An urban constructed wetland is a type of blue infrastructure (i.e. consisting of a permanent body of water) that can provide a range of ecosystem services. They are different from ponds in that they have more shallow zones in which bottom-rooted vegetation can grow. Wetlands consist of different zones that are either permanently wet, permanently dry or periodically wet. The periodically wet zone provides room for storing surplus water in high rainfall events. Release of water can be controlled through structures at the outlet of the wetland.
In permanently wet zones, vegetation acts as a filter slowing and stabilising suspended solids and adsorbing pollutants. Pollutants are also destroyed by microbial processes or UV radiation. Artificial as well as natural wetlands store water and provide habitat for different species.
Wetlands can be designed to suit various sites and functions, however they generally need a comparatively big area of land to function and keep costs low.
Can provide highly valuable recreational areas (has been shown to be up to ~63,400£/ha/a) that encourage physical activity and have positive health impacts. (17,22, 31)
Potential to reduce air pollution significantly, but few studies on constructed wetlands. (8)
Reduction of volume and peak flow potential >80%. Storage area needs to be provided (high land take). Helps to reduce flood impact by delaying high flows but not necessarily reduction in volume. Varying success. Can increase peak flow due to saturation if capacity full. (5,6,7,9,11,14,22, 23, 28)
Can provide flood prevention if positioned upstream/in floodplain areas. Few studies on constructed wetlands. (23, 25)
High carbon storage potential (up to 2.4kg/m2/yr net), can regulate air temperature and have significant positive effect on UHI. Dense vegetation increases carbon sequestration potential. However, GHG release can potentially occur. (12,16,21,22)
Potentially very high but depends on design. Can provide important stepping stones for migratory birds, but depends on size. However, high pollutant loads can compromise this. (18, 19, 22, 24)
Wetlands can increase water flow during dry seasons but may also decrease it. (25)
Effective pollutant reduction: sediment ~90%, nutrients avg. 60% depending on retention time and season. Reduction of hyrdocarbons 50-80%, heavy metals varying but up to 99%. During dry seasons, storm events can wash out pollution w sediment. High water temperature may be an issue. (2,5,6,9,11,13,14,18, 26, 32)
Potentially very high if open water is visible. Water bodies have been shown to provide sense of place, restorative environments and so many cultural benefits. (17,22, 30, 31)
Potential very high, can be used for angling, birdwatching etc, but depends on design. (17,22, 29)
Can increase property value by up to 28%. Some studies even show up to 300% increase. Increased spending in commercial areas. (11,20)
Taking up water from their own area and surrounding areas can help reduce the risk of flooding and the extent of flooding on a larger scale.
Considering the Bigger Picture
Wetlands are most suitable as the last stage of the treatment process (secondary and tertiary treatment). They provide infiltration (but only above non-vulnerable groundwater) to an extent and storage.
To function, a wetland needs a continuous water flow. Artificial as well as natural wetlands store water and provide habitat for different species. Wetlands can be designed to suit various sites and functions, however they generally need a comparatively big area of land to function and keep costs low.
They should always be preceded by other treatment interventions or sediment forebays to ensure aesthetic and hydrologic benefits, and also to keep costs low.
On the left, you can find an example of how different interventions can be incorporated into the urban landscape.
To provide a comprehensive treatment and management of surface water, wetlands should be seen within the wider landscape. While they are able to store and treat runoff, it is important to understand that their ability to reduce pollution is limited by the retention time and the pollutant load.
Especially where large quantities of pollution runoff are expected, filter strips or sediment forebays should be incorporated. Wetlands can take up water from other interventions, either as part of the standard runoff path or to deal with flows exceeding the design of other interventions.
Below, you see some of the multitude of additional benefits that trees provide in the context of the urban landscape.
20-35£/m3 or £15,000-160,000 per wetland. The exact costs depend on design, with high land take and planning costs. (4, 11)
Residential, Industrial (Retrofit – if site conditions make it possible or pocket wetland) Sufficient base flow needs to be provided, low infiltration rates of soil. They are best used to take runoff from multiple areas after it has undergone primary/secondary treatment. (5,14, 27)
0.1£/m2/a. Removal of litter and potentually silt/sediment, vegetation (pruning etc.). Fences, landscape maintenance. Costs are likely to decline after the first few years. (4)
Trade-offs and Potential Dis-services
Blue spaces have high impacts on stress levels, and emotional connection to blue spaces is higher than to green spaces. This can strengthen the sense of place and identity and so improve wellbeing.
wetlands can provide highly biodiverse, unique habitats and if designed and maintained correctly can be used to educate children and adults about various nature-related topics. The spaces can also be used as outdoor classrooms.
if not enough pre-treatment is provided, pollution of sediments might occur and wildlife might be negatively impacted by the heavy metals etc in the water.
If not designed correctly, it can be perceived as a hazard mainly for children.
Water stored in wetlands can potentially be re-used for other purposes, e.g. irrigation. This may save energy and water costs.
Standing bodies of water could release GHG like methane. This is dependent on their design, and should be considered when constructing the wetland.
Danger of pollutants being washed out of wetland, higher water temperatures in water body can have impact on aquatic species downstream