As mentioned, road runoff is a major reason for erosion and sedimentation. This flow may come from the road surface itself as well as from the surrounding area, with the road typically acting as a drain for the area around it. The volume of water conveyed from the road surface can be considerable and can cause considerable erosion to the road surface and the areas adjacent to the road.
Several measures may be considered to curtail this erosion. Along rural road alignments, trees, shrubs and grasses may be planted to reduce the erosive effect of runoff from the road template, in particular if no side drain is provided and the road template is out-sloped or crowned. This is discussed in Chapter 12, which deals with roadside vegetation.
An alternative or complement to roadside vegetation is the use of infiltration bunds. These may be more appropriate in arid areas because of the difficulty for roadside vegetation to take root. Infiltration bunds can be placed on the downhill side of the road or at any other location in the catchment where they intercept sheet flow. The stone bunds disperse water and slow runoff. They ensure more infiltration of the runoff, contributing to improved soil moisture and recharge of groundwater.
Based on the work of Bender (2009), the following recommendations apply (Figure 9.4 presents an overview):
- Rock infiltration bunds are best placed within three meters of the road border.
- The stones in the rock infiltration bunds should be placed at sufficient density to disperse the water and avoid erosion. In other words, if only a few stones are placed the slowdown effect will not work.
- Large stones should be placed upstream.
- A single layer of stones is sufficient in medium erodible road slopes, but good compaction is critical. An initial compaction under dry conditions followed by a second compaction under wet conditions is recommended. A double layer of stones may be used in highly erodible soils.
- The thickness of the infiltration bund is 15 cm for terrains of medium sediment transport capacity. This can be increased in areas with higher transport capacity. Wider infiltration bunds with larger head stones or a second bund are recommended for loamy soils, vertisols, and areas with concentrated runoff.
- Masonry uphill protection for the infiltration bunds may be considered in areas with highly erodible, very soft loamy soils, concentrated runoff, and long-standing water. A T-shape layout may also be considered.