A polder is a low-lying tract of land enclosed by embankments (barriers) known as dikes that forms an artificial hydrological entity, meaning it has no connection with outside water other than through manually operated devices. There are three types of polder:
- Land reclaimed from a body of water, such as a lake or the sea bed
- Flood plains separated from the sea or river by a dike
- Marshes separated from the surrounding water by a dike and subsequently drained
The ground level in drained marshes subsides over time and thus all polders will eventually be below the surrounding water level some or all of the time. Water enters the low-lying polder through water pressure of ground water, or rainfall, or transport of water by rivers and canals. This usually means that the polder has an excess of water, which is pumped out or drained by opening sluices at low tide. Care must be taken not to set the internal water level too low. Polder land made up of peat (former marshland) will sink in relation to its previous level, because of peat decomposing in dry conditions.
Polders are at risk from flooding at all times and care must be taken to protect the surrounding dikes. Dikes are typically built with locally available materials and each material has its own risks: sand is prone to collapse owing to saturation by water; dry peat is lighter than water and potentially unable to retain water in very dry seasons. Some animals dig tunnels in the barrier, allowing water to infiltrate the structure; the muskrat is well known for this activity and actively hunted in certain European countries because of it. Polders are most commonly, though not exclusively, found in river deltas, former fenlands and coastal areas.
Polders and the Netherlands
The Netherlands is frequently associated with polders. This is illustrated by the English saying: God created the world but the Dutch created Holland.
The Dutch have a long history of reclamation of marshes and fenland, resulting in some 3,000 polders nationwide. About half the total surface area of polders in north-west Europe is in the Netherlands. The first embankments in Europe were constructed in Roman times. The first polders were constructed in the 11th century. As a result of flooding disasters, water boards called waterschap (when situated more inland) or hoogheemraadschap (near the sea, mainly used in the Holland region) were set up to maintain the integrity of the water defences around polders, maintain the waterways inside a polder and control the various water levels inside and outside the polder. Water bodies hold separate elections, levy taxes and function independently from other government bodies. Their function is basically unchanged even today. As such they are the oldest democratic institution in the country. The necessary cooperation between all ranks in maintaining polder integrity also gave its name to the Dutch version of third way politics - the Polder Model.
The 1953 flood disaster prompted a new approach to the design of dikes and other water-retaining structures, based on an acceptable probability of overflowing. Risk is defined as the product of probability and consequences. The potential damage in lives, property and rebuilding costs is compared to the potential cost of water defences. From these calculations follows an acceptable flood risk from the sea at one-in 4,000–10,000 years, while it is one-in 100–2,500 years for a river flood. The particular established policy guides the Dutch government to improve flood defences as new data on threat levels becomes available.
Some famous Dutch polders and the year they were laid dry are:
- As part of the Zuiderzee Works:
Examples of polders
- De Moeren, near Veurne in West Flanders
- Polders of Muisbroek and Ettenhoven, in Ekeren and Hoevenen
- Polder of Stabroek, in Stabroek
- Kabeljauwpolder, in Zandvliet
- Scheldepolders on the left bank of the Scheldt
- Uitkerkse polders, near Blankenberge in West Flanders
- Prosperpolder, near Doel, Antwerp and Kieldrecht.
- Altes Land near Hamburg
- Blockland & Hollerland near Bremen
- Nordstrand, Germany
- Bormerkoog and Meggerkoog near Friedrichstadt
Black Bush Polder, Corentyne, Berbice
- Delta of the river Po such as Bonifica Valle del Mezzano
- Hachirogata in Akita Prefecture
- Isahaya Bay in Kyushu
- Kojima Bay in Okayama Prefecture
- Rusnė island
- Alblasserwaard, containing the windmills of Kinderdijk, a World Heritage Site
- Anna Paulownapolder
- Beemster, a World Heritage Site
- Haarlemmermeer, containing Schiphol airport
- Prins Alexanderpolder
- Zijpe- en Hazepolder
- Eastern and Southern Flevoland polders, containing the cities of Lelystad and Almere, respectively. Together these polders are also known as the Flevopolder.
- The Ankaran Polder (Slovene: Ankaranska bonifika), Semedela Polder (Semedelska bonifika), and Škocjan Polder (Škocjanska bonifika) in reclaimed land around Koper
- Parts of the coast of Ganghwa Island, adjacent to the river Han in Incheon
- Delta of the river Nakdong in Busan
- Saemangeum in Jeollabuk-do
- Traeth Mawr
- Sunk Island, on the north shore of the Humber east of Hull
- Parts of The Fens
- Parts of the coast of Essex
- Some land along the River Plym in Plymouth
- Some land around Meathop east of Grange-over-Sands, reclaimed as a side-effect of building a railway embankment
- The Somerset Levels and North Somerset Levels
- Romney Marsh
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