Improving water quality
Effective and well designed sustainable drainage systems not only reduce surface water flooding, but also provide opportunties to improve water quality and local biodiversity by reducing the impact upon water levels and qulity of nearby water bodies and water courses.
The following guidance sets out the various sustainable drainage solutions. Further information on improving water quality is also given in the Climate Change Adaptation module.
Sustainable urban drainage systems (SUDS) mimic natural drainage from a site and enable rainwater to run back into natural systems, rather than the stormwater drainage network. SUDS also treat run-off water to remove pollutants.
The SUDS water management hierarchy is:
- prevention – using good site design and housekeeping measures to prevent run-off and pollution (e.g. minimise impermeable paved areas)
- source control – controlling run-off at or very near its source (e.g. rainwater harvesting, permeable paving, green roofs or soakaways)
- site control – attenuate and/or treatment of surface water for a group of buildings on site or a commercial park or a stretch of highway. The principle SUDS devices for a site control may involve a combination of devices including: detention ponds, swales, soakaway and infiltration trenches and basins.
- regional control – attenuate and/or treatment of surface water for a region, serving a number of sites. Retention ponds and wetlands are the major regional treatment facilities.
There are a number of sustainable drainage techniques. Choosing which solution(s) to pursue depends on the attributes of the site and project, including:
- local hydrology and hydrogeology
- ground contamination
- depth of water table
- soil permeability
- ground stability
- size of catchment area
- development type
natural on-site retention
Natural onsite retention includes drainage techniques that use natural features to infiltrate water, such as filter strips, swales, detention basins and balancing ponds.
swales and filter strips
Swales and filter strips are vegetated surface features that drain water evenly off impermeable areas (e.g. roads). Swales are long shallow channels while filter strips are gently sloping areas of vegetated land on which runoff is directed.
Both solutions mimic natural drainage patterns by allowing rainwater to run through vegetation, slowing and filtering the flow.
Swales work by attenuating and slowing down water flow to allow sedimentation and infiltration of pollutants.
Filter strips only attenuate the flow slightly but they can be used to reduce the drained impermeable area.
Swale
Filter strip
basins and ponds
An infiltration basin is a vegetated depression, which is normally dry except after storm events. Infiltration basins are built to store water temporarily to attenuate flows. They can also allow infiltration of water to the ground.
A balancing pond attenuates flows by storing run-off during the peak flow and releasing it at a controlled rate during and after the peak flow has passed. The pond always contains water.
Basins and ponds can be designed to control flow rates by storing floodwater and releasing it slowly once the risk of flooding has passed (a balancing pond). Basins and ponds should be designed to function in both dry and wet weather.
Basins and ponds treat run-off in many ways:
- sedimentation
- microbial activities
- Sorption and uptake by plants
- absorption by soil
- settlement of solids - plants in the water can promote settlement
- adsorption by aquatic vegetation/soil
- biological activity
engineered on-site retention
Common engineered solutions for onsite retention are soakaways, infiltration basins and filter drains.
- Infiltration devices allow surface water to soak and percolate into the soil, thereby re-charging the ground water and maintaining the water levels in local waters.
- Ground water and soil type can limit the infiltration systems, especially in high groundwater and clay soil areas. The base of an infiltration system should have sufficient unsaturated soil immediately below it to allow filtering of stormwater. The soils around the base and sides of an infiltration system should not be compacted which would reduce permeability or the infilteration efficiency of the system.
cross-section through a traditional soakaway
cross-section through an infiltration basin
Infiltration devices treat run-off in different ways:
- physical filtration to remove solids
- sorption of pollutants by soils in infiltration devices
- biochemical treatment using micro-organisms
Infiltration systems are easy to integrate into a site. They are ideal for use as playing fields, recreational areas or public open space. Infiltration systems can be planted with shrubs and other plants, which improves their appearance and provides a wildlife habitat. Infiltration systems also increase soil moisture content and help to recharge groundwater, which can mitigate problems of low river flows.
permeable surfaces
Pervious surfaces are key techniques in SUDS for surface water management and source control of he quantity and quality of runoff. Surface water is infiltrated through the surface and into the underlying construction layers where water is stored prior to infiltration to the ground, reuse or being released to the watercourse or other surface water drainage system. Pervious surfaces are often used for pavement, walk paths, driveways, car parks, cycle routes and sports ground.
Permeable pavement used for infiltration
Pervious surfaces can be either porous or permeable involving the following materials and techniques:
- Porous surfacing infiltrates water across the entire surface of the material forming the paving/car parking areas; e.g. grass and gravel surfaces, porous asphalt and porous concrete.
- Permeable surfacing consists of impervious material to water, however, voids are built-in to these materials that allow infiltration of water through the minute void channels; e.g. concrete paving blocks.
They are effective to provide attenuation of water flow treatment. Pervious paving would ameliorate the need for surface water drains, allowing runoffs to permeate through porous pavements, such as permeable concrete surfaces, crushed stones or porous asphalts. Pollutants removal by filtration occurred within the surfacing or sub-base material itself, or by the filtering action of the reservoir or sub-soil. Some biological breakdown of organic pollutants can also occur.
Permeable surfaces can be designed to fit in with a variety of environmental settings, e.g. hard surfaces of a car park, town centre or gravel surfaces for light traffic. They can be grass-crete or soft landscape surfaces for rural areas. Infiltration devices can be incorporated into open space areas, e.g. playing field or car parks as part of a flood management scheme.