Polder

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 groundswell owing to water pressure on ground water or rainfall and transport of water by rivers and canals. This usually means that the polder has an excess of water, which needs to be pumped out or drained by opening sluices at low tide. However care must be taken not to set the internal water level too low. Polder land made up of peat (former marshland) will show accelerated compression owing 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 owing 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 behaviour. For this reason in the Netherlands it is actively hunted to extinction. No such care is taken in neighbouring Germany though, causing the population to be maintained.

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