Sustainable drainage systems (SuDS)
Surface water drainage methods that take account of water quantity, water quality, biodiversity and amenity issues are collectively referred to as Sustainable Drainage Systems (SuDS).
Using SuDS to manage surface water has a number of benefits, such as reducing the risk of flooding of homes and businesses, as well as adjacent or downstream properties, as a result of heavy rain. However, they also provide an important function in improving water quality and the local environment.
Conventional surface water drainage systems
It is often perceived that modern conventional forms of drainage; gullies, manholes, pipework and storage, are sustainable drainage systems.
However whilst often delivering the goals of reducing flood risk and dealing with rainfall from the development, these systems do not meet the basic requirements for SuDS.
When to use SuDs
The Local Planning Authority (LPA) will have to ensure that the SuDS are used for 'Major Development'. There is, however, still an expectation within the National Planning Policy that SuDS will be used on 'Minor Development' as well.
As Bracknell Forest is a unitary authority we are responsible for determining the SUDS elements of any planning application and are the statutory consultee with respect to surface water flooding and drainage.
The Environment Agency (EA) will continue to be the statutory consultee where development affects 'Main River' or is within flood zones 2 and 3.
The LPA will determine the application in accordance with the national and local policies whilst taking into account advice on technical matters from the LLFA and EA (if appropriate).
Further information can be found in Annex F of Local flood risk management strategy.
When making planning applications, developers will get the best results if they consider the use of SuDS options early in the site evaluation and planning process, not just at the detailed design stage.
Trying to retrofit a sustainable drainage system into a layout which has already been designed is very difficult. It can lead to a design which compromises the benefits of SuDS, results in excessive land take and usually costs more than a conventional drainage system.
It is therefore important to engage in early discussions with the SuDS team, who work alongside their colleagues in the highway and planning authorities, preferably by using the planning pre-application process. This will ensure that surface water management is integrated into the development, leading to an effective drainage design with costs adequately considered at the start of the development.
SuDS solutions and tools
There will be a SuDS solution to suit the site, due to the wide range of components available.
To determine the right technique it is necessary to first establish the soil conditions and hydrology of the site and use the results of the investigations to support the drainage proposals. The choice can also be significantly influenced by:
- the quality of the land (whether it is affected by contamination)
- the need to protect vulnerable groundwater sources
- the permeability of the soil
The flood risk from surface water mapping provides a useful tool to establish flow routes through sites and identify areas where water might naturally pond. This provides a valuable means of selecting the appropriate locations and types of SUDS.
All development sites are required to ensure that runoff rates and volumes post development are not greater than the pre-development runoff rates and volumes. This usually requires a volume of storage to be provided in the site. This can be quickly estimated using Micro Drainage software or the tools available on the UK Sustainable Drainage website.
This establishes the volume of storage required to be provided. Consideration should be given to the proposed outfall and this will help determine the area that may need to be set aside for the provision of storage. Whilst underground storage may be preferable in terms of the land take, it is quite often more costly to construct, requires replacing every 25 to 50 years and does not provide any water quality of biodiversity benefits.
SuDS solutions are most cost effective when designed to work with the natural drainage pattern of the site, for example designed to use existing ditches or natural depressions for swales and ponds or designed to form part of hard and soft landscaped areas. Ponds and green spaces will provide habitats for wildlife to flourish, reduce pollution and provide areas for people to enjoy, adding value to the site.
In the early stages of the site design, consideration should be given as to how the drainage system will be adopted and maintained in the future. It is likely these decisions will influence the design just as much as the technical considerations.
The local planning authority will determine the application and will need to be satisfied that:
- any proposals meet national and local policies
- any proposals clearly identify who will be responsible for maintaining the SuDS and funding for maintenance should be fair for householders and premises occupiers
- minimum standards are set out to which the SuDS must be maintained
Bracknell Forest Council (BFC), being the LPA and LLFA, encourages the use of pre-application discussions to ensure that any planning application can be determined as smoothly as possible.
Validation of application
In order to ensure the LLFA can make a timely response to the LPA and for the LPA to be able to determine the application, a local list will be used to validate any application.
This list requires a submission of a drainage strategy for all major applications and other information for some minor applications. The guidance will be published soon.
National and local policy standard and guidance for SuDS
Planning practice guidance giving further explanation can be found in National Planning Policy Framework - Communities and Local Government.