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The continuing flow of fresh river water soon flushed out the salt water. Part of the IJsselmeer was later closed off to form the Markermeer . From 1929 till 1967, over half the IJsselmeer was drained, creating 1,979 km 2 (764 sq mi) of polders : [ 4 ] Wieringermeerpolder , Noordoostpolder , East and South Flevoland .
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 potential damage in lives, property and rebuilding costs is compared to the potential cost of water defences.
The salt water inlet changed into a fresh water lake now called the IJsselmeer (IJssel Lake) after the river that drains into it, and by means of drainage and polders, an area of some 1,500 km 2 (580 sq mi) was reclaimed as land. This land eventually became the province of Flevoland. [1] Part of the IJsselmeer was also divided into the Markermeer.
This body of water is then pumped up into the IJsselmeer. It is sometimes called the Gelderse IJssel (IPA: [ˌɣɛldərsə ˈʔɛisəl] ⓘ; "Gueldern IJssel") to distinguish it from the Hollandse IJssel. It is in the provinces of Gelderland and Overijssel, the latter of which was named after this river. The Romans knew the river as Isala.
Nowadays, eels are rare in the IJsselmeer; the most probable causes are water pollution, and the industrial fishing of their offspring ("glass eels" due to their transparency) during their travels back to Europe from the eels' breeding waters in the Sargasso Sea for rearing in land-based containers.
In preparation for this flooding, the water level of the IJsselmeer was deliberately raised. [3] The level in the IJsselmeer was 3.62 m (12 ft) above the ground level of Wieringerpolder at the site of the breach, and varied between 3.12–5.12 metres (10–17 ft) above, across the polder. [4]
Almost all of the land belonging to Flevoland was reclaimed in the 1950s and 1960s [5] while splitting the Markermeer and Bordering lakes from the IJsselmeer. As to dry land, it is the smallest province of the Netherlands at 1,410 km 2 (540 sq mi), but not gross land as that includes much of the waters of the fresh water lakes (meres) mentioned ...
The inner berm slope toward the future water level of the IJsselmeer (or Amstelmeer) was designed to be steeper than the outer slope, with a ratio of approximately 1:3. The majority of the dam's core consisted of sand, while boulder clay on the exterior provided resistance against water flow, limiting seepage and enhancing dike breach protection.