Bart Schultz
I was somewhat surprised by several of these announcements, as there was no indication that the persons involved were aware of the existing knowledge in this field. Nowadays, this is often the case in our fast-paced world, but this time I feel the need to explain the process of land subsidence in more detail.
With regard to land subsidence, there are significant differences between clay and peat. In the clay soils of Polder Flevoland, land subsidence is a finite process. At the beginning of the reclamation, it proceeded quite rapidly, and later became increasingly slower until it eventually stopped. At the former IJsselmeerpolders Development Authority (RIJP), we assumed at the time that this would be the case after approximately one hundred years, but that the major share would occur in the first thirty years. By then, we had experienced land subsidence of nearly one metre to approximately 1.25 metres in the rural area. If an old channel was present in the underlying sand, it could be even more.
The relevant processes were monitored for years by R.J. de Glopper and described by him in 1973 in Subsidence after drainage of the deposits in the former Zuyder Zee and in the brackish and marine forelands in The Netherlands.
To enable construction during both summer and winter, urban areas were typically raised by one metre of sand prior to the development, and the building sites were provided with a subsurface drainage system. The weight of the sand caused an additional land subsidence of approximately fifty centimetres. Ideally, construction of buildings did not start until about eighty percent of the land subsidence had occurred. Because the buildings were founded on concrete piles in the sandy subsoil and therefore did not sink any further, while the surrounding soil did, the house connections had to be designed to be flexible so that they could accommodate the remaining land subsidence.
The RIJP has always attached great importance to a thorough documentation of how the IJsselmeerpolders were reclaimed and developed. For Polder Flevoland, in addition to the aforementioned publication by De Glopper, this was also done in the books Oostelijk van de Knardijk and Westelijk van de Knardijk, among others. Furthermore, there are numerous Van Zee tot Land publications, Flevoberichten, RIJP reports, and Working documents, as well as a multitude of articles in the professional literature. All these publications are neatly archived at Museum Batavialand. It seems sensible to me to consult the relevant publications if one is involved in the further development of Polder Flevoland. In this way, the wheel does not have to be reinvented.
In Polder Flevoland, peat layers were present only to a limited extent, so they actually played no significant role in the land subsidence. However, this is the more the case in the many peat polders in the Netherlands. This involves compaction and oxidation of the peat. As a result, most peat polders experience a subsidence of approximately one centimetre per year, meaning the total subsidence in the older peat polders has now amounted to about five metres.
One of the polders where this land subsidence is greatest is the Zuidplaspolder. In a previous contribution to Flevolands Geheugen about The Lammefjordpolder and the Zuidplaspolder, the deepest polders in Europe, I wrote about the situation in this polder. The deepest point in the Netherlands is located in this polder, and a very thick layer of peat is present. Yet, there are serious plans to build 8,000 houses there. The saying Look before you leap certainly applies here.
