Polder

A polder (Dutch pronunciation: [pɔldər]) is a low-lying tract of land enclosed by 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:

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 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 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 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.

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.

From Dutch polder ("polder"), from Middle Dutch polre, from Old Dutch polra, ultimately from pol- "part of land, elevated above its surroundings"; with augmentative suffix -er and epenthetical -d-.

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