Climate change and urbanisation are increasing the risk of flood and drought phenomena. Retention reservoirs are one effective solution to manage rainwater.
The experience of September 2024 has shown that the construction of large retention reservoirs makes sense. Despite the high costs, including environmental costs, the construction of new reservoirs, which play an important role in the flood and drought prevention system, should not be abandoned.
In Brief:
Types of Flood Retention Reservoirs
Open water reservoirs
Open reservoirs are the most common solutions that take the form of artificial lakes. They often have other functions as well, such as electricity production or water reservoir for large agglomerations, as is the case with Goczałkowice and Dobczyce. Often these types of reservoirs, in addition to water retention, serve as a place for recreation and leisure.
An example of a dry retention reservoir is Racibórz Dolny. Its main purpose is to protect against flooding by collecting excess water during floods. This flattens the flood wave and delays its arrival in the Oder Valley and nearby cities such as Wrocław. The reservoir proved its usefulness in September.
It has a capacity of 185 million cubic metres and there are plans to increase this to 320 million cubic metres in the future. Other examples include the four new reservoirs of the Klodzko Basin in Szalejów Dolny, Krosnowice, Roztoki and Boboszów. During the construction of the first two, HydroBIM engineers acted as author’s supervisors.
Closed tanks
Closed tanks are solutions used primarily in densely built-up urban areas where there is no space for the construction of open tanks. These types of tanks can be installed under car parks, squares or parks.
There are several cities in Poland that have invested in underground retention tanks to manage rainwater and increase the efficiency of its use.
In some cases, hybrid solutions that combine open and closed reservoirs are used. Such systems make maximum use of the space available in cities. These reservoirs can be used for a variety of purposes, such as rainwater and water recycling for industrial purposes or irrigation of green spaces.
Benefits of using flood retention reservoirs
As we wrote above, one of the main tasks of retention reservoirs is to protect against floods. By storing excess water during heavy rainfall, reservoirs reduce the risk of urbanized areas being flooded and sewerage systems being overloaded. An excellent example is the Racibórz Reservoir, which has unquestionably proved its usefulness.
Flood retention reservoirs are also an important source of water during periods of drought. An example is the Solina Reservoir, which, in addition to flood protection, serves as a water reservoir for hydropower and recreation. During summer periods, water from the reservoir is used to stabilise flows in the San River.
A similar function is performed by Lake Mucharskie, which supplies water to the surrounding area while protecting it from flooding. This is an example of a multi-functional retention reservoir that caters for both residents and industry.
Retention reservoirs can also contribute to improving water quality by filtering pollutants and through the process of sedimentation.
Challenges and limitations
Despite the numerous benefits of reservoirs, their construction and operation are also associated with inconveniences.
One of the biggest challenges associated with retention basins is the high cost of construction, as well as subsequent maintenance and monitoring costs. Retention projects, especially on a large scale, such as the Racibórz Reservoir or the Reservoirs of the Kłodzko Basin, require huge financial outlays. The cost of building the Racibórz Dolny Reservoir amounted to more than one billion zlotys, one of the most expensive hydrotechnical projects in Poland.
Maintenance costs include regular maintenance, monitoring of technical condition and protection against possible damage to dams and water discharge systems. After the floods in September, there was a lot of publicity about this issue. It is a problem that cannot be ignored.
Need for site-specific adaptation
Every reservoir must be adapted to the local geological and hydrological conditions. Not every site is suitable for the construction of a large reservoir. In regions with unfavourable terrain, such as mountainous areas, it is necessary to build smaller, local retention reservoirs. Examples are the reservoirs in Szalejów Górny and Krosnowice.

Reservoirs built in areas of high natural value affect ecosystems and require the implementation of measures to minimize the negative impact on the environment.
Flood Retention Reservoirs – summary
The experience of the September flood is strong evidence in favour of building new retention reservoirs in Poland. Investment in retention reservoirs, although having significant material and environmental costs, must be included in long-term strategic planning and budgeting on a national scale.
Proper management and monitoring of the technical condition of existing and newly designed retention reservoirs is also essential. Digital Twin technology, with which we have good experience in HydroBIM, can be put to good use here.
Contact us if you are in search of support in solving water engineering problems in your project.