The primary function of motorway drainage is to get water off the carriageway quickly so that the risk of accidents is minimised. That’s fair enough. But is equally important to prevent flood risk downstream, and to control pollution to an acceptable level.
These functions can all be delivered by well-designed and constructed filter drains in many locations. They have a huge capacity to receive water quickly, to capture pollutants and to attenuate flows, controlling flows downstream. They are not always appropriate, because of lack of space, or the vulnerability of underlying groundwater, but they should always be considered as a first option.
However, many new and retrofit drainage systems in England use concrete slipform drainage channels. These don’t seem to be much good at any of the functions set out above. They convey water off the carriageway quickly, but not if they are blocked, and I have seen scores of blocked gullies on these channels recently.
They don’t capture any pollution, and they don’t attenuate flows. They are a poor selection for motorway drainage, and yet they are being installed on many of the Smart Motorway projects that I drive through. This is short-sighted and will lead to increased pollution and flood risk in some locations.
There are also slot drains in use on many Smart motorway schemes and these are also prone to blocking. They don’t capture any pollutants and they don’t attenuate flows.
The schemes that I have recently seen in Cheshire and Lancashire drain straight into local rivers and streams, with no pollution treatment or attenuation tanks, ponds or basins. So how can we carry on designing drainage systems like these, that don’t deliver the essential functions of motorway drainage, and that lead to increased pollution of the water environment?
