Towns throughout Germany are struggling with high water levels. Rivers, streams and lakes have massively swollen, particularly in parts of Saxony, Thuringia, Lower Saxony and North Rhine-Westphalia. Days of continuous rain hit soil that had sometimes been parched for months. In many cities and communities, water is already standing in fields and flooding streets. In some districts, people had to leave their homes as a precaution to avoid being trapped by the floodwaters.
For days now, the situation has been described naturally using terms such as flooding or storm surge. But when do these terms even begin to be used? The star provides information.
Basically, floods are natural events. In maritime waters and connected rivers, the term describes the highest water level during the flood (mean high water). On the contrary, low tide is the lowest level during low tide. The Domestic Water Act defines flooding as “temporary inundation of land not normally covered by water by surface water or by seawater entering coastal areas.”
According to the Federal Environment Agency, regular floods have an important ecological function. Animals and plants on river floodplains, for example, have adapted to the interplay between flooding and drying out. In areas where floods occur frequently, residents are usually well prepared for these events and damage is very rare. An example of this is the city of Hamburg, where floodwaters from the Elbe flood the St. Pauli fish market several times a year.
The reasons for flooding of non-tidal bodies of water such as lakes or smaller rivers are usually due to an interaction between precipitation and sealed soils. Particularly long-lasting or shorter but particularly intense rainfall hits soil that can no longer absorb the amount of water. This is particularly common in winter, when the ground is frozen and water cannot seep away, while precipitation or melting snow causes rivers and streams to swell into raging torrents.
In Germany, a DIN standard even defines what exactly a storm surge is. Accordingly, such an event is described as “a rise in water on the sea coast and in estuaries in the coastal area caused by strong winds when water levels exceed a certain value.” Or to put it more simply: Storm surges occur when strong winds push seawater against coasts or into the “German Bight”, thus increasing water levels.
From a water level of 1.5 meters above mean high water, the Federal Maritime and Hydrographic Agency (BSH) warns of a storm surge. In Hamburg, for example, such a storm surge is only perceived as a significantly higher water level. From 2.5 meters above the mean high water the classification “severe storm surge” applies; from 3.5 meters above the mean high water it is referred to as a very severe storm surge.
However, it cannot be said in general terms whether water levels at this height have any impact and whether flooding will occur. This depends on the structural conditions and the surrounding area.
The spring tide is commonly, although incorrectly, referred to as a “spring tide”. It is a particularly strong form of tide, occurs twice a month and is the result of a special sun-earth-moon constellation. Ebb and flow are influenced by the gravity of the moon and sun. If the sun and moon lie on an imaginary line with the earth, the attractive forces add up.
The result is a stronger ebb or flow, which “pulls back” the water in seas or adjacent rivers particularly strongly and then “pushes it forward” again. The spring tide is a natural phenomenon and usually happens without any particular impact.
Anyone who thinks of tidal waves as rising masses of water or even something like tsunamis is on the wrong track. The very waves that break when they hit land are often incorrectly referred to as tidal waves. The explanation is obvious: a wave that triggers a flood, i.e. a flood.
But this explanation is a misconception. Hydrography defines a tidal wave simply as a “regularly recurring shallow water wave caused by the effects on the ocean of gravitational interactions between the Sun, Moon and Earth.” A tsunami, on the other hand, is triggered by an external event such as an underwater earthquake and is not caused by the tides.
A flash flood is a particular challenge for disaster and water conservationists. Scientists define it as a sudden flood, for example due to a dam bursting or extreme rainfall, in which neither the ground nor the sewage system can absorb the precipitation, and surface waterways form.
Flash floods are considered dangerous because they occur quickly and unpredictably. They are particularly dangerous when heavy rain falls in dry areas. The waterways can have destructive power, sweeping away entire buildings and moving for several kilometers. Water levels as low as 50 centimeters can wash away cars.
Sources: Hamburg.de, Federal Environment Agency, German Weather Service, Bavarian State Government
