A polder is a low-lying tract of land enclosed by embankments known as dikes, that forms an artificial hydrological entity, meaning it has no connection with outside water other than through man operated devices. There are three types of polders:
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 ground swell due to water pressure on ground water or rain fall and transportation of water by rivers and canals. This usually means that the polder has an excess of water that needs to be pumped out or drained by opening sluices at low tide. However, care must be taken in not setting the internal water level too low. Polder land made up of peat (former marshland) will show accelerated compression due 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 due 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 behavior. For this reason in the Netherlands it is actively hunted to extinction. No such care is taken in neighboring Germany though, causing the stock to be constantly resupplied across the border.
Polders are most commonly found, though not exclusively so, in river deltas, former fen lands and coastal areas.
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 of all polder surface within northwest Europe is located within the Netherlands. The first embankments in Europe were constructed in Roman times. The first polders were constructed in the 11th century. Due to flooding disasters water boards called waterschap (below sealevel) or hoogheemraadschap (above sea level) were set up to maintain the integrity of the water defenses 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 through this day. As such they are the oldest democratic institution in the country. The necessary co-operation 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, it is 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 defenses. From these calculations follow an acceptable flood risk from the sea at 1/10,000 years - 1/4,000, 1/2,500 years - 1/100 years for a river flood. For comparison the risk of the New Orleans (repaired) dike system in the future collapsing due to sea flooding is estimated at 1/100 years (i.e. flooding risks in New Orleans remain 100 times higher compared to Rotterdam which is set at 1/10,000) The established policy forces the Dutch government to improve flood defenses as new data on threat levels becomes available.