Polder

Wikipedia:

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:

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 stock to be constantly resupplied across the border.

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

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 nationwide. About half the total surface area of polders in north-west Europe is in the Netherlands. The first embankments in Europe were constructed in Roman times. The first polders were constructed in the 11th century. As a result of flooding disasters water boards called waterschap (when situated more inland) or hoogheemraadschap (near the sea, mainly used in Holland) were set up to maintain the integrity of the water defences 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 even today. As such they are the oldest democratic institution in the country. The necessary cooperation 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, 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 defences. From these calculations follows an acceptable flood risk from the sea at 1 in 4,000–10,000 years, while it is 1 in 100–2,500 years for a river flood. The established policy forces the Dutch government to improve flood defences as new data on threat levels becomes available.

Examples of polders

Belgium

Canada

France

Germany

Guyana

Black Bush Polder, Corentyne, Berbice

India

Italy

Japan

Korea, Republic of

Lithuania

Monaco

Netherlands

Poland

United Kingdom

United States