National Highways drainage rules: design implications for highway runoff assets
Reviewed by Tom Sullivan
First reported on New Civil Engineer
30 Second Briefing
Engineers from nine specialist drainage consultancies have criticised National Highways’ current design rules for road runoff assets, warning that the standards constrain effective sizing and placement of features such as attenuation ponds, swales and filter drains. They argue the prescriptive approach limits designers’ ability to account for site-specific hydrology, climate change allowances and exceedance routing, potentially leaving trunk road schemes with underperforming systems during extreme rainfall. The dispute signals likely revisions to design guidance and may affect ongoing and future highway drainage designs and approvals.
Technical Brief
- For geotechnical teams, the criticism implies more iterative drainage–earthworks interfaces and potential rework on current schemes.
Our Take
National Highways appears frequently in our infrastructure coverage not just for standards but also for contentious asset treatments, with the Hertfordshire overbridge infill protests signalling that prescriptive design rules can quickly translate into reputational and heritage‑related risk when applied rigidly on the ground.
The fact that engineers from nine separate companies are criticising drainage asset design rules is notable given that many of the same supply‑chain players are simultaneously delivering major schemes such as the M3 Junction 9 upgrade and A38 works, suggesting potential friction between delivery imperatives and perceived technical robustness of current standards.
Prepared by collating external sources, AI-assisted tools, and Geomechanics.io’s proprietary mining database, then reviewed for technical accuracy & edited by our geotechnical team.
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