a lecture by filmmaker Louis van Gasteren, who, among other things, made the documentary Een zaak van niveau (A Matter of Level), which explains the workings of the Normal Amsterdam Ordnance Datum (NAP). He began his introduction with the words:
"I was conceived four and a half meters below sea level."
He was born in the Watergraafsmeer.
Around the 1970s, when high sand prices and the size of the required dikes made raising the surface to a certain level above the water level in the collection systems prohibitively expensive, a start was made with raising the surface level in the new urban quarters to a certain level above the polder water level. One of the problems encountered was that excess water from the new urban quarter could no longer be discharged directly into the adjacent collection system, but was instead discharged into the polder water. Because of the faster discharge from the new urban area, the polder's drainage system generally had to be reinforced.
Another problem is the soil subsidence that is generally expected after raising the surface level, which can be particularly significant in peat polders. Because the buildings are built on concrete poles and the adjacent ground is subsiding, the connections of cables and pipes to the buildings must be flexible.
One advantage of the subsidence around buildings is that excess rainfall doesn't cause problems in the houses, which become relatively high compared to the surrounding area. As a resident of one of the older houses in Lelystad, I've seen this occurring around my house. My garden and the street in front of my house are now about twenty centimetres lower than my ground floor. No matter how hard it rains, only a small amount of water will accumulate in my garden and on the street temporarily.
In the polders where new urban quarters have been raised to a certain level above the polder water level, urban water is generally not directly discharged to the water in the adjacent collection system, but discharged to the rural area via one or more weirs or by pumping. If everything is properly constructed and maintained, this allows for effective water management. However, more intense rainfall is now occurring, which can overload the sewer system. Therefore, it is important to apply higher design standards than in the past when constructing or repairing sewer systems.
In summary, the Netherlands is dealing with urbanization in many of its polders, on the one hand, and more extreme rainfall, on the other. Although publicity primarily focuses on the more extreme rainfall, urbanization often has a significantly greater impact on water management in the polders. It will therefore be crucial that the necessary adjustments are made in a timely manner in the coming years, depending on the processes at play. If the Water Authorities, in particular, remain capable of doing so, sound water management in the polders of the Netherlands can be assured in the future.
