Sustainable Drainage Systems Design Guidance

One of the primary aims of SuDS is to mimic the natural greenfield characteristics of a site. Drainage strategies should be designed to match greenfield discharge rates and follow natural drainage routes as far as possible.

The greenfield runoff should be calculated from IH124 or a similar approved method. Standard Average Annual Rainfall (SAAR) and any other rainfall data used in runoff storage calculations should be based upon most recent Flood Estimation Handbook (FEH) rainfall values.

The proposed change in impermeable area should by clearly detailed on a suitable drawing.

Surface water should be discharged using a staged discharge approach with flows limited to the greenfield Q1, Q30 and Q100 year rates for the corresponding storm events. The Q100 rate should be used for the 1 in 100 year + climate change event. The Environment Agency (EA) have published Climate Change allowances for peak rainfall in England.

The greenfield Q1 or QBar rural rate could also be used as the fixed discharge rate for the positively drained areas of the site. Alternatively, the whole site can be evaluated and the total discharge pro-rata'd between the positively drained and retained green spaces.

Brownfield Sites should be limited to as close to greenfield rates as is reasonably practicable. If greenfield rates cannot be achieved, clear written evidence must be submitted as to why a lower rate cannot be achieved. Information will be considered on a site by site basis. For brownfield sites robust justification could include; analysis of storage requirements required to achieve greenfield runoff rates in comparison to a site area, a cost analysis, spatial or level constraints.

Discharge rates should never exceed the rates prior to redevelopment for the corresponding Q1, Q30 and Q100 year storm rates.

In meeting the above, discharge rates below 1.0 litre/sec may not be achievable, depending on self cleansing velocities. However, low flow control devices are available on the market to achieve such discharge rates. 

Through proper design and maintenance such devices should not be prone to blockage. If proposed discharge rates are greater than the equivalent greenfield runoff rates based on there being a greater risk of blockage, full justification should be provided as to why blockage cannot be designed out and is considered to be a risk. Such sites will be considered on site by site basis.