For those expats in Holland who don’t understand how dikes the Dutch build actually work in keeping Holland dry, despite most of it being below sea level, here is a super-simple explanation…
How Dutch dikes function to keep Holland from constantly flooding becomes clear looking at the photo above. On the right is the Vliet canal which runs through a long stretch of South Holland, all the way from Delft in the south to Leiden in the north. This portion visible in the photo is in the village of Leidschendam.
The water level of the canal is just slightly lower than the top of the dike which runs along it on the left (east). A single lane road which connects the town center to the Vlietland lake has been paved on top of the dike. This particular dike is fairly wide in order for it to support the huge amount of pressure being applied by the water in the canal. The higher the surface level of the water compared to the surrounding land, the higher the pressure. In addition, the wider the canal, the greater the pressure on the dike. Since the Vliet is both wide and the water level is high, the dike needed to support it has to be wide.
On the left, you can see the grass-covered downward slope of the dike until it ends at the point where the ground levels out. That level ground point is substantially lower than the water level in the canal. If this dike hadn’t been built, the entire grass-covered area on the left (called a ‘polder’) would be under water. The building of the dike allowed the polder to be drained over the course of years, until it was finally dry enough to be farmed.
It was windmills that originally provided the power needed to pump the water from the lower level of the polder to the higher level of the canal. During the Industrial Revolution, windmills were replaced by steam engines, which were then later replaced by electric motors.
The surface level of the water in Dutch canals is not something left to chance. It is closely monitored by local water boards, of which there are some 25 in the country. The standard water level for canals in Holland is NAP -60 cm (60 cm below sea level).
Below is a graphic showing a cross-section of a typical Dutch dike and its composition.
And that’s our super quick, very basic explanation of how Dutch dikes generally work in keeping polders, which are at lower elevations than sea level, from flooding.