In response to Brian Bartlett's letter (NCE 15 July), I am writing in the capacity of the consultant commissioned by the Highways Agency for developing guidance on the use of grassed surface water channels. However, the following comments are my personal views.
The grass channels appearing on the highway network are not swales, as such, but grass lined channels of similar dimension to, and follow the Agency's design procedures for, concrete channels.
Although they offer some of the benefits of swales, they differ in that the side slopes are very shallow and there is no gravel bottom.
Grass channels, along with many other roadside features, may be prone to damage. The Agency commissioned full-scale trials at TRL that showed the channel could support a fully loaded 38T vehicle (see photo).
A range of vehicles from a small car to the 38T HGV was driven through the channel with no adverse effect.
The Agency is satisfied that this new alternative to concrete channels not only has environmental benefits but will also perform better under vehicle overruns when compared to any of the other conventional alternatives, with the exception of concrete and kerbed channels.
The Highways Agency expects to publish advice and guidance on the construction and use of grass lined channels in 2005.
Andrew Todd (M), principal engineer, TRL, Crowthorne House, Nine Mile Ride, Wokingham, Berkshire, RG40 3GA