What do we see on the risk map?
Large parts of the Netherlands are protected from water by flood defences (dykes, dunes and engineering structures). A successive ring of flood defences which are more or less the same height is referred to as a dyke ring. Higher areas may also be part of a dyke ring. The land within the ring is referred to as a dyke ring area. The dyke ring area and the required protection level of every dyke ring are laid down by law in the Dutch Flood Defences Act [Wet op de Waterkering].
The risk map shows the dyke ring areas that could be flooded. The risk map also shows how high the water will rise in the worst case.
buitendijks gebied = river side of the dyke / rivier = river / dijk = dyke / dijkringgebied = dyke ring area / overstromingsgebied = flood area
/ overstromingsdiepte = flood depth
There are a great many factors that determine which parts of the land are flooded. Computer calculations have been developed for many areas to show how a flood could unfold. In the case of such calculations, assumptions are made about the location and size of a dyke breach and the water levels of the river or the sea. Calculations are then made as to which areas will be submerged and as to which flood depths will be reached at each location.
By combining the results of the calculations, a maximum flood depth can be determined for each location. In this way, the flood depth map indicates the areas that may be flooded and the water depths.
What is the risk?
A flood occurs if an unmanageable amount of water flows inland. Floods pose a real danger to the Netherlands, because:
large parts of our country are below sea level;
the Netherlands has a number of large rivers;
the climate is changing: sea level is rising and it is raining more often and more heavily.
The water may come from several sources: the sea or the large rivers and from inland waterways such as canals and lakes. A dyke may subside, a dune may be swept away, or the water may come over the dykes. An additional danger is that the power supply may fail, so that no communication by telephone, Internet and television is possible. Public health may also be at risk, which may be a reason for the government to advise evacuation.
What safety measures are there?
All public authorities – the state, provinces and municipalities – work together to avoid and to limit floods. Rijkswaterstaat – the Directorate General for Public Works and Water Management – and the water authorities are jointly responsible for maintaining and inspecting dykes, dams and dunes, and rivers are given more room. The municipalities inform and warn the population. Together with the emergency services (police, fire brigade and ambulance), the public authorities regularly rehearse the actions to be taken in the event or the risk of a flood. Water levels are monitored continuously, so there is, in principle, time to take action and to raise the alarm.
Up-to-date water levels can be found at www.actuelewaterdata.nl.
Rijkswaterstaat’s storm tide warning service issues warnings for high water at www.svsd.nl.
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