13.3.1 Road Alignment and Routing
For new roads, before drainage can be considered, the routing of the road must be fixed. Road routing is a complex procedure that involves very many factors including social, economic, engineering and environmental criteria as well as public acceptance. However, proper selection of the road corridor is one of the most important factors in the protection of water bodies. Technical measures aimed at mitigating a problem provide a poor substitute for efficient routing and alignment which could have avoided the occurrence of the problem altogether. Nevertheless, in those cases where conflicts between roads and water bodies cannot be avoided,
design of the road’s elements, together with technical measures, plays an important role. The design of the road’s alignment has a major influence on the inclination of the road surface. Although the main criterion for road surface dewatering is usually traffic safety (e. g. to prevent aquaplaning) water protection criteria must also be included into design practice. The selection of the longitudinal and transversal cross fall of the road can
• Direct the water from different parts of the highway surface and sub-surface to different drainage outlets, thus allowing “isolation” of spillage-affected run-off to localised collection points rather than allowing the contaminant to affect an entire drainage system,
• Lead water to less sensitive ground — or surface-water zones where it is more easily handled,
• Be an influence on the recharge area and, at the same time, on the total pollutant loads flowing from the particular section of the road.
Horizontal alignments can be selected to take a road away from a place where it may be expected to have an undesirable impact. Vertical alignments can permit the drainage of water collected from above the pavement’s formation level (e. g. water deriving from cutting slopes and from pavement runoff) to be led out of a local catchment where it might be undesirable, to another location where it can be handled with less risk to the environment (Fig. 13.1). Less obviously, but more practical in many situations, horizontal curves may be introduced into a road’s alignment in order that the pavement has to built with some super-elevation. This will then provide a cross-fall across the pavement to the margin on the inside of the bend (Fig. 13.2). By carefully choosing the road’s horizontal and vertical alignments, both surface and sub-surface waters can be moved to a desired outlet point where the impacts will be minimised.
