WETLANDS IN THE 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. They should always be preceded by other treatment interventions or sediment forebays to ensure aesthetic and hydrologic benefits, and also to keep costs low.
CASE STUDIES Glenbrook Wetlands, London
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.
Thames 21 created wetlands as part of a SuDS scheme to improve the health of a local river system (Salmon Brook). The wetlands remove nutrients and coliforms and provide amenity value, flood protection and a habitat for wildlife at the same time. www.goo.gl/RvZacK
In this new development, a Health Centre was build using SuDS treatment to manage surface water. The landscape design involved the creation of areas of new, chiefly native, planting and grassland as well as a series of wetlands acting as part of the storm-water management system on the site. Employees and patients of the Health Centre are able to enjoy the landscape, including the swales that are located in the staff gardens. www.goo.gl/JXoXgb
Lamb Drove, Cambridgeshire Lamb Drove is a residential development of 35 homes on a one hectare site. A range of SuDS components were used, including permeable pavements, green roofs, swales and detention basins. When the capacity of source control components (e.g. water butts) are exceeded, water flows through swales and detention basins into a retention pond before being released into a drainage ditch. The overall savings are estimated at £11,000. www.goo.gl/teySj4
WATER REUSE Water stored in wetlands can potentially be reused for other purposes, e.g. irrigation. This may save energy and water costs.
EDUCATION 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.
COSTS & MAINTENANCE
Instalment costs are likely to be 20-35£/m3 or £15,000160,000 per wetland. The exact costs depend on design, with high land take and planning costs. Maintenance amounts to £0.1/m2 per year, and mainly consists of removal of litter and potentially sediment. They are likely to decline after the first few years.
FEASIBILITY
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.
INTERVENTIONS TOOLBOX
The Surgery, Kington, Herefordshire