| SluicesA sluice is a dike between two bodies of waters with different water levels. A sluice which is closed on one side is called a floodgate and is only used to regulate the water level. A sluice which can be closed at both ends is called a lock and is also used to bring ships from one water to another. |
- Locks
- A Lock, Ecomare
A lock is a connection between two bodies of water which have different water levels. The first lock is assumed to have been developed by Dutchmen during the first half of the 13th century. The level difference is called the fall, drop or rise. The fall can be as little as several dozen centimeters or four meters, such as found when sailing into some IJsselmeer polders. The procedure for a ship heading upstream is as follows: the ship sails into the lock, after which the doors are closed. Water is pumped into the chamber until the other water level is reached. The other doors then open and the ship can move on. For a ship heading downstream, water is removed from the waiting chamber. This kind of sluice is called a single retaining lock . When a lock forms a connection between inland water and open sea (whereby the sea can be lower or higher than the inland water), then a bi-directional sluice is built, so that the doors restrain water during high and low tide.
- Discharge sluices
- Discharge sluice, low and high tide, Ecomare
A discharge sluice must drain off inland water and stem offland water. The discharge sluices in the Afsluitdijk ('Closure Dike') by the IJsselmeer is a good example. The IJsselmeer is constantly being fed fresh water from rainwater as well as the rivers IJssel, Vecht and Eem. In order to get rid of the excess water, water is spouted into the Wadden Sea during low tide. The Afsluitdijk has a total of 25 discharge sluices. The same principle is used by Lauwersoog, where water is spouted from the Lauwersmeer into the Wadden Sea. Other discharge sluices in the Netherlands can be found in polders that discharge into rivers or the sea. Discharge sluices are by far the most important type of sluice for managing inland water levels.
In Groningen and Northern Overijssel, a sluice is called a 'zijl' (no difference in English). The sluices in Groningen often discharge into the sea and restrain high-tide water. Many towns thank their name on such a 'zijl', for example Delfzijl, Termunterzijl, Noordpolderzijl and Dokkumer Nieuwe Zijlen. In Frisian a sluice is called a 'syl'. Many towns in East Friesland (Germany) end with -siel (German syl), such as Bensersiel, Harlesiel and Neuharlingersiel.
A discharge sluice located under the dike is called a culvert. These sluices were built in the Netherlands during the Roman Times. They were made from hollowed-out trees with a lid so that the water could exit but not enter. In Friesland, a discharge sluice is called a pump. - Drainage sluices
- Sluice by Oudebildtzijl in Friesland, Ecomare
Drainage sluices are for preventing waterways from silting up, however nowadays they are rarely found anymore. Dredging ships have taken over this function. A drainage basin is located behind the sluice, which is filled with water during flood. When the sluice is opened at low tide, a large force of water washes away the mud on the bottom. This keeps the waterway open at the necessary depth for shipping. The only sluice with a drainage basin is the sluice complex Nieuwe Termunterzijl by Termunterzijl. The sluices which are presently called drainage sluices are actually discharge sluices.
- Other sluices
- Culvert, low and high tide, Ecomare
Other kinds of sluices include inundation sluices, go-out sluices, evacuatior sluices, inlet sluices, guard locks and irrigation sluices. During wartime, inundation sluices were opened to flood the land very rapidly. This was a very effective defence strategy (Holland Defence Line) before there were airplanes. Go-out sluices make sure that the water in flooded areas cannot leak into other areas. Evacuatior sluices are for emergency purposes, to help water exit an area. Inlet sluices are the opposite of discharge sluices: they allow water to enter dry polders. Guard locks make sure the water level behind the sluice remains at the desired level. For example, they are used to prevent flooding or to make sure the water level in a harbour does not sink too low. And finally, irrigation sluices are for allowing water to move through farmland.
- Flood-control dams
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