Sometimes it is desirable to combine surface water runoff collection and sub-surface seepage water into one, combined, drainage system. Typically, such systems are in the form of lateral trench drains (see Section 13.4.1) that are open to the surface. These are the traditional means of draining roads and continue to be used on lower — trafficked roads where run-off contamination is lower and runoff volumes smaller (because the road is narrower).
Their principal drawbacks are: •
The Highways Agency (2006) publishes a table that indicates when it is sensible to consider a combined system and when not to do so.
