Highways England has published details of thousands of Sustainable Drainage Systems (SuDS) in use on the road network in the Greater London area.

The organisation responsible for operating, maintaining and improving England’s motorways and major has published the information in response to a Freedom of Information request about the use of SuDS made under the Environment Information Regulations Act.

The inquirer had asked for details on how many outfalls on highways in Greater London area, within and including the M25, have SuDS installed, including swales and filter strips, so that the run-off is treated before it discharges to a local watercourse or river.

The term ‘outfall’ is used by HE to describe an asset that discharges to a surface waterbody - as opposed to the term ‘outlet’ which is used to define discharge into another component of the organisation’s drainage network. Soakaways, which fall within a different asset class, are not included in the definition.

HE says a total number of 663 assets are defined as ‘Outfall’ within the M25 and spur roads.

The response provides further details on the number of outfall asset records within the M25 and spur roads where a point asset flow control device is reported for managing run-off discharging from the strategic road network:

• 61 outfall assets reported with flap valve
• 53 outfall assets reported with a weir
• 24 outfall assets reported with a baffle
• 22 outfall assets reported with screens
• 4 outfall assets reported with penstock
• 1 outfall asset reported with sluice gate

It also points out that a singular outfall asset can have multiple control devices. In terms of continuous / linear drainage assets within the M25 and spur roads, HE says its records show that there are approximately:

• 6,000 combined surface and sub-surface filter drain systems (380,000 linear metres);
• 3,870 ditch systems (217,500 linear metres);
• 4,590 filter drain systems (207, 400 linear metres);
• 2 linear cellular storage systems (59 linear metres) (not used as soakaway);
• 1 grassed surface water channel / swale (10 linear metres).

HEalso has a further 790 soakaway asset records, 96 detention pond, 85 retention pond, 8 wetland, 7 infiltration basin and 4 sediment pond records.

In response to a further question on whether the performance of the SuDS is monitored, the roads agency says that SuDS drainage asset performance parameters are set out in its published standards in accordance with water quality requirements.

London | Connect Plus is the Area 5 Service provider which manages the assets under a Design, Build, Finance & Operate (DBFO) contract. 

In response to a request for details on the proportion of the SuDS which are monitored, HE said that SuDS are managed in accordance with specified performance criteria and not individually monitored. SuDS assets are only monitored where a pilot scheme is being implemented to assess whether an alternative way of managing surface water runoff would be effective.

Commenting on how often SuDS are maintained and whether there is a maintenance system in place, HE said the maintenance regime for SuDS is based on asset type according to the performance requirement of the asset. The approach to maintenance regime is based on standardised SuDS performance parameters and the regime is contract dependent rather than prescriptive.