Where there are environmentally sensitive areas or high traffic flows, increasing the risks of accidents and generating contamination from wear, water flowing over the surface of the road and the embankment should be collected before it can soak into the ground in an uncontrolled manner. Water seeping through the earthworks and collected by a drainage layer has to be led, by virtue of a fall in the drainage layer and by shaping it, to collection points. At the collection point the water quality can be monitored and, according to the quality measured, it can be fed to a soakaway (Section 13.4.7) or piped away for treatment.
Water that arrives at an outlet from a drainage system may need treatment to bring the water quality to an acceptable level. In extreme cases, conventional waste-water treatment systems can be installed, but such a level of treatment is seldom required, except where water has passed through some contaminated soil body. If the water could, following a traffic accident or similar, contain spillages of fuel or cargo from
traffic, then it will be necessary to install a oil/fuel separator. If these are installed then it will usually be sensible to provide a retention pond on the upstream side of the separator. This lagoon can act as a location to temporarily store the spilled fluid from which it can be extracted and taken away for off-site handling. The aim will be to give the road operator or environmental manager sufficient time to respond to the spill before the fluid is allowed to enter a surface water body or allowed to soak down to the groundwater. Storage lagoons also provide a zone in which solids can sediment from water and provide a means of attenuating the peaks of hydrographs.
Man-made wetlands and reed beds can also be provided as water purification areas (see Chapter 12, Section 12.3.1). They are particularly suited to groundwater outlets as the water arrival will be more consistent, at a low flow volume rate, than for surface water runoff and there will be much less need to provide storage for excess water arriving after a rain storm. Reed growth is much more suited to this consistent supply of water, while the area to be occupied by the water purification planting can remain relatively small. They often have the additional advantage of providing an enhancement to the pre-existing environment. Naturally occurring wetlands are under serious threat in most countries and they should not be used for this purpose.
The WATMOVE questionnaire sent to European road authorities (see www. watmove. org) revealed that settlement of solids and oil separation are the most common treatment methods used in practice, see Fig. 13.10.
Fig. 13.10 An overview of treatment methods used in Europe. The number is the percentage of countries in the survey using the treatment
