Urban floods? Hydrologist from the University of Warsaw: We must take into account that they will become more and more frequent

Urban floods are a special type of flood because they are caused not by overflowing rivers, but by sudden and intense rainfall. As a result, various elements of cities are flooded in spots – streets or crossings under viaducts. Therefore, these floods are usually not life-threatening. However, they can destroy infrastructure or cause serious traffic disruptions.

– It is still very difficult for us to predict this type of floods because they depend on various factors – in addition to the amount of rainfall, the terrain and its permeability also need to be taken into account – says a hydrologist from the University of Warsaw, Prof. in an interview with PAP. Artur Magnuszewski.

According to the scientist, we should expect more and more of them, also in Poland. – Taking into account global climate change, we should take into account that urban floods will become more and more frequent – ​​he emphasizes. As he explains, this is due to the fact that the temperature of the oceans and air is rising.

– Rising ocean temperatures result in better evaporation. The stream of water that enters the atmosphere is therefore increasing. And a warmer atmosphere can hold more water vapor, which causes more rainfall – explains the hydrologist.

The intensity of rainfall is also increased by the cities themselves because they are warmer than the surrounding areas. They are sometimes referred to as “heat islands”. Therefore, when a precipitation cloud is above the city, the precipitation intensifies due to the phenomenon of convection.

Urban floods are most often caused by precipitation that comes from puffy storm clouds (cumulonimbus). The scientist points out that their occurrence in Poland is becoming more and more frequent, which results, for example, from the analyzes of Prof. Elwira Żmudzka from the Faculty of Geography and Regional Studies, University of Warsaw.

How can cities protect themselves?

How can cities protect themselves against floods caused by heavy rainfall? The hydrologist indicates that it is important to slow down the flow of water. – The key is to have as much green space as possible – lawns, green areas and trees. They capture excess water and slow down its flow into the sewage system, which is often simply overloaded in the event of heavy rains, says the scientist.

Lawns growing on the roofs of buildings are also helpful. According to prof. Magnuszewski, green areas have a double benefit: better quality of life for residents and protection against the effects of heavy rainfall. – Moreover, the more plants in the city, the better living conditions of residents, better thermal comfort and less air pollution – adds the hydrologist.

But green areas are not the only solution. In China, where the problem of intense rainfall is related to the monsoon climate, even more advanced solutions are used.

– For example, playgrounds or playing fields, which are built in the form of basins, in the event of heavy rainfall, turn into retention reservoirs where excess rain is collected – the scientist points out. Care is also taken to ensure that large concrete surfaces, e.g. parking lots, are more permeable and absorb water.

According to prof. Magnuszewski, awareness of this disturbing phenomenon is growing in Poland. They are noticed not only by municipal employees, but also by developers in the designs of new housing estates.

– More and more often, not all surfaces are covered with paving stones or covered with asphalt, but we are also thinking about large enough green areas. Sometimes tanks are installed to store rainwater, which is then used to water plants. And since the building is referred to as “ecological” at the same time, it arouses greater interest among potential customers, he notes.