A map of the Netherlands reveals that one-fifth of the country is below sea level, as indicated by the dotted line. The combination of stormy weather and a high tide could bring the water high enough to flood half the country. No other country has ever had to deal with a problem of this magnitude before. Is it possible to explain how the Dutch were able to not only keep the floodwaters at bay, but also to reclaim land from the sea and convert it into fertile crop-producing soil? Dutch settlers began constructing seawalls and draining land as early as the thirteenth century, allowing them to create new land from existing waterways. They referred to these new tracts of land as 'polders.' At first, these polders were only small patches of land, but by the seventeenth century, methods of draining larger tracts of land had been discovered: the invention of the rotating turret made it possible to point the sails of a windmill in any direction, according to the direction of the wind, and this constant power could be used to pump water out of flooded areas such as lakes and marshes; the invention of the rotating turret made it possible to point the sails of a wind The first steam-driven pumps were installed to drain Lake Haarlem, which is located south of Amsterdam, in the mid-nineteenth century.
Besides providing protection against flooding when there were gales, it also made 19,000 hectares (47,000 acres) of new and fertile land available for agricultural use in the area. There is now an airport here, with a control tower that is 4:5m (15 ft) below the surface of the water. Land reclamation continued throughout the nineteenth century, but the most significant advancements have occurred in the last sixty years, with the two most ambitious projects still in the process of being completed - the Delta Project and the Zuyder Zee Works. At the turn of the twentieth century, the Zuyder Zee was a deep inlet of the North Sea with a maximum width of 59 kilometres (37 miles) and a maximum length of 80 kilometres (50 miles). The land had to be protected all along its perimeter by dams, which, in bad weather, had to withstand the full force of the storms and tides that pounded the area. This inlet has a mouth that is 32 kilometres (20 miles) wide, and the first step in the project was to construct a dam across it. This task took from 1927 to 1932 to complete. A freshwater lake (now known as Lake Ijssel) developed gradually on the Zuyder Zee, which was cut off from the open sea but fed by inland rivers. This lake was capable of supplying water supplies to the surrounding provinces during times of drought because it was isolated from the open sea.
Keeping up the approximately 290 km (18o miles) of inland dykes around the Zuyder Zee, which are no longer exposed to the ebb and flow of the North Sea tides, has become less expensive since the Barier Dam was completed; in addition, the dam provides an important link between the provinces of North Holland and Friesland through which a district road can be built. The level of water in Lake Ijssel is maintained at a level that is approximately equal to the mean sea level (half way between high and low water). When the lake's water level is too high, sluices are opened and the excess water drains into the sea. The construction of the polders within the freshwater lake was the next stage. Because the lake was no longer tidal, the task of constructing the new dykes was made much easier to accomplish. The Wieringermeer Polder, covering an area of approximately 20,000 hectares (50,000 acres), was the first to be completed. First, dykes were constructed around the perimeter of the area to be drained. Dredgers excavated the soft mud from the sea bed where the dyke was to run and filled the trench with sand to provide a more stable foundation for the dyke. Dredgers excavated the soft mud from the sea bed where the dyke was to run.
In order to form the outer and inner sides of the dyke, boulder clay mounds were built up along the outside edge of the foundation and filled with sand to form the core of the structure. Where it was thought necessary, the exposed slopes of the dyke were faced with stone to protect the portion of the dyke that would be submerged by brushwood mattresses covered with rubble. The next step involved putting the two pumping stations into operation. It took six months for these to completely drain the area. Of course, even after it had been drained, the ground remained salinity-laden and infertile. When it came time to put the land back into use, the salt had to be completely washed out of the soil. This was accomplished by constructing a network of drain trenches and drainage pipes throughout the area, as well as by lowering the water level in the ground to as much as 1.5m (5 ft) below the lowest point in the polder's ground surface. Finally, houses and farms were constructed, as well as electricity and water lines were installed.
The first major setback occurred during World War II. Following their discovery that the Allies were attempting an invasion, the Germans, who were occupying Holland, decided to flood the Wieringermeer by breaching the dykes on the lake Ijssel side. The residents of the polder were able to flee because it took two days for the area to flood completely. However, nearly all of the houses and farms were destroyed and were submerged under 4m (13 ft) of water when the flooding began. Work on the breaches began as soon as the Germans surrendered, and with the help of auxiliary pumps, the polder was completely drained in four months after the end of the war. Two additional polders are now in operation. The North-East Polder, which is twice the size of the Wieringermeer, was drained in 1942 and is now a protected area. Approximately 30,000 people now reside and work in the area. It was decided in 1957 to drain approximately 54,000 hectares (133,000 acres) of eastern Flevoland, with 85 percent of the land now under cultivation.
Markerwaard, a project that will cover 60,000 hectares (148,000 acres) in southern Flevoland, will be completed in 1978 after 42,500 hectares (105,000 acres) in southern Flevoland became dry in 1968. In this case, Lake Ijssel will cover approximately one-third of its original area, and narrow arms of the lake will continue to divide the last three polders from their respective portions of the Dutch mainland. Due to the fact that the ground water level of these polders is so low (in the case of Eastern Flevoland, between 5m and 6m (16-20 ft) below sea level), if they were connected to the main land, the ground water level there would sink significantly, resulting in the crops not receiving adequate irrigation. The Zuyder Zee Project will result in an increase in the arable land area of the Netherlands of approximately 10%, as well as new space for towns to relieve congestion in the country's congested areas, new industrial sites, and new places for sailing, swimming, and sunbathing for holiday-makers, among other benefits.