Flash floods – how to protect yourself from the elements

Progressive climate change means that extreme weather events are occurring more frequently and with greater intensity. They are also appearing in regions that were previously considered safe. Intense rainfall, causing flash floods, is a particular threat. Analyses conducted by IMGW-PIB indicate that by the end of the 21st century. The probability of daily precipitation above 20 and 30 mm will increase in most Polish cities. This means that as a society we need to be well prepared for the dangers of flash floods (flash floods) and urban floods (urban floods).

Flash floods occur when a large amount of rain falls on a small area in a short period of time. Typically, these types of events produce heavy rainfall associated with localized cloud bursts or slow-moving and stationary storm cells. But flash floods can also be caused by the failure of hydroelectric facilities or the breach of a dam or levee. Such construction disasters are extremely rare, but when they do occur, the damage is enormous – the rushing mass of water destroys everything in its path and there is no escape from it.

A number of environmental factors determine the scale and course of flash floods. The steeper the slope and the more sealed the land surface, the faster the rainwater runoff. The same is true when precipitation occurs, for example, after a period of drought or in an area devoid of vegetation. The soil then does not absorb excess water, which accumulates in depressions and “seeks” the shortest and fastest way down. For this reason, flash floods can occur virtually anywhere, and human activity – among other things – can occur anywhere. deforestation, surface sealing, wetland drainage or neglect in the area of water management – increases the risk of its occurrence even where this phenomenon has not been experienced before.

In Poland, flash floods are increasingly affecting urban areas. The stormwater drains in operation there – often outdated and inadequately maintained – are not adapted to changing climatic conditions. In addition, in many cities, the way space is developed reinforces the dangers of rainfall – even if its intensity is not exceptionally high. As a result, water is beginning to pool in the streets and seep into lower-lying areas, such as subway stations, tunnels, basements, etc. This is when urban floods – so-called urban floods– occur, which completely destabilize city operations and the lives of residents. In 2022. Such incidents have occurred several times.

In July, among other things. In Szczecin and Czestochowa, where nearly 70 mm of rain fell in a day. In August in Opole, after a rainfall total of 100 mm was recorded for 24 hours. This month was exceptional for the historic capital of Upper Silesia in general, as the rainfall norm was exceeded five times (a total of 273 mm of rain fell for the month). Even heavier precipitation, which caused flooding and waterlogging, occurred on September 7 in Gorzow Wielkopolski – more than 130 mm of rain fell there in 24 hours.

Monitoring and forecasting

Flash flooding is so dangerous and unpredictable because it occurs several hours after the rainfall occurs (3 to 6 hours), and there are known cases where the water runs off after a few tens of minutes. Therefore, monitoring this type of threat and warning of it is extremely difficult. We would have to have a very dense measurement and observation network – both meteorological and hydrological – to track weather developments in real time and monitor the condition of local rivers, streams and creeks. Even the best-organized services in the world do not have such an extensive system, and some areas remain out of control.

Nonetheless, modern technology is greatly improving our safety and providing the knowledge needed to make appropriate decisions. In Denmark, after the flash urban flooding that hit Copenhagen in 2011, a so-called bluespot map was developed – a digital map of low-lying areas that could be filled with water as a result of a cloudburst, for example. GPS systems and Lidar technology were used to prepare it, which made it possible to generate a terrain model that was not only highly detailed, but also high-resolution.

In the United States, on the other hand, since 2003. a method of studying flash floods is being developed, in which catchments are classified according to their potential for water runoff. The FFPI index uses historical flood data and maps the physical characteristics of the catchment that are relevant to its formation: surface slope, land use, soil type and vegetation cover. Another interesting indicator is the Flash Flood Vulnerability Index used by many countries around the world, which makes it possible to delineate areas at risk of flash floods, but taking into account the losses they can cause.

In Poland, the problem of the threat of flash floods has been on the rise for several years. Many research centers are carrying out scientific work to improve our knowledge and increase the security of society and the economy. At IMGW-PIB, for example, the PMAXTP project has been completed, providing planners and engineers with a set of modern tools for designing infrastructure to drain excess water. More than 2,500 have been created. maps of maximum precipitation characteristics and an interactive map of Poland for visual and numerical presentation of results.

All these methods have advantages and disadvantages. However, since it is virtually impossible to fully protect against flash floods, any tool that helps reduce their negative effects and minimize losses is useful for services and local communities. Awareness of the risk and notifying residents of the danger as soon as possible are priorities. In this context, an initiative by the UN and the World Meteorological Organization called “Early Warnings for All,” an integrated global warning system that by 2027. is to ensure that every person on Earth has access to information about weather and climate risks.

According to the WMO, in the last fifty years, statistically every day there has been a disaster related to hazardous weather, causing 115 deaths and economic losses of $202 million. Although the number of disasters has increased fivefold in recent decades, thanks to early warning systems and efficient emergency management, the number of deaths has decreased by almost three times. This shows that the development of certain procedures can significantly reduce the negative effects of extreme weather. According to the Global Commission on Climate Change. Adaptation spending $800 million on such systems in developing countries would avoid losses of between $3 billion and $16 billion a year. That’s the equivalent of 50 cents a year per person for the next five years.

Flash floods – how to protect yourself?

Public awareness is crucial in this case. Residents in areas where the risk of flash floods is higher should know this and be prepared. If you live in a mountainous region or with significant differences in relative altitude, in a depressed area or adjacent to a stream or canal, and there are water damming facilities in the area – you must consider it likely that one day you will experience a flash flood.

You should also pay attention to how land use is changing in your area. Are there no activities and investments that may increase the risk of flooding. Take an interest in current meteorological and hydrological conditions in the region. If forecasts indicate the possibility of thunderstorms or a stationary front with heavy precipitation, prepare accordingly and follow developments. It is worth reading the guides available on the web, which show in a simple and accessible way how to safely survive a flash flood and what to do after it has receded.

Protecting property from flood damage requires a considerable amount of money, and not everyone can afford it. The construction of barriers, embankments or basement of a residential house will effectively increase our safety, but such investments may not be feasible, if only because of building regulations or environmental requirements. Therefore, it is worth directing efforts to support local policies, responsible land use or urban space management. This is because very often the intensity of flash floods is amplified by faulty administrative decisions.

First, let’s protect natural ecosystems, such as forests, wetlands, small bodies of water, meadows and green wastelands, which slow down rainwater runoff, protect soils and increase the retention capacity of catchment areas. Second, let’s not allow the concreting of our cities. The more greenery there is, the better the chance of avoiding a catastrophic rainfall flood.

Of course, don’t forget to maintain and properly expand the city’s stormwater drainage system, because even the best-designed green roofs or water parks won’t take all the excess water. Let’s also pay attention to surrounding developments – building a road without culverts, baffling a dried-up riverbed, and planting a massive structure over shallow groundwater can have disastrous consequences for flood safety.

The phrase that most often falls in conversations with those affected by natural disasters is: “as I live, I haven’t seen anything like it.” Modern climate change means that we will increasingly experience extreme weather events of unprecedented magnitude, intensity and frequency. And since the climate is not a constant thing, you have to constantly keep track of what is happening to it. And be ready for change.

Rafal Stepnowski, UW graduate, geographer, hydrologist, popularizer of Earth sciences. Specialist in the field. communications and content marketing.