Road drifts

This chapter discusses the siting, design, and construction of road crossings in dry riverbeds to harvest and retain floodwater. The use of road drifts, particularly culvertless “non-vented drifts” functioning as sand dams in semiarid and arid areas, is an important and often underutilized opportunity to harvest flood water. Excellent (2018) estimates that there are 156,000 and 233,000 such crossings in semiarid parts of Africa and Asia, respectively. These road crossings can be used to stabilize riverbeds and spread floods. This chapter draws on the experience of rural road construction in Kenya. The techniques are equally suitable for all semiarid areas, in particular with dry and sandy or gravelly rivers.

Ephemeral rivers, which range in span from 5 m to 300 m, dry up quickly after rains cease in arid and semiarid areas. They typically flow for only a few days, or even a few hours, each year. Even when there is no rain, the rivers transport water. Although the rivers are dry on the surface for most of the year, they are a reliable source of water because of their subsurface flow. In fact, the volume of subsurface flow in ephemeral rivers in many cases exceeds the water carried during the occasional floods.

Moreover, the transport of water in the riverbed reduces evaporation and minimizes water losses. The water quality in the riverbed is usually improved with the riverbed material acting as a sand filter.1 In the absence of other reliable sources, the water from the dry river, accessed through scoop holes, infiltration galleries, or wells, serves as a source of domestic water or as water for livestock or irrigation. In addition, the subsurface flow in an ephemeral river recharges shallow groundwater. By constructing a well upstream of the river, people can extract this water more conveniently.

The importance of these rivers is evident, especially in arid and semiarid areas where the single river flood may be the only source of water for an entire year; all the more reason to harness these rivers in the best manner possible and to increase their water-retaining capacity. This can be done by dual-purpose road drifts. On low-volume roads, road drifts are more economical and preferred over conventional bridges across expansive dry rivers with occasional floods. They may not be passable during floods, but the construction cost of a road drift is substantially lower than that of a conventional bridge. If constructed well, they will retain subsurface water upstream. Moreover, they may stabilize the riverbed and control gullying and rutting. The latter will make it easier to divert water for irrigation from the surface of the riverbed.

• River crossings as sand dams and bed stabilizers
• Siting of a non-vented drift
• Design of non-vented drifts
• Measures to prevent failure

All materials to be used for the construction of the drifts and water-retaining structures should be tested to ensure that they meet the standard specifications for road and bridge construction. The recommended specifications are:

• Drifts should be constructed with reinforced concrete with twisted Y12 steel bars in a single layer spaced at 250 mm center to center.
• Structural concrete should be class 25/20 and blinding concrete should be class 15/20, with 1:1½:3 and 1:3:6 ratios of cement, fine aggregates, and coarse aggregates, respectively (as detailed in the drawings supplied by the design engineer). The concrete should be mixed well in the concrete mixer and compacted in place using a poker vibrator.
• The water/cement ratio should be well controlled. Ideally, the water should be half the amount of cement in the mix.
• The concrete design mix should be prepared by a qualified engineer and tested to meet the required strength before application. Well-compacted rock fill should be placed up to a minimum depth of 1 m to give the drift sufficient mass.