Where Should the Rain Go? Into a Shallow Ditch Full of Grass

A drain takes water away as fast as possible and delivers all of it to a watercourse at once, which is exactly what causes flooding downstream and what carries pollutants straight into the river. A shallow vegetated channel does the opposite in every respect. It conveys water slowly, because grass and uneven ground resist the flow. It lets a proportion soak into the ground as the water travels. It allows sediment to settle out. And it delivers what remains later and more gradually, which reduces the peak that the receiving watercourse has to handle.

The form follows from the job and the dimensions are calculated rather than guessed:

• •A broad flat or gently dished bottom, not a narrow V
• •Side slopes shallow enough to mow and to walk out of
• •A gradient low enough that water moves slowly and does not scour
• •Dense grass or planting, which does the filtering
• •Check dams across the channel where the ground is steeper
• •An overflow route for storms larger than the design case

The technique appears under several headings and is now standard in new development in many countries. Motorway verges carry them alongside the carriageway, treating road runoff which contains oil, rubber, metals and salt before it reaches a watercourse. Car parks and industrial sites use them for the same reason. Housing developments use them as part of drainage systems designed to mimic how the site behaved before it was built on, which regulations increasingly require. In agriculture a broad shallow channel following the contour of a slope intercepts runoff, slows it and encourages it to soak in, which is a long-established soil conservation measure.

The planting is not decorative and choosing it badly undermines the whole thing. Dense fine-leaved grass gives the greatest resistance to flow and the best filtering, which is why an ordinary mown sward performs well if it is thick. Deep-rooted species keep the soil open and maintain infiltration, where a shallow-rooted mat can seal the surface over time. Plants tolerant of both flooding and drought are needed, since the channel is wet for hours and dry for weeks. Salt tolerance matters beside roads. And where the design allows longer standing water, wetland planting is used deliberately, which takes up more nutrients and changes the maintenance regime completely.

The technique is cheap and looks undemanding, which is precisely why installations perform poorly. Compacted ground during construction destroys the infiltration the design depended on, and this is the commonest fault. Steep side slopes erode and cannot be mown safely. Too steep a gradient scours the channel into a gully. Sediment from a construction site clogs the surface permanently if it is allowed in before the site is stabilised. And maintenance is frequently nobody's responsibility once a development is handed over, so channels fill with litter and silt, the vegetation is lost and the system reverts to being a rough ditch that does none of the intended work.

A broad vegetated channel slows surface water, lets some soak away, settles out sediment and delivers the rest later, which reduces both flood peaks and pollution compared with a pipe that removes everything at once. Motorway verges, car parks and new housing use them routinely. Compaction during construction, over-steep gradients and abandoned maintenance are why so many installed ones do not work.