Opportunities Recommended practice: River crossings as sand dams and bed

OPPORTUNITIES

Ephemeral rivers, which range in span from 5 meters to 300 meters, 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 flows. 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 because the riverbed material acts 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 a single river flood may be the only source of water for an entire year. The lack of alternative water sources is all the more reason to harness these rivers in the best manner possible and increase their water-retaining capacity. This goal can be accomplished using dual-purpose road drifts. on low-volume roads, road drifts are more economical and are preferred to conventional bridges across expansive dry rivers with occasional floods (photo 9.1). The drifts may not

PHOTO 9.1

Culvertless road drift combined with sand dam with scope to further raise sand deposition upstream by gradually closing the gap in the side wall, Ethiopia