Water management of a polder

The design, operation and maintenance of the water management system of a polder are largely determined by the soil condition and by the land use. However, the period in which the polder in question was created is also of great importance. The original initial situation has in many cases led to a structure of the water management system that is still recognizable as such in the landscape.

The water management system of a polder must be designed in such a way that the precipitation surplus, possibly increased by the seepage, can be stored and discharged according to the set standards and that in times of drought - if necessary - water can be supplied. In order to gain insight into the circumstances that can be expected, precipitation and evaporation data must be available. The course of both over the year is important here. Furthermore, precipitation under design circumstances is important for the determination of the discharge capacity.

The water management system of a polder can consist of the following components:

• field drainage system: subsurface drains, trenches, open drains;
• main drainage system: collector drains, sub-main drains, main drains or canals, weirs, bridges, culverts;
• discharge system: discharge sluices, pumping stations.

Drainage is understood as the drainage of water from the ground and the drainage of water that may be present on the surface in the form of puddles. In drainage, the excess water is discharged through open drains or subsurface pipe drains to the outfall point. Discharge occurs when the water to be drained is discharged from the polder in such a way that it does not flow back. With the help of these systems, the water within the polders can be controlled independent of the surrounding. To this end, the systems must be coordinated with each other in a balanced manner. If one of the components does not function optimally, this manifests itself as the weakest link in a chain and the water management system is not optimal.

The location and dimensions of the various components are not always determined exclusively by water management considerations within the polder, other factors can also play an important role. Within the polder, this concerns aspects such as agricultural economy, cities, recreational areas, nature reserves, shipping, cattle barriers, discharge of waste water, parcel separation, etc. Outside the polder, ebb and flood, regime in the collecting and transport system for superfluous water, or river regime can have a significant influence on the dimensions, operation and maintenance of the water management system in the polder.

The water management system has a number of characteristic variables that can vary depending on the circumstances, namely:

• capacity of the drainage system;
• the discharge capacity of the main drainage system;
• the percentage of open water;
• the preferred water level(s) in the polder;
• the pumping capacity or discharge capacity.

These characteristics are often directly related to each other. For example, a lower pumping capacity will lead to a higher required percentage of open water.

The history of water management clearly shows a shift from a large percentage of open water in combination with a small discharge capacity to a small percentage of open water, in combination with a large discharge capacity. This development has gradually occurred over the centuries and has been determined mainly by technical and socio-economic changes. The water management system in the first, mostly small polders that were developed from around the tenth century onwards, usually consisted of only a discharge sluice, in combination with a few open drains. Gradually, water management within the polders was improved by the construction of trenches, open drains, main drains and sometimes also canals.

Due to measures taken in favour of other interests, discharge possibilities were disadvantaged in a number of places outside the polders. An important role was played by damming the outflows to the rivers or the sea. This gradually created a system for collection and transport of superfluous water from the polders with a level that was approximately 0.30 - 0.60 metres below mean sea level and usually still is. A distinction is made between open systems and closed systems. In the case of an open system, the polders may discharge their water at all times. In the case of a closed system, a fixed water level applies; the pumping level. When this level is reached, the polders must stop discharging their water.

The rise of discharge means in the form of first hand and horse mills and later windmills, led to an increase in the drainage capacity and the reliability of the drainage means. This also made it possible to drain lakes. In the beginning, this concerned shallow lakes and ponds. At the beginning of the seventeenth century, the development of pumping in series of windmills made it possible to drain deep lakes, achieving a pumping height of 4 - 6 metres. The drainage possibilities were further improved shortly afterwards by the use of Archimedes screw pumps.

The invention of the steampower pumping station, the electric pumping station and diesel pumping have not only led to the possibilities to carry out the large-scale land reclamation works - the Haarlemmermeer and the IJsselmeerpolders - but also to a better drainage of existing polders and especially of the collection and transport systems. In addition to these changes in the drainage, not only the construction and maintenance possibilities of the watercourses have gradually improved, but especially the drainage systems could be improved from the middle of the previous century by the increasing use of subsurface drainage in clay polders.

For many centuries, the water management system in polders has been constructed exclusively for the purpose of drainage for agricultural land use. Particularly since the Second World War, clear changes have taken place with respect to this. In both the IJsselmeerpolders and the older polders, in addition to or instead of agricultural use, urban expansions are being realised, forests and recreational areas are being created and nature reserves are being developed. When designing and adapting the water management systems, account must therefore be taken of various drainage regimes. Since the early seventies of the previous century, various water quality aspects have also played an important role.