Water harvesting and drainage from unpaved roads

Key message

  • Most damage to unpaved roads – as much as 80% – is caused by rain run-off;
  • Related to this unpaved roads surfaces are a major cause of sedimentation (up to 35%, excluding gullies) in mountain catchments;
  • This can be minimized when water is systematically directed to the land rather than running along the road surface;
  • This requires the consider design of unpaved road alignments – regular slope reversal, avoid sunken roads and include basic road drainage
  • Several additional measures preserve unpaved roads, enable water harvesting and reduce erosion: water bars, rolling dips and infiltrating bunds

Main applications

  • Providing water harvesting measures combined with road drainage is essential to unpaved roads all over the world. Maintenance of unpaved feeder roads is in many countries heavily underfunded. Measures that have the double purpose of reducing damage and promoting beneficial water use will contribute to longevity of vital rural road connections.

This chapter discusses basic measures that reduce damage to unpaved roads while guiding water away from the surface to use it productively or for recharge. Maintenance of unpaved roads is a challenge in many countries. The ballpark figure is that 80 percent of the damage to unpaved roads is water related. Unpaved roads are usually built to much lower standards than paved roads. For instance, they may not be equipped with a road drainage system.

Moreover, funds for maintenance of lower-tier roads are chronically insufficient. Because the financing of maintenance is a challenge, repairs may take a long time, with a major impact on connectivity in remote areas. The lack of maintenance is compounded by the fact that in some areas unpaved roads are made from highly erodible material for lack of a better choice. The rutting of such unpaved roads constitutes an important road safety issue. For these reasons, preventive measures that reduce the degradation of the unpaved part of the road network are extremely important.

Ethiopia: water damage in action on an unpaved road; note the gully at the end of the slope
Ethiopia: water damage in action on an unpaved road; note the gully at the end of the slope
Mozambique: roads constructed with highly erodible red sand washed away in each major runoff event
Mozambique: roads constructed with highly erodible red sand washed away in each major runoff event

Preventing damage to unpaved roads by combining adequate road drainage with water harvesting is essential. More than 75 percent of the roads in many countries are unpaved. They are the largest single intervention in the rural landscape. As discussed in Chapter 3, they are also main contributors to the sediment in a catchment: ranging from 12 percent to 40 percent, according to the literature. Several studies on forestry development in the United States establish that road development, more than logging, accelerated sedimentation in local streams. This is because roads start to behave as tributaries of the streams, “creating a more efficient sediment delivery system” (Castro and Reckenhoff 1995). Practices that keep the sediment out of the stream, like vegetation buffers, are bypassed and sometimes even destroyed during the construction of rural roads.

Where a rural network is expanding in the upper catchment of a hydropower dam and no water management measures are taken, the accelerated sedimentation may considerably shorten the lifespan of the hydropower reservoir. There is a risk of this happening in the upper catchment of the Grand Ethiopia Renaissance Dam, Africa’s largest dam: even though part of the catchment consists of deep and highly erodible soils, new rural roads are constructed without adequate drainage and water harvesting facilities.

High sediment loads in water have other effects as well. Turbidity affects much of the aquatic life. It can reduce the spawning of fish (Noss, no date). Higher sediment loads have an optical effect by blocking light penetration and slowing biological activity. Second, many fish, such as salmonids, spawn in the gravelly bed load of a river, placing their eggs at different depths (Castro and Reckenhoff 1995). The eggs require fresh, fast-flowing water. However, medium-textured sediment that is suspended in water at medium speeds will settle and clog spawning grounds. The same can affect benthic organisms. The effect of sediment loads depends on the type of stream as well: in fast-flowing streams, much of the additional sediment may be carried further along, but in streams with lower gradients the riverbed morphology will change much more under the impact of sedimentation. Another variable is the sediment composition that is released: the size of the sediment as well as the shape. Flat particles (like clays) are more likely to form a relatively immobile film on the riverbed.

In the construction of new unpaved rural roads there are several dos and don’ts to reduce sediment release and improve the capacity to harvest and recharge water. The measures taken are usually low cost and will help to preserve the integrity of the road. This chapter discusses three sets of measures that help to preserve the road:

  1. Planning the alignment of unpaved roads by avoiding long and steep slopes without drainage facilities.
  2. Using basic road-surface drainage: a series of rolling dips (small depressions with a small bump) or water bars (small slanted humps) to divert water from the road surface to the land for productive use.
  3. Using infiltration bunds to slow own the side erosion from the roads and promote recharge.


Box 9.1. Creating youth employment with sand harvesting

In Ethiopia, organized groups of youth (with equal numbers of males and females) are given permits by the government to mine sand for a period of time. This activity is very successful and may be considered for more countries. For one year, the group is expected to retain the profit from sand harvesting as seed capital for business activities. For this business graduation, the government contribution is 80 percent provided as a loan on top of the US$850/year that the group member may have saved. In total, each member may establish his/her own business with about US$4,250. This employment opportunity has resulted in better regulation of sand and gravel mining through the local government; the business had previously been captured by local thugs.

Controlled sand mining from road hydraulic structures can enhance the safety of the road and provide livelihood opportunities for nearby communities.

  • Planning road alignments for adequate drainage and water management
  • Use basic road-surface drainage
  • Use infiltration bunds to control erosion and enhance recharge