It is a grassy trough designed in the lowering of the terrain or a specially profiled grass surface, which during rainfall stores rainwater, which then seeps into the deeper layers of the earth. It is important to remember that the bottom of the basin is flat and the ground is permeable enough for the water in the basin to stay in it for more than 24 – 48 hours.
Establishing retention basins in urban areas brings many benefits:
Environmental benefits: rainwater retention, rainwater purification, reduction of water runoff to the sewage system, reduction of the urban heat island (lowering the air temperature), air purification.
Social benefits: new spaces for recreation, beneficial effects on mental and physical health of local communities.
Economic benefits: reduction of costs related to the discharge of rainwater to the rainwater sewage system.
A solution for professionals and local communities.
Location: Retention basins are preferably located on extensive lawns in park and housing estate spaces. We do not use these solutions in industrial areas, near construction sites, on slopes above 20%. Ideally, it should be up to 5%.
Before the facility is constructed, tests related to the soil stability of the soil should be carried out and the permeability of the existing substrate in the area of the project should be examined. We also determine the height of the groundwater.
There should be drainage directly under the infiltration basin, which is to facilitate the infiltration of rainwater into the deeper layers of the soil, as well as to maintain aerobic conditions for bacteria decomposing pollutants (Exemplary combination of filtering layers 30 cm/h – of gravel with a fraction of 7 mm, on it – a layer of sand with soil in the proportion of 1:4 – 20 -30 cm/h). The water table should decline at a rate of 64 mm per hour.
The area of the retention reservoir should be allocated to no more than 2 ha of the catchment area. To accept rainfall at the level of 25-year rainfall. Depth no more than 90 – 120 cm.
For the possibility of rains of greater intensity, it is recommended to build a special drainage system, the so-called emergency drainage and emergency flow. To ensure the patency of the system and to protect it from clogging, a geotextile should be placed on the drain.
The whole should be covered with good-quality vegetation. The grass here needs to be trimmed regularly.
Important:
– water from the rainwater drainage system should not be directed to the basins during dry periods, as it may contain a high concentration of pollutants.
– these basins should not be installed next to facilities whose operation is associated with the production of large amounts of pollutants.
Example: Södertälje
The infiltration basin was built on the terrain of a housing estate where in the past there used to be problems with the rainwater collection, which resulted in local flooding. Therefore, it was decided to create a space where water could infiltrate the ground and delay the moment of its discharge into the rainwater drainage system. For this purpose, the natural topography and depressions existing in it were used. By digging up a portion of the earth, the capacity where rainwater can be stored has been increased. A special sewage system has also been installed, through which rainwater is supplied from nearby areas and delivered to grassy basins. From them, the water infiltrates the ground or during heavy rains it is discharged to the proper rainwater sewage system (the main task is therefore to slow it down before reaching the proper storm sewage system). This space plays a recreational role on a daily basis.
An example, Copenhagen
The project is the revitalization project of the Inner Nørrebro area. Its aim is to adapt the city to climate change, create new ecosystems, as well as create new spaces for the integration of the local community. In this project, a system of retention ponds and basins (during dry periods, acting as a football field) was established in a small park. These components are to protect the space from flooding during heavy rains or hailstorms. The park will be able to store 18,000 cubic meters of water. During exceptionally heavy torrential rains, excess water will be transported to a nearby lake, it will be cleaned of pollutants by specially shaped greenery.
https://iwa-network.org/greater-copenhagen-water-utility-hofor-as/
Example of New York
Post-rail areas transformed into a park with a rebuilt riverside habitat. An important component of the project is protection of the riverbank against flooding. The former swamp areas were rebuilt and a recreational and cultural place was created that can be used by all the city’s residents. The lawn and the bioswales set up around it are designed to purify and store rainwater.
Sources and additional information:
https://www.maine.gov/dep/land/stormwater/stormwaterbmps/vol3/chapter6.pdf
https://www.nj.gov/dep/dwq/highway/pdf/NJ_SWBMP_9.5.pdf
https://www.water.wa.gov.au/__data/assets/pdf_file/0018/5355/84985.pdf
https://uslugiekosystemow.pl/wp-content/uploads/2017/07/05_niecki.pdf
https://mwik.bydgoszcz.pl/index.php/component/attachments/download/445