Polders

Polders

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. In the Netherlands the muskrat is actively hunted to extinction; in neighbouring Germany though, the policy allows the muskrat population to endure. Polders are most commonly, though not exclusively, found in river deltas, former fenlands and coastal areas.