Polder
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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 man operated devices. There are three types of polders:
- 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, 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.
Polder are at risk from flooding at all times and care must be taken to protect the dykes surrounding a polder. Dykes are mostly built using locally available materials and each has its own risk factor: sand is prone to collapse due to oversaturation by water; 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.
Polders are most commonly found, though not exclusively so, in river deltas, former fen lands and coastal areas.
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[edit] 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 [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 through this day. 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, it is 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. Typical acceptable flood risks from these calculations are 1/10,000 years - 1/4,000 years for flood from the sea, 1/2,500 years - 1/100 years for a river flood. The established policy forces the Dutch government to increase defenses as new data on threat levels becomes available.
[edit] Examples of polders
[edit] Belgium
- De Moeren, near Veurne in West Flanders
[edit] Canada
[edit] France
[edit] Germany
- Altes Land near Hamburg
- Blockland & Hollerland near Bremen
- Nordstrand, Germany
- Bormer- und Meggerkoog near Friedrichstadt
[edit] Italy
- Delta of the river Po such as Bonifica Valle del Mezzano
[edit] Netherlands
- Alblasserwaard, containing the windmills of Kinderdijk, a World Heritage Site
- Beemster, a World Heritage Site
- Haarlemmermeer, containing Schiphol airport
- Noordoostpolder
- Wieringermeer
- 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
[edit] External links
[edit] References
- ^ TKijk naar de geschiedenis. Rijkswaterstaat. Retrieved on 2008-01-21.
- 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.
