Bart Schultz
“A polder is a level area, in its original state subject to high water levels (permanently or seasonally, originating from either groundwater or surface water), but which through impoldering is separated from its surrounding hydrological regime in such a way that a certain level of independent control of its water table can be realised.”
As far as reclaimed low-lying land is concerned, a distinction can be made between marshy land and river polders. Lands gained on the sea are polders that have been created by land reclamation along the coast. These areas used to be dry during certain periods of the day, depending on the tide and the elevation. Drained lakes are polders that have been created by draining areas that were previously permanently under water. Together with the dikes, which directly or indirectly protect the reclaimed areas from the adjacent water, the water management system is essential to make and keep these areas suitable for the various forms of use. In the case of drained lakes, a distinction can be made between the draining of more or less natural lakes and the draining of areas where peat had been excavated for fuel.
The oldest polders in the Netherlands are low-lying lands that have been reclaimed. In total, these polders cover approximately 1,335,000 hectares. These are often peat polders that are still subject to subsidence of approximately one centimetre per year. In the river areathis concerns, for example, the Betuwe and the Prins Alexander Polder. The lands gained on the sea are of a later date and cover approximately 315,000 hectares. These polders mainly exist in the provinces of Friesland, Groningen and Zeeland. The history of the drained lakes begins in 1533 with the reclamation of the Achtermeer - 35 hectares - near Alkmaar. The total area of drained lakes is approximately 350,000 hectares. An important part of this concerns the IJsselmeer polders - Wieringermeer, Noordoostpolder, Oostelijk Flevoland and Zuidelijk Flevoland.
In total, the polders in the Netherlands cover around two million hectares, which corresponds to about two-thirds of the land surface. Due to the low location, the subsidence and the rising sea level, the Netherlands has become increasingly vulnerable and good management and maintenance with, when necessary, adjustments to the water management system and protection against flooding is an absolute necessity.
Abroad there are two other types of polders. These are the so-called horseshoe shape polders and polders in depressions with drainage through a tunnel. Horseshoe shaped polders are polders in river areas that are protected from flooding by dikes at the upstream side and at the sides. However, the slope in the terrain allows the excess water to flow out freely at the downstream side. Such polders can be found, for example, in the Irrawaddy Delta in Myanmar, as described in my earlier contribution to Flevolands Geheugen.
Polders in depressions with drainage through a cave, or a tunnel are also a special type of polders. So far I have found three examples of this. One example concerns reclamation in Lake Copais in Greece where the excess water is discharged through a cave. The first works started here already at 1300 BC. Discharge through a tunnel has been applied for reclamation in Lake Fucino in Italy at 668 meters above mean sea level. This lake was already reclaimed in the Roman times. Over the years, a number of improvements have been made to the drainage and a new tunnel has also been built. The third example concerns Mexico City at 2,240 metres above mean sea level. Here too, the excess water is discharged through a tunnel. As far as I have been able to determine, this is the highest polder in the world. The city has millions of inhabitants.
