Satellite image of Noordoostpolder, Netherlands (595.41 km²)

A polder is a low-lying tract of land enclosed by embankments 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 consequently 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 ground swell due to water pressure on ground water or rain fall and transportation of water by rivers and canals. This usually means that the polder has an excess of water that needs to be pumped out or drained by opening sluices at low tide. However, care must be taken in not setting the internal water level too low. Polder land made up of peat (former marshland) will show accelerated compression due to the 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 mostly built using locally available materials and each has its own risk factor: sand is prone to collapse due to oversaturation by water while dry peat is lighter than water, making the barrier potentially unstable in very dry seasons. Some animals dig tunnels in the barrier, undermining the structure; the muskrat is notorious for this behavior. For this reason in the Netherlands it is actively hunted to extinction. No such care is taken in neighboring Germany though, causing the stock to be constantly resupplied across the border.^{[citation needed]}

Polders are most commonly found, though not exclusively so, in river deltas, former fen lands and coastal areas.

[edit] Polders and the Netherlands

Windmill in North-Holland, the polder lies lower than the surrounding water on the other side of the dike

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 ^[1] nationwide. About half of all polder surface within northwest Europe is located within the Netherlands. The first embankments in Europe were constructed in Roman times. The first polders were constructed in the 11th century. Due to flooding disasters water boards called waterschap (below sealevel) or hoogheemraadschap (above sea level) were set up to maintain the integrity of the water defenses 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 currently. As such, they are the oldest democratic institution in the country. The necessary co-operation 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 damage in lives, property and rebuilding costs is offset against the cost of water defenses. From these calculations follow an acceptable flood risk from the sea at 1 in 4,000–10,000 years, while it is 1 in 100–2,500 years for a river flood. The established policy forces the Dutch government to improve flood defenses as new data on threat levels becomes available. For comparison, the risk of the (repaired) New Orleans dike system in the future collapsing due to sea flooding is estimated at 1 in 100 years^[2] (i.e. flooding risks in New Orleans remain 100 times higher compared to Rotterdam which is set at 1/10,000)

[edit] Examples of polders

[edit] Belgium

• De Moeren, near Veurne in West Flanders
• Polders van Muisbroek en Ettenhoven, in Ekeren and Hoevenen
• Polder van Stabroek, in Stabroek
• Kabeljauwpolder, in Zandvliet

[edit] Canada

• Holland Marsh

[edit] France

• Marais Poitevin
• Les Moëres, adjacent to the Flemish polder De Moeren in Belgium.

[edit] Germany

• Altes Land near Hamburg
• Blockland & Hollerland near Bremen
• Nordstrand, Germany
• Bormerkoog and Meggerkoog near Friedrichstadt

[edit] Guyana

Black Bush Polder, Corentyne, Berbice

[edit] Italy

• Delta of the river Po such as Bonifica Valle del Mezzano

[edit] Netherlands

(This is by no means an exhaustive list)

• Alblasserwaard, containing the windmills of Kinderdijk, a World Heritage Site
• Andijk
• Anna Paulownapolder
• Beemster, a World Heritage Site
• Bijlmermeer
• Haarlemmermeer, containing Schiphol airport
• Mastenbroek
• Noordoostpolder
• Ooijpolder
• Prins Alexanderpolder
• Purmer
• Schermer
• Rosandepolder
• Watergraafsmeer
• Wieringermeer
• Wieringerwaard
• Wijdewormer
• Zijpe- en Hazepolder
• Zuidplaspolder
• Eastern and Southern Flevoland polders, containing the cities of Lelystad and Almere, respectively. Together these polders are also known as the Flevopolder.

[edit] Poland

• Vistula delta near Elblag and Nowy Dwor Gdanski
• Warta delta near Kostrzyn nad Odra

[edit] United Kingdom

• 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

[edit] United States

• New Orleans

[edit] See also

• Flood control in the Netherlands
• Afsluitdijk
• Land reclamation
• IJsselmeer
• Zuiderzee Works
• Windpump

[edit] References

1. ^ "TKijk naar de geschiedenis". Rijkswaterstaat. https://www.rijkswaterstaat.nl/rws/waterinnovatiebron/cgi-bin/toonlijst.pl?config=config&var=volgnr&val=1&layout=index-forceframe&confignr=1&menu=1067260347&menuitem=1120115781. Retrieved 2008-01-21. 
2. ^ [1]

• Farjon, J.M.J., J. Dirkx, A. Koomen, J. Vervloet & W. Lammers. 2001. Neder-landschap Internationaal: bouwstenen voor een selectie van gebieden landschapsbehoud. Alterra, Wageningen. Rapport 358.
• Morten Stenak. 2005. De inddæmmede Landskaber - En historisk geografi. Landbohistorik Selskab.
• Ven, G.P. van de (red.) 1993. Leefbaar laagland: geschiedenis van waterbeheersing en landaanwinning in Nederland. Matrijs, Utrecht.
• Wagret, P. 1972. Polderlands. London : Methuen.

[edit] External links

• Polder landscapes in the Netherlands within a northwest European and is also a landmark context
• How to make a polder - online film
• http://watercanon.nederlandleeftmetwater.nl/view/canon/professionalisation