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Selection

Sustainable drainage systems are designed using the same underlying principles of hydrology and hydraulics as conventional drainage systems, but applying them in a different way. Alongside the purely technical issues of draiange are wider considerations of environmental enhancement, amenity and urban design.

Principles

When selecting SUDS it is important to consider water quality, water quantity and amenity design criteria equally. There will not be a single "correct" answer: several options may meet the design criteria, and judgement will be needed. It may not be possible to maximise all three design critera as this is likely to reflect the opportunities and constraints on site.

Selection and design of SUDS are multi-disciplinary processes. Unlike conventional drainage systems, factors that influence the final choice will include planning, water quality, water resource, architectural and landscape requirements.

Design criteria

Setting the design criteria at the start of the project and establishing a clear SuDS vision ensures that the drainage 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 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.

Design criteria	Key principles
Water quantity (hydraulics, flooding, runoff etc)	People and property must be protected from all flooding sources, including watercourses, the 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 the SuDS management train. 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.

Table 1 Typical design criteria

The selection process

Once an initial assessment of the site has been made and the design strategies for the drainage system have been agreed, drainage components can be selected.

The selection process in bassed on the principles that:

The drainage scheme complements the overall vision for the site and management of surface water
Drainage components will be used in series to meet the design criteria for water quantity, quality, amenity and biodiversity

Drainage components at the top of the management train are generally to be preferred to those further downstream

There is no single correct solution; selection may be the result of site opportunities and constraints

Minimise the use of impermeable surfces wherever possible
Source control is preferred providing more natural drainage and integrating water into the built environment
The SuDS scheme should be inspired by the original drainage pattern and characteristies of the local area.

The SuDS scheme should be designed to facilitate wase of maintenance and operation

The process is cyclical, and various factors have to be considered in increasing detail as a final solution is approached.

The selection of individual components and the SuDS scheme will depend on a variety of site attributes and factors including:

local hydrology and hydrogeology
ground contamination
depth of water table
soil permeability
ground stability
Sensitivity of receiving waterbody (either surface water or groundwater)
size of catchment area
development type, density and required layout
Requirements for local flood risk management
Other opportunities within the overall site
Affordability of scheme (capital and operational)

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