Design

SuDS and design

Good design of SuDS is crucial as there is no ‘one size fits all’ solution. Often the best SuDS schemes and components are those that have involved a range of expertise (and the community) in their design. Planners play a vital role in involving the right experts at the right time ensuring SuDS are effectively integrated within the wider development.

Design is an important consideration at all levels of the planning process with different information (and detail) required at different planning stages.

With early engagement and imaginative and innovative design sustainable drainage should be possible on any site (for further information please see FAQs).

Getting the most from a site

To fully exploit opportunities of a site and overcome potential challenges sustainable drainage should be considered at the start of the project. At this stage planners and designers should seek to understand the following aspects to inform SuDS design:

1. What are the developer’s aspirations (type, quantity and quality of the development)?

2. What is the likely mix of impermeable and permeable surfaces?

3. What is the local planning authority seeking from the site, ie are there policy requirements or local guidance?

4. How can development design improve and be part of addressing water in a sustainable way?

5. How can SuDS inform the development and how can the design of the landscape, and external areas integrate water in a sustainable way, eg public open space used as detention basins?

6. How could SuDS enhance biodiversity on site?

7. How will land-use on site create pollution of runoff and influence treatment requirements for SuDS?

8. How will long-term maintenance and management of SuDS be ensured?

9. Are overland flow routes (and drainage exceedance) incorporated into the master plan?

Answering these questions is likely to require input from a range of disciplines including engineers, urban designers, architects, landscape architects and ecologists.

Master planning and outline planning applications

The inclusion of SuDS at the master planning or development site planning stage has a significant effect on the viability and cost-effectiveness of SuDS integration and the ability of SuDS to deliver multiple benefits. Master planning provides a strategic approach to consider the (sometimes competing) requirements for a development. It is an inclusive urban design approach that brings together stakeholders, to regenerate or develop areas that create high quality places.

Integration of SuDS with spatial design is an important aspect to include within the requirements of a design and access (D&A) statement in support of an outline planning application. D&A statements explain the considerations behind a planning application and the aspirations and ambition for a development. When reviewing a master plan at outline planning stage, the planner should ensure that the spatial layout both minimises runoff, and incorporates SuDS components.

For further information about SuDS master planning see Planning for SuDS - making it happen (C687)

Design criteria

Setting the design criteria at the start of the project and establishing a clear SuDS vision ensures that the SuDS scheme and/or components are not secondary to other requirements for the site. Sustainable drainage should be integrated into the urban form, whether using hard engineering or soft landscaping features.

Design criteria (as set out in table 1) for the SuDS scheme provide a useful framework for delivering a scheme that meets objectives agreed by the client and design team. These will ideally relate to flood risk management (water quantity), water quality, and the provision of biodiversity and amenity. It may not be possible to maximise opportunities for all three objectives and the extent that this happens should be discussed with the relevant stakeholders. More specific guidance on the design criteria can be found in The SuDS Manual (C753F)

Table 1: Considerations for design criteria

Design criteria	Key principles
Water quantity (hydraulics, flooding, runoff etc)	People and property must be protected from all flooding sources, including local watercourses, the development’s drainage system and overland flows. Development should not exacerbate flood risk within the wider catchment. The flow rate and volume of runoff should be managed to agreed levels and drainage exceedance should be considered.
Water quality (pollution control, management etc)	Potential pollution risks should be mitigated by the use of source control, the SuDS management train with the appropriate number of treatment stages. Adequate retention time should be provided to enable pollutants to be treated.
Amenity and biodiversity	SuDS can positively influence urban design and landscape value through provision of green space, vegetation integrating water into the built environment, providing opportunities for biodiversity. This is essential if sustainable drainage is to be included in public open spaces and contribute to green infrastructure. Healthy and safety concerns should be addressed and designed out. SuDS components should have shallow side slops, and ponds should have shallow shelving edges, and make good use of vegetation to prevent access. Often and amenity and biodiversity is dependent on water quantity and quality being effectively managed.

The design process

Good urban design is only possible where the design and management of water in a scheme is integrated as an important part of the whole concept. SuDS can be designed without urban design input, but this leads to the creation of water management features that are disconnected with the character and aesthetic of the place, known as the ‘context’. Having an inter-disciplinary team (if not transdisciplinarian) will help deliver a robust scheme with multiple benefits.

An ideal design team will vary from project to project but could include:

Urban designers
Landscape architects
Drainage engineers
Highway engineers
Geotechnical engineers
Architects
Town planners
Ecologists
Developers

Making SuDS attractive and integrating their design with other urban features (ie roads and public open space) is vital to their acceptance, performance and longevity. Communities and stakeholders are more likely to approve, operate and maintain SuDS when they are delivered using good urban design principles.

It is also useful to structure the design process around the delivery of the SuDS management train. It should start by considering what can be done to reduce impermeable surfaces (prevention) and then consider source control.

Often these are excluded from the management train due to limitations with space and SuDS can become token features. Runoff is conveyed (transported) via conventional drainage networks to off-site storage and the benefits of SuDS are lost, with often unsightly end of pipe ponds.

It this stage it’s also useful to consider how the SuDS scheme will be constructed and maintained as much can be done during the design stage to enable ease of construction and maintenance.