Written question WQ100860

There is no requirement to install specific stormwater treatment devices on public-sector highway schemes. However, most highway schemes must demonstrate compliance with the Welsh Sustainable Drainage Systems (SuDS) Standards. Standard three requires that surface water runoff should be treated to prevent negative impacts on any receiving water quality and to protect downstream drainage systems. This typically requires the incorporation of appropriate treatment measures within the drainage design.

Regarding highway upgrades or refurbishments, requirements are scheme specific. If the works constitute new construction with drainage implications, and exceed the relevant thresholds, SuDS Approving Body (SAB) approval for compliance with the SuDS standards may be required. For maintenance-only or like-for-like renewal works, there is generally no automatic requirement to retrofit treatment devices unless other environmental, planning, permit, or funding conditions impose them.

Trunk roads and motorways adopted and maintained by the Welsh Government are exempt from the statutory SuDS approval regime. For such roads, the requirements of the Design Manual for Roads and Bridges (DMRB) will apply. The DMRB applies outcomes-based assessment standards and is not prescriptive about any specific interventions or treatment solutions. The choice of treatment method is normally left to the designer, subject to environmental assessment results and regulator acceptance.