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Flash floods in urban areas are becoming an increasingly frequent and severe phenomenon. The rise in the frequency and intensity of rainfall, combined with the sealing of soil in cities, leads to a rapid water run-off and flooding.

In some cases, urban flash floods occur even dozens of times more often than just 15-20 years ago. What should be done to be better prepared for such situations?

For years, the definition of a flash flood only included river flash floods recorded by dedicated institutions mainly in mountainous and highland areas. Nowadays, in the era of widespread access to social media, the term “flash flood” is primarily associated with so-called urban flash floods, which not only happen more and more often but are also becoming increasingly destructive.

It does not rain but it pours

There are two main factors contributing to the increase in flash floods, to put it simply. The first is climate change and the associated change in rainfall patterns. Although the total annual rainfall doesn’t change significantly, the frequency of rain decreases. It rains less often, but when it does, it is much more intense. This leads to more frequent and deeper droughts interspersed with heavy downpours – that’s why flash floods occur faster and more violently.

High-intensity rainfall events are becoming more frequent, which means that a significant part of the area can be flooded, especially in summer. A single rainfall event can cause even dozens of local floods, whereas rainfall with the same intensity caused less severe consequences in the not-so-distant past.

The second factor is very local and depends on the land cover structure. Sealing the ground intensifies the effects of the rainfall events, as water is unable to infiltrate the subsoil. Additionally, this doesn’t go hand in hand with the development and proper planning of greenery in many cities.

Urbanized areas are characterized by climatic conditions that differ from the surrounding areas. It rains more in cities than outside their borders. This happens due to, among other things, the presence of urban heat islands, which contribute to the formation of vertically developed clouds that are the source of heavy rainfall. However, it’s important to remember that each such urbanized area has different conditions.

On the other hand, it’s a fact that a 1% increase in surface sealing translates to a 3.3% increase in flood risk (Blum, 2020). Other factors that play a role include elements such as land slopes, the condition of underground infrastructure, the degree of soil moisture, and soil permeability. Therefore, it’s important to remember that two spatially close areas can have completely different characteristics and generate completely different reactions to the same intensity of rainfall.

The Role of Modeling

Highly precise and locally focused modeling studies are necessary to determine the threat.

Modeling also allows designers to find "tailor-made" solutions, meaning precisely matched retention measures that are hydraulically and environmentally effective, while also financially optimal.

Below we share a few examples of where our comprehensive, tailor-made solutions are used in urbanized areas in projects related to flood protection on the one hand, and increasing retention capacity on the other.

Gdańsk, the sponge city

Let us look at Gdańsk, an important port city located in Northern Poland, which has begun implementing a flood-resilience strategy based on small retention systems and green-blue infrastructure. Green areas are being gradually adapted to also fulfill retention functions. Municipal authorities are heavily investing in rain gardens, retention basins, and infiltration trenches. These solutions have become so widespread that the agglomeration is taking on the character of a sponge city.

Each city has its own unique conditions and predispositions. The first step is to thoroughly assess the existing situation, and then use the modeling results to plan investments.

Due to the dense development and underground infrastructure, urban areas are very difficult and expensive to invest in, so solutions need to be tailored to very specific local conditions.

Flood protection infrastructure of the city of Cracow

As part of the task "Improvement of flood safety in the Serafa river basin in the XIIth Bieżanów-Prokocim district of Cracow ", the investor, i.e. the state owned Polish Waters, decided to construct  additional flood protection infrastructure and our vinyl sheet piles were used in this project.

As in the case of the Złocień housing estate, also in Bieżanów the same vinyl sheet profile GW-610/9 with an integrated gasket was used, to ensure the tightness of the lock connections along the entire length of the sheet pile immediately at the time of the installation.

The wall made of vinyl sheet piles was intended to raise the maximum level of safe damming of water in the Serafa River bed, but also to extend the water filtration path in the ground. A total of 3 818 square meters of 1.5-2.0 m long sheet piles were used for the entire project.

The sheet piles were driven into the ground on one side of the riverbed, and in sections where it was deemed necessary according to the geotechnical analyses, on both sides of the Serafa river. In order to additionally reinforce the construction give it a more aesthetic appearance, the structure made of sheet piles was crowned with a system vinyl cap, also delivered by the Pietrucha Group.

In this project, the sheet piles were installed using the most popular method of installation, i.e. using a vibratory hammer. It is worth emphasizing, however, that the assembly took place in sections in a very densely built-up area. That is why, at some spots with limited access, the sheet piles were installed in a narrow excavation.

Thanks to the use of vinyl sheet pile technology with their numerous advantages, the densely developed areas along the Serafa River have been effectively and quickly protected against flooding.

In order to maintain accessibility and undisturbed pedestrian and vehicle traffic, where necessary the structures made of vinyl sheet piles were complemented with our FloodWarden modular barriers.

Poznań

Vinyl sheet piles and modular FloodWarden barriers complemented each other in a revitalization project of a reservoir with an area of 6,010 m2, located in a housing estate near Poznań, a large city located in Western Poland.

The well-worn and obsolete infrastructure in this area made it impossible to maintain a stable water level in the reservoir, and the sliding embankments posed a threat to the safety of the inhabitants.

The objective of the project was to develop the pond and the adjacent area to create a recreational spot for the local community, one that would also be an important element of water retention infrastructure.

Profiles GW 610/9 with a length of 3 m were used to create a continuous water-tight palisade protruding above the water level to a height of 15 cm so as to ensure the water retention quality of the reservoir. At the same time, the embankments of the pond were profiled to restore the recreational function.

1,090 square meters of sheet piles were used to construct the tight-wall, and the structure was topped with our system cap. The cap covered the sharp edges of the sheet piles, helped mask any irregularities, giving the wall an aesthetic appearance and protecting the structure against weather conditions.

In order to restore the water retention function of the reservoir, the original water discharge was also modernized by installing Flood Warden mobile barrier in the existing weir.

Bold decisions, integrated actions

In conclusion, the problem of flash floods in cities requires a comprehensive response. Combining retention infrastructure with green solutions, as well as taking into account the specifics of local terrain conditions, can significantly improve the safety and comfort of residents.

It is important to remember that flood protection is a task for everyone - local authorities, urban planners, architects, and residents themselves. Together, we can create cities more resilient to extreme weather events.

#floodprotection #floodbarrier #flashflood