Urban areas contribute to a drastic decline of wetlands that are often the place of river spills or rainwater storage. The wetlands are characterized by high biological diversity. Urban wetlands are specially designed wetlands located in urban spaces. They are formed by specially shaped chambers through which water flows.
In cities, they play the role of storing rainwater and delaying the run-off of water to the rainwater sewage system. Water purification is also their important role (this solution is 10 times cheaper than investment in a conventional rainwater purification plant).
Urban wetlands can be built at any point in the surface water catchment area. (The optimal location – receiving water from the rainwater drainage system from the largest possible area of the catchment area).
These areas can be used as recreational areas, however appropriate infrastructure should be created.
Environmental benefits: mitigating climate change, lowering air temperature, rainwater etention, mitigating surface water runoff, rainwater purification, creating ecological niches for urban fauna.
Social benefits: new spaces for recreation, beneficial effects on mental and physical health of local communities, increasing the esthetics of the place, education of local communities.
Economic benefits: increasing the value of the space concerned, reducing the costs associated with rainwater drainage and treatment.
They are part of creating green networks in the city:
– rain gardens, bioswales, swales, green walls, green roofs.
Construction:
– The area of urban wetlands should be from 1,000 to 3,000 m2, which allows the collection of surface water from an area of 8 ha. Other factors that must be taken into account when determining the wetland surfaces will be: the amount of rainwater, the type of substrate and the degree of water pollution (More on this subject at: https://www.zsl.org/sites/default/files/2021_Urban%20Wetlands_FINAL % 5B125594% 5D.pdf). They are often set up in places where there used to be water facilities (i.e. where the soil is not permeable).
– Water in wetlands should stay for 12 to 24 hours. It should flow smoothly without creating a pool, for example to prevent the multiplication of mosquitoes.
– The length of the chambers that make up the structure of urban wetlands should have a ratio of 4:1, have meandering shapes created in accordance with the topography of a given terrain, which allows for an increase in water flow time (it promotes better water purification). The more varied the space – the greater the degree of biodiversity. The cells should not be too long and too narrow so as not to increase the rate of water outflow from the system. The wetland area should consist of at least 3-4 cells. The depth of a single cell should be between 10 and 30 cm.
– Water to the system should not be supplied directly from the rainwater drainage system – this solution will counteract erosion. Controlling the flow of rainwater in the system is guaranteed by the plants present in it (they slow down the flow).
– When the area where the wetland is to be located is quite flat, the space should be diversified with weirs, which will control the amount of water in the individual cells of the system. Weirs should be constructed in such a way that they cascade downwards (so as not to be an obstacle for animals moving in the wetlands). In turn, when there is a large height difference between individual weirs, we should protect the structure against erosion, e.g. by stacking stones.
– Excess water from wetlands is removed to retention reservoirs or open waters. To prevent the flow from being too fast, block the outflow in a controlled manner. Therefore, it should be built in such a way that there is easy access to it.
– Sedimentation of pollutants in rainwater takes place to a large extent in the first cell of the system. So it should be a little deeper (up to 50 cm) than the others, it should also be cleaned of the silt excess every 10-20 years.
– We use native species resistant to pollution to green the system. The species used should be diverse in order to be able to create a varied mosaic of habitats.
More on the design of urban wetlands at the link:
https://www.urbangreenbluegrids.com/measures/urban-wetlands/
https://www.zsl.org/sites/default/files/2021_Urban%20Wetlands_FINAL%5B125594%5D.pdf
https://soils.ifas.ufl.edu/media/soilsifasufledu/sws-main-site/pdf/technical-papers/Sexson_Nick_Immediate_Release.pdf
Examples:
Walthamstow Wetlands, London
The urban wetlands built in London are one of the largest of its kind in Europe. The whole area covers 211 hectares. This reservoir supplies water to 3.5 million inhabitants of the city. It is also a recreational area freely available without restrictions to residents (there is an accompanying infrastructure in the form of paths, bicycle routes, viewing platforms or cafes) and a nature reserve, where various animals species have found their habitat.
Fig.
Wetlands are also an important component of many new urban parks, e.g. in New York.