Rivers and streams are referred to as floods when they reach a level which is significantly higher than normal. A stream or a river can overflow its banks, even flood the dams that were built specifically to protect against flooding. A flood can have two causes: a large amount of snow melting in the mountains due to the sudden onset of warm weather, or steady heavy rain that lasts for a long time.
Floods occur along coastlines, too, where they are caused by storms increasing the movements due to tides, which is why they are not referred to as floods but as storm floods.
A flood that has been caused by the snow melting only occurs in spring. Because mountain streams have increased their water-level every year, certain precautions have been taken, which includes moving buildings further back away from the river zone in order to make sure that the water can flow away freely. Therefore a higher water mark rarely causes damages in areas like this.
Sometimes floods occur in river valleys when the rain in a certain area is heavy and lasts for a long time. A lot of rivers have been straightened and regulated over the course of time which is why there wasn’t enough land space to take up a larger volume of water. Dams along the rivers to keep the water away from residential areas were the first measures to serve as protection against the floods. The sealing of land
and the faster draining of water is frequently not enough nowadays, which is why rainwater retention basins must be built. Polders, which are protected by these kinds of artificial basins and dams, and which can be flooded when necessary, re able to hold excess amounts of water and thus contribute to the distribution of the outflow of the water over a longer period of time.
A flood is referred to as a hundred year flood if it has a magnitude that can only be expected once every one hundred years. Current flood protection in different countries is based on even higher values. But what is also important is that this protection is achieved along the whole course of the river. If a dam breaks at its weakest point the hinterland is also flooded by the masses of water, even if its dams
are still intact.
Flood that can not be subdued by protective measures causes great damage. Not only basements but even the lower floors of buildings in the vicinity of the river are flooded occasionally. If dams break there is often enough time to bring the people out of harm's way and to safety, so that ultimately property damage is the only loss to be mourned.