Guidance for developers
When making planning applications, developers will get the best results if they consider the use of sustainable drainage systems (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 make sure 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.
Finding the right solution
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 and 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 make sure 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 Microdrainage software or the tools available on the UK Sustainable Drainage website.
In establishing the volume of storage required, consideration should be given to the proposed outfall. 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 or biodiversity benefits.
SuDS solutions are most cost effective when designed to work with the natural drainage pattern of the site. For example, they could be designed to use existing ditches or natural depressions for swales and ponds or designed to form part of hard and soft landscaped areas. In addition, 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.
National and local policy standard and guidance
Planning practice guidance giving further explanation can be found in National Planning Policy Framework - Communities and Local Government.