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Hydrologist: flash floods will become more frequent

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We cannot escape the sudden rainfall. We will also not avoid flash floods and sudden river surges, said hydrologist Professor Artur Magnuszewski from the University of Warsaw. He stated that situations like the one in Bielsko-Biała on Tuesday, when many streets and properties were under water in a short time, would occur more and more often. This is the result of the expansion of cities and their sealing.

Heavy rainfall on Tuesday caused flooding in numerous places in the south of the country. In Bielsko-Biała, during a downpour, a month's rainfall fell within a few hours, which caused a flash flood. According to the voivode's services and firefighters, from 100 to 150 liters of water per square meter were recorded in the city and surrounding areas. The last time such intense rainfall was recorded in the region was in 1972.

– We cannot escape the sudden rainfall. We will also not avoid flash floods and sudden river surges, although what happened in Bielsko-Biała is symptomatic of mountainous areas – said Prof. Magnuszewski. – In science we talk about the orographic effect. This phenomenon means that the flow of air over the mountains intensifies precipitation. When we look at the distribution of rainfall and its annual total, Tatra Mountains we have 1,200 millimeters of rain, and in Central Poland only 550-600 mm per year – he explained. He added that high mountains are not needed to cause the orographic effect. Observations show that even the Świętokrzyskie Mountains have the ability to activate this phenomenon.

The terrain and the orographic effect cause much more water to fall from the same air mass than in lowland areas. Tight river valleys and steep slopes cause sudden surges, high water levels and fast currents. This means that the risk of flooding and flooding in mountainous areas such as the Bielsko-Biała area is high.

“These are systems built in the 19th and 20th centuries”

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However, a hydrologist from the University of Warsaw noted that we will encounter flash floods more and more often.

– The growth of cities and their sealing result in the formation of heat islands. In summer it is very visible. A concreted city heated by the sun turns into a heat island, which causes stronger convection currents, which increases the dynamics of the atmosphere and the ability to form storm clouds and cause sudden, heavy rainfall – he explained. Heavy rainfall in a small area and in a short time means that the sewage system is unable to remove excess water. It rises and flows into basements, flooding low-lying areas, drowning houses and cars.

The Biała River in the center of Bielsko-Biała on Tuesday after heavy rainfall PAP/Jarek Praszkiewicz

– Sewage systems in cities collect rainwater and sewage. These are systems built in the 19th and 20th centuries. Considering the area of ​​the cities and their structure at that time, there was no problem with capturing rainwater given the density of buildings and large unconcreted surfaces existing at that time – said the expert.

He added that over a hundred years of urban development, the situation has changed. – Cities have expanded and become more sealed. We have sidewalks, asphalt roads, concrete squares. Even the roofs of large buildings collect water. The sewage system is unable to 'swallow' this mass in a short time. The sewage treatment plant will not accept it either, because it may result in flooding of the equipment and a breakdown – explained the hydrologist. Therefore, in extreme cases, rainwater together with domestic sewage is directed through storm overflows straight to rivers, bypassing treatment plants, and this causes surface water pollution.

Blue-green infrastructure

– It is hard to imagine that we will completely forget about existing solutions, such as sanitary or storm sewage. But an addition to the traditional system may be the so-called blue-green infrastructure. Ecological solutions and micro-retention reduce the amount of water that reaches the tight surface of the city during a downpour. They will not prevent floods, but they can significantly reduce their effects – said the professor.

Solutions used around the world include, for example, green areas that serve a recreational function and turn into retention reservoirs during rainfall. – An additional advantage of this solution is that rainwater does not flow into the sewage system, but soaks into the ground, feeding groundwater. In the idea of ​​blue-green infrastructure, we go in the direction that part of the water is directed into the ground to rebuild aquifers – described the hydrologist.

Flooded streets in Bielsko-Białasid/bielsko.info

The expert mentioned designers of sewage systems for Warsaw Whether Łódź from the turn of the 19th and 20th centuries. – Even a visionary like Lindley could not have predicted that in a hundred years cities would be so tight with concrete, and the climate would change so much that then rare phenomena, such as flooding, torrential rains or flash floods, would become common – said Prof. Magnuszewski. – That's why we should take a fresh look at design standards, because storm sewage systems are inefficient, but we won't build entire storm sewer systems with the diameter of subway tunnels again, because it's impossible, the expert pointed out.

What else to do to reduce the effects of downpours?

He added that there are other ways to deal with downpours. For example, build tanks that will retain rainwater during downpours, when the sewage system is under the greatest load. When the water subsides, the water collected in such reservoirs will flow to the treatment plant. As an example, the hydrologist gave the construction of underground retention tanks for rainwater, which are supposed to capture the largest waves and release excess water when its level in the sewage system normalizes.

There are more ideas to reduce the effects of heavy rainfall. These may be designed depressions in the ground or lawns in parks, which in the event of a downpour will capture and discharge into the ground, or at least retain some of the rainwater until the end. storm. Urban devices can also be designed to have dual functionality. Prof. Magnuszewski gave an example of basins which under normal conditions are used as skate parks, but in the event of a downpour they turn into retention reservoirs.

Main photo source: PAP/Jarek Praszkiewicz

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