Sustainable Urban Drainage Systems (SUDS) are the most appropriate current solutions but SUDS can only be implemented with good policies, supportive stakeholder groups and partnerships so that these new ideas, which cut across existing methods and practices, can be accepted.
With rapid urbanisation there is an urgent need of city development and to reduce the impact of flooding on residents and in other places, and the worsening of the water quality in streams, rivers and lakes caused by the expansion of cities. Sustainable Urban Drainage Systems (SUDS) provide a natural approach to managing drainage in and around sites. SUDS work by slowing and holding back the run-off from a site, allowing natural processes to break down pollutants. SUDS deal with run-off close to the source, rather than transporting it elsewhere via the public sewer system. Sustainable drainage systems have several benefits; slow down surface water run-off to help reduce chances of flooding, reduce risk of sewer flooding during heavy rain and recharge ground water to help prevent drought, among others.
The New Urban Agenda (NUA) adopted at the HABITAT-III Conference in Quito last October is also supportive.
Sustainable Urban Drainage Systems require several changes in thinking and practice in city planning. There are many barriers to progress including the perceived costs added to development, the increased maintenance activities required, the attractiveness of big infrastructure projects to politicians whereas drainage projects are very often just ‘normal work’. The barriers to more sustainable drainage are high but a whole portfolio of potential ‘Green’ infrastructure solutions are available to be applied in any city in the world.
To achieve more widespread use of sustainable drainage principles, greater integration into Green Infrastructure is necessary, and its multiple benefits need to be clear. Major developments and redevelopments give the opportunity for the reallocation of open space to improve its use through multiple functions. Sustainable drainage has the potential to provide habitat improvements which provide places for breeding, give connectivity between SUDS and with natural areas, and link directly to zones of natural habitats thus providing more sustainable solutions and greener solutions to drainage problems.
SUDS are physical structures built to receive surface water run-off and provide a drainage system thatdeals with run off as close to source as possible, seeks to mimic natural drainage and minimises pollution and flood risk, among others. Traditional systems on the other handmove rainwater as rapidly as possible from where it falls to a point of discharge e.g. watercourse. This causes a number of problems including increased flooding, poor water quality as run off can contain pollutants and less infiltration to ground leading to poor groundwater recharge.
Therefore, using SUDS is most important for sustainable development. All developments must carefully consider appropriate sustainable surface water drainage options. Careful design of drainage systems and/or the provision of treatment facilities prior to discharge will assist in reducing the environmental impact of new development. This range of techniques is known as SUDS. They can be successfully applied to most development and can even be fitted to existing development. There are considerable environmental and economic benefits of incorporating SUDS techniques in local development. SUDS can be applied to large or small developments due to the variety of techniques available.
Building Regulations
Incorporation of SUDS should be reinforced as part of the development process by changes to Building Regulations. Such regulations may stipulate that, in order of priority, rainwater run-off should discharge into one of the following:
- an adequate soakaway or some other adequate infiltration system; or where that is not reasonably practicable;
- a watercourse; or where that is not reasonably practicable;
- a sewer
Methods of Sustainable Urban Drainage Systems
Permeable surfaces and filter drains
Filter drains and permeable surfaces are devices that have a volume of permeable material below ground to store surface water. Run-off flows to this storage area via a permeable surface. Examples of this are: grass, reinforced grass; gravelled areas, solid paving blocks with large vertical holes filled with soil or gravel, solid paving blocks with gaps between the individual units, porous paving blocks with a system of void within the units and continuous surfaces with inbuilt system of cavities.
Permeable surfacing encourages surface water to permeate into the ground. Depending on the ground conditions, the water may infiltrate directly into the subsoil, or be stored in an underground reservoir (e.g. a crushed stone layer) before slowly soaking into the ground. Pollutant removal occurs either within the surfacing material itself, or by the filtering action of the reservoir or subsoil.
Infiltration Devices
Infiltration devices drain water directly into the ground. They may be used at source or the run-off can be conveyed in a pipe or swale to the infiltration area. They include soakaways, infiltration trenches and infiltration basins as well as swales, filter drains and ponds. Infiltration devices can be integrated into and form part of landscaped areas. Soakaways and infiltration trenches are completely below ground, so water should not appear on the surface. Infiltration basins and swales for infiltration store water on the ground surface, but are dry except in periods of heavy rainfall.Soakaway is an underground chamber lined with a porous membrane and used to store surface water, and then allow its gradual infiltration into the surrounding soil.
Swales and Basinsare dry channels or ditches and basins are dry “ponds”. They can be created as features within the landscaped areas of the site, or they can be incorporated into ornamental, amenity and screen planted areas where they would be maintained as part of a normal maintenance contract. They provide temporary storage for storm water and reduce peak flows to receiving waters. Swales and basins are often installed as part of a drainage network connecting to a pond or wetland prior to discharge to a natural watercourse.
Infiltration trenches and filter drains: Infiltration trenches are stone filled reservoirs to which stormwater runoff is diverted and from which the water gradually infiltrates into the ground. Filter Strips: An area of gentle sloping, vegetated land through which surface water run-off is directed. Filter drains are similar to infiltration trenches but have a perforated pipe running through them. They are widely used by highway authorities for draining roads.
Basins and Ponds
Basins are areas for storage of surface run-off that are free from water during dry weather conditions. These structures include flood plains, detention basins and extended detention basins. Ponds contain water in dry weather and are designed to hold more when it rains. Ponds and Wetlands can be particularly beneficial during time of storm due to their capacity to hold large amounts of water and therefore reduce flood risk. Algae and plants in wetlands can significantly assist with filtering and nutrient removal. The ponds and wetlands can be fed by swales, filter drains or piped systems.
Choosing the right SUDS
Of the various methods, large ponds and wetlands are generally more appropriate for larger sites in excess of 5ha. Infiltration trenches, swales and porous pavements are suitable for both large and small sites. Many large sites may incorporate a mix of different mechanisms.
SUDS and Planning
It is important that developers establish the soil conditions and hydrology of the site (storm water run-off, water table height, water quality) and consider appropriate SUDS at an early stage in the site evaluation and design process. This will ensure that the best drainage solution for a particular site is found and incorporated into the layout, development costs and timetable for implementation. Planning conditions or legal agreements should be used to secure implementation of SUDS where appropriate.
New Urban Agenda (NUA)
At the Habitat-III Conference in Quito in October 2016, New Urban Agenda (NUA) was adopted which aims at enhancing effective urban planning and management, efficiency, and transparency through e-governance, information and communications technologies assisted approaches, and geospatial information management. Further, New Urban Agenda underscores the need to promote adequate investments in accessible and sustainable infrastructure and service provision systems for water, hygiene and sanitation, sewage, solid waste management and urban drainage among others.NUA seeks to ensure that this infrastructure is climate resilient and forms part of integrated urban and territorial development plans, including housing and mobility, among others, and is implemented in a participatory manner, considering innovative, resource efficient, accessible, context specific, and culturally-sensitive sustainable solutions.
SUDS should be part of local economic development strategies which also coordinate land use, infrastructure and investment planning. Financing and investment planning are also important driving concerns. Urban Local Bodies (ULBs) should identify set of policies that will allow cities and their surrounding regions to reap the benefits of economies of urbanisation and localisation, attract and leverage private investments while minimising risk hazards. To be successful, SUDS have to be an integral part of the local and regional urban planning and strong building bye-laws.
Available evidence indicates that in low and middle income countries, urban drainage sector is among few other sectors including sanitation and solid waste management that has made little progress in addressing the need for institutional reform and financial sustainability. New approaches are needed in urban drainage sector in delivering services to the informal settlements. The whole life costs of the systems of drainage infrastructure can be correlated to the pattern of urbanization, with compact cities providing the most cost-effective solutions to drainage infrastructure investments.