Drainage Design

Sustainable flood drainage systems are processes which regulate water in a way that imitates natural drainage systems.

Designed to control water flow and the distribution of polluted water in developed, urban areas, SuDS work by enabling water to soak into the ground (infiltration systems), slowing down the flow of water into watercourses (retention systems), storing and transferring water from vegetation into the atmosphere (evapotranspiration) and removing pollutants from water (filtration).

Faced with a future of climate uncertainty, SuDS are a far more competent water drainer, container and manager than their traditional counterparts like underground pipe systems, borehole soakaways and drainage bricks..

Planning applications need to demonstrate that impacts to the environment, both on site and in the vicinity, have been considered.

Sustainable drainage design is viewed favourably and can unlock a site through control of the quantity and the quality of the discharge.

If you intend to develop property or land, a detailed drainage design will inform your builders (or perhaps yourself) to know what drainage to build, where. The design has to consider all relevant guidance, such as from the council and SuDS manual, and demonstrate a reduction in discharge rate where required.

Fundamentally planners are interested in reducing the impact your development will have on the environment – both on your site and the local vicinity. Sustainable drainage design is viewed favourably and some of the issues planners are interested in include:

• What is the quantity of the discharge from the site? Flood risk is a key concern – both on site and further afield.
• What is the destination of the discharge, where will you send it?
• What are the discharge rates locally permitted for greenfield development?
• If your site is a brownfield development – how will you improve the environment?

Every project is different and therefore the drainage design is unique. In some cases, planners may require a significant level of detail to ensure the feasibility of the design and drawings are used by the regulators to ensure the approved scheme is installed correctly. Sustainable drainage is viewed favourably and can unlock planning permission for a difficult site.

If you intend to build a house, or any kind of development, you will need a detailed drainage design in order for your builders (or perhaps yourself) to know what drainage to install. The design has to consider all relevant guidance, from the Local Authority, DEFRA and best practice from the SuDS manual (CIRIA, 2015) and demonstrate a reduction in discharge rate where required.

Every project is unique and so every project’s drainage design is unique. Every site has different topography, (site levels and slopes) and ground conditions. Some projects may need to discharge to a watercourse but others to sewers. Some projects may have very favourable ground conditions where infiltration is feasible.

Planners need to be confident that the design has considered all of these factors and that the development has met their criteria for sustainable drainage.

A drainage strategy considers all options for sustainable drainage and advises on the most likely solution and incorporates an outline strategy. This will include attenuation volume required, a proposed discharge rate or an infiltration rate.

Some local authorities require more detail and therefore a ‘detailed drainage design’ is required with a planning condition. This design gives a lot more detail such as gradients and sizes of pipes, cover and invert levels.

A detailed drainage design is often thought of as the engineering drawings and technicalities of how the system will work for a proposed SuDS scheme.

Developments should demonstrate that they will not increase the risk of flooding on site or in the local area. This can be achieved by storing rainfall on site and then restricting the flow off site to replicate what would have happened before anything was constructed. A Detailed Design Drainage gives all the technical details for how this is achieved and where on the site the infrastructure should be placed.