| definition |
- A generally fertile tract of flat, low-lying land (as in Netherlands and Belgium) reclaimed and protected from the sea, a lake, a river, or other body of water by the use of embankments, dikes, dams, or levees. The term is usually reserved for coastal areas that are at or below sea level and that are constantly protected by an organized system of maintenance and defense.
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| Abstract from DBPedia | A polder (Dutch pronunciation: [ˈpɔldər]) is a low-lying tract of land that forms an artificial hydrological entity, enclosed by embankments known as dikes. The three types of polder are: 1.
* Land reclaimed from a body of water, such as a lake or the seabed 2.
* Flood plains separated from the sea or river by a dike 3.
* Marshes separated from the surrounding water by a dike and subsequently drained; these are also known as koogs, especially in Germany The ground level in drained marshes subsides over time. All polders will eventually be below the surrounding water level some or all of the time. Water enters the low-lying polder through infiltration and water pressure of groundwater, 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 when exposed to oxygen from the air. 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 known for this activity and hunted in certain European countries because of it. Polders are most commonly, though not exclusively, found in river deltas, former fenlands, and coastal areas. Flooding of polders has also been used as a military tactic in the past. One example is the flooding of the polders along the Yser River during World War I. Opening the sluices at high tide and closing them at low tide turned the polders into an inaccessible swamp, which allowed the Allied armies to stop the German army. Netherlands has a large area of polders: as much as 20% of the land area has at some point in the past been reclaimed from the sea, thus contributing to the development of the country. IJsselmeer is the most famous polder project of the Netherlands. Some other countries which have polders are Bangladesh, Belgium, Canada and China. Some examples of Dutch polder projects are Beemster, Schermer, Flevopolder and Noordoostpolder.干拓（かんたく）とは遠浅の海や干潟、水深の浅い湖沼やその浅瀬を仕切り、その場の水を抜き取ったり干上がらせるなどして陸地にすること。主に農地として開拓する時に用いられる。干拓された土地を干拓地（蘭: polder）と呼ぶ。 水域に土砂や廃棄物等を投入して土地を造成する埋立とは異なる。 方法として、まず、干拓堤防（潮受け堤防、潮受堤防）で水域を仕切り、堤防の随所に水門を設ける。その上で動力によって強制的に仕切内の水を排水し干上がらせる。または海の場合、潮の干満を利用する方法も取られる。干潮時に水門を開き海水を排し、満潮時には水門を閉じて干上がらせる。 こうしてできた土地は海面よりも低くなることが多く、塩分を含んだ土地であるため、農地化する際には、塩分とともに水を排水する設備を作る必要がある。また地盤も軟弱であるため、宅地としては適していない。 (Source: http://dbpedia.org/resource/Polder) |
