Regulations now state that sustainable drainage systems should be included in new housing schemes where feasible. This AG sponsored CPD explains how these systems work and the specification considerations
According to CIRIA C609 – Sustainable Drainage Systems: Hydraulic, Structural and Water Quality Advice, the concept of sustainable drainage systems (SuDS) is to mimic, as closely as possible, natural drainage of a site in order to minimise the impact that urban development has on flooding and pollution of rivers, streams and other water bodies.
What are sustainable drainage systems? Each will be developed individually in a site-specific manner, but the focus will be to meet four sets of key aims, known as the four pillars.
The four pillars
- Water quantity
- Reduce the volume and rate of urban runoff
- Recharge groundwater (where appropriate)
- Reduce erosion
- Control the quantity of runoff both to support the management
of flood risk and to maintain and protect the natural water cycle.
- Water quality
- Reduce pollution in groundwater and water courses
- Manage the quality of the runoff to prevent pollution.
- Provide recreation and landscaping for the community, creating and sustaining better places for people.
- Improve and enhance wildlife habitats, creating and sustaining better places for nature.
The implementation of a good SuDS system requires the creation of a “management train” that allows drainage to occur effectively and without damaging the various environments likely to be affected. In the built environment, such as on housing developments, this could include harvesting of rainwater.
Good site design and housekeeping measures within a development should also help prevent flooding. Driveways and other domestic areas could have permeable surfaces – this is called “source control” and covers management of water close to where it falls.
“Site control” of drainage can be achieved through dedicated filter strips (areas of permanent vegetation such as grass used to reduce sediments and other contaminants from runoff to maintain or improve water quality), community ponds and swales, which are shallow vegetated channels that encourage the run-off or storage of rainwater. Such management practices within a site’s boundary are desirable.
Further out, “regional control” is management of run-off from several sites. It can make use of regional ponds and wetlands, where these exist locally, as well as water courses. Other natural water features, such as natural lakes, rivers and the sea, can also play a part in drainage management.
Why the need for sustainable drainage?
Many surface water drains are operating close to full capacity, and with the continuous new-build developments our drainage system needs extra drainage capacity.
Climate change is undoubtedly a factor. Since 1961, the antecedent precipitation index (API) shows that incidences of three consecutive days of rainfall (API 3) have risen by 50%.
Typical storm pattern graphs show low volumes of runoff in the early stages, increasing to the centre of the bell curve which represents the heaviest part of the storm, then decreasing as the storm passes or gradually stops.
Three consecutive storms of the same duration and peak intensity creates a cumulative effect, however. The graph curves show much greater volume and/or rate of surface water runoff overall (compared with three storms less close together), exceeding capacity and leading to surface flooding.
The UK government’s expectation is that sustainable drainage systems will be provided in new developments wherever this is appropriate.
To this effect, local planning policies and decisions on planning applications relating to major development will be designed to ensure that sustainable drainage systems for the management of run-off are put in place, unless demonstrated to be inappropriate.
The current requirement in national policy that all new developments in areas at risk of flooding should give priority to the use of sustainable drainage systems will continue to apply.
Through the use of planning conditions or planning obligations, there should be clear arrangements in place for ongoing maintenance over the lifetime of the development.
Paragraphs 163 and 165 of the National Planning Policy Framework (NPPF) support the applications of SuDS among other flood risk and water quality related matters.
This section of the NPPF says that when determining any planning applications, local planning authorities should ensure that flood risk is not increased elsewhere. Where appropriate, applications should be supported by a site-specific flood risk assessment.
Development should only be allowed in areas at risk of flooding where, in the light of this assessment it can be demonstrated that it incorporates sustainable drainage systems, unless there is clear evidence that this would be inappropriate.
It also specifies that major developments should incorporate sustainable drainage systems unless there is clear evidence that this would be inappropriate. The system used should:
- Take account of advice from the lead local flood authority
- Have appropriate proposed minimum operational standards
- Have maintenance arrangements in place to ensure an acceptable standard of operation for the lifetime of the development
- Where possible, provide multifunctional benefits.
Meanwhile, the Planning Practice Guidelines note that “sustainable drainage systems are designed to control surface water run-off close to where it falls and mimic natural drainage as closely as possible”.
The guidelines say that they such systems provide opportunities to:
- Reduce the causes and impacts of flooding
- Remove pollutants from urban run-off at source
- Combine water management with green space with benefits for amenity, recreation and wildlife”.
According to the SuDS Statutory Guidance, from January 2019 all new developments of at least two properties, or where the construction area is 100m2 or more, require SuDS for surface water.
Approval from the local authority’s SuDS Approving Body (SAB) is required, alongside planning approval, before construction work begins.
Local development plans and local flood risk management strategies will outline requirements for SuDS, and if formally adopted these are enforceable through the planning process. These will vary from local authority to local authority.
There are also a series of non-statutory technical standards which need to be considered, which outline the flood risk protection requirements for SuDS along with considerations in relation to construction, operation and maintenance. These non-statutory technical standards vary from one local authority to another, depending on how built-up an area is and the rainfall intensity in that part of the country.
Which blocks to use?
An effective measure against flooding in certain circumstances is the use of sustainable paving systems. Concrete block permeable pavements (CBPP), for instance, provide a structural surface while allowing water to pass straight into the sub-base for temporary storage and dispersal into the ground, or for collection.
A typical implementation might consist of 80mm paving blocks (depending on loading) with 7mm joints filled with 2mm to 6.3mm clean-washed angular grit. The laying course beneath these paving blocks is generally between 30mm and 50mm, comprised of 2mm to 6.3mm clean-washed angular grit, which is the same material between the block pavers. Below this laying course is a transfer layer made up of 4mm-20mm clean-washed angular stone, which creates a 33% void ratio for water storage and from which water is slowly dispersed naturally into the ground or else channelled away at a controlled rate depending on design.
Whatever aggregate particles are used in the laying course need to comply with BS EN 12620, which requires that they be angular in order to provide a good interlock, thereby maximising friction and increasing strength and durability. Their size should range from 4mm to 20mm for the transfer layer to prevent the migration of the 2mm to 6.3mm laying course stone.
There are three SuDS system design types: total infiltration, partial infiltration, and no infiltration.
Total infiltration, which has the benefit of achieving zero discharge from the site, can be used where the water table is low, the subgrade is of high permeability sub-grade, and the soil scores 5% or better in the CBR strength test. It uses a permeable geotextile liner that is non-woven. Total infiltration requires awareness and mitigation of pollution risk.
Partial infiltration features partial discharge from site, and involves the use of collector systems and a permeable geotextile liner. While some of the water permeates into the ground, some passes through a series of pipes that connect up to an attenuation tank or nearby water course to be temporarily stored for use as irrigation purposes or else to be released into the drainage system when the storm event has passed and the system is no longer at maximum capacity.
No infiltration is used in situations where there is a high water table and a low-permeability subgrade. It uses an impermeable membrane and implements attenuation of flow, water harvesting, and trapping/removal of pollutants.
A typical hydraulic design can feature three structural improvement layers installed below the permeable paving design:
- Dense bitumen macadam
- Cement-stabilised coarse-graded aggregate
- A structural slab.
Hydraulically-bound coarse-graded aggregate (HBCGA) can be installed to give structural support to the design. HBCGA is a mixture of coarse aggregate (usually 20mm nominal size), cement and water. It is covered by BS EN 14227 – 1. Its typical compressive strength is deemed to be Class C5/6, in accordance with IS EN 14227 – 1, Table 2 Line 4. Other strength classes will be available, however.
Other specific site conditions must be taken into account, including those that are subject to change. Design considerations include the degree of gradient: there should be a maximum surface gradient of 5%, or 1 in 20.
Where necessary, baffles or dams can be installed across a slope to slow the flow of water. Terracing the site to create a number of flat areas might also be required to prevent the lower areas from becoming overwhelmed.
In some cases tree pits may be advantageous. Observation of root development after a five-year growth period has shown that permeable pavements are less prone to damage by shallow root growth than are conventional, impermeable, pavements.
Structural support, in terms of pavement build-up, can involve a layer of asphalt being used to protect the sub-base while it is used for construction traffic – then cored before final finishes.
Water pollution considerations
Sources of pollution include cars, the general atmosphere, maintenance, agriculture and de-icing salts. Types of pollution include heavy metals, hydrocarbons and oils, herbicides, sediments, chlorides and organic matter.
A number of removal practices and mechanisms can be used to remove water:
- Filtration Water moves slowly due to the large volume of stone causing sediment to store at base of structure.
- Adsorption A chemical bond of hydrocarbons to the upper layer of stone and particles of sediment. As the water passes through the transfer layer, the pollution molecules bind to the outer surface of the aggregate particles – once binded the aggregate particles break down.
- Aerobic bacteria An oxygen-based organism that breaks down the adsorption effect. This prevents the geotextile liner which sits directly beneath the transfer layer from blocking up over time. Aerobic bacteria engulfs the bacteria that have gradually built up in the geotextile liner pores.
- Biodegradation Takes place by naturally occurring process.
According to Construction Industry Research and Information Association (CIRIA) information, based on worldwide research, sustainable paving can be used as part of a pollution control system.
As pollution control systems, sustainable paving can remove between 60% and 90% of total suspended solids, 70% to 90% of hydrocarbons, 50% to 80% of phosphorous, 65% to 80% of nitrogen, and 60% to 95% of heavy metals.
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