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These walls are called dykes or dams. (dijken in Dutch)
they reclaim land from the sea so that they have more land to build on and so that it makes japan flatter with the land its using to reclaim the land which also gives them more space to build on.
Because the land mass of their country is small, rclaiming lad from te ea increases the area available for agriculture, although it is on ongoing roject to prevent the sea frm taking it back again.
Dykes built to help reclaim land from the sea.
he North Sea and land below sea level
The Netherlands is known for reclaiming land from the sea through extensive land reclamation projects. Around half of the country's land is reclaimed from the sea, using systems of dikes, canals, and pumping stations to manage water levels and create new land.
ondergelopen grond
The dutch province of Flevoland was reclaimed from the 'Zuiderzee' (South Sea).
The Dutch call their area of land reclaimed from the sea "polders." These are low-lying lands enclosed by dikes where water is drained to create arable land. Polders are a notable feature of the Dutch landscape and have helped expand their habitable territory.
zeeland
Growing crops
First build a dike around the area which you want to claim from the sea. Then pump the water away threw canals. In the early days we used windmills to pump the water, nowadays we use electric machines.