Bart Schultz
The collector drains serve a dual function. On the one hand, they determine the preferred groundwater table, and on the other, they form the first link in the discharge of excess water. The excess water in this system is discharged to the outside waterbody through discharge sluices or pumping stations.
Trenches are typically used in peat polders. Their aim is to maintain a relatively shallow groundwater table - 20 to 40 centimetres below the surface. In clay polders, subsurface drainage is typically used, aiming for a groundwater table of approximately one metre below the surface. In some areas, the subsurface drainage system has been equipped at the outlet with a provision to raise the groundwater table in summer or during dry periods.
Collector drains, sub-main drains and canals may contain various structures. These structures may be related to crossing the watercourse in question, such as dams, culverts, and bridges. They may also be related to achieving the desired water table in the watercourse, particularly adjustable weirs and pumping stations.
When crossing watercourses with roads, paths, and dikes, a solid dam is used if the water level on either side of the dam does not need to be connected. If this is the case, a culvert is usually installed in the dam. The culvert - usually a round pipe or a structure - can be open, but it can also be closed if the goal is for water flow in one direction. This is the case, for example, with culverts along coasts, where excess water from the polders is discharged into the sea during low tide, but the flow of salt water from the sea into the polders is prevented.
It is also possible that sections of a polder have different preferred water levels, and the culvert is intended to allow flow only from a higher section to a lower one. A bridge is generally used when free flow is desired at the intersection, for example, in the case of shipping on the watercourse. This can be a fixed bridge or a bridge that can be opened when necessary for shipping.
Weirs or pumping stations can be used in watercourses when different water levels are desired in different sections of a polder. This can be important due to differences in surface level within the polder, but also due to different uses, such as agriculture, urban areas, nature and recreation. Weirs can be fixed, or adjustable. Fixed weirs are applied when only certain water levels are preferred. Movable weirs are applied when requirements also concern the discharge from one section to another. Pumping stations are used when a deeper water level is preferred in certain, usually lower, parts of a polder.
With all these structures, more or less perfect water management can be achieved in the polders during the year, tailored to the needs and requirements of land use and any differences therein.
