Road construction and gully erosion in West Pokot, Kenya

The study of soil erosion in Kenya is largely limited to agricultural and pastoral land. Little attention has been given to the effects of roads on soil erosion, although they cause more inconvenience than any other form of soil erosion. The object of this study is the B4, an unpaved road leading from the Marich Pass up the Kerio valley. This road runs across the footslopes of a steep scarp fault, in a climate with torrential rainfall. The footslopes consist of alluvial fans, pediments and terraces. Aerial photographs from 1963 show no erosion along the road: the 162 culverts and drifts were apparently adequate to cope with the drainage of the footslopes. Roadside gully formation is now a big problem for the road engineers. The 1 km sections of the road marked by the Ministry of Public Works have been used as the units of research, to facilitate exchange of information with the road engineers. In each section of the first 42 km of the road, the volumes of the roadside gullies were measured, along with erosion factors related to type of cross‐drainage construction, stability of the roadside material, geomorphological parameters and decrease in soil cover above the road since 1963. There is a strong correlation between roadside gullies, alluvial fans and decrease of soil cover. The road engineers were guided by geomorphological principles when designing the road. They preferred drifts, although these dips in the road surface are uncomfortable to pass by car. Drifts resemble natural drainage channels more closely than culverts and cause less damage in the fields below the road. The later settlers also showed geomorphological sense by preferring the smooth surfaces and well drained fine‐grained soils of the alluvial fans. The research demonstrates a common problem of road design in developing countries: however carefully the measures against erosion are designed, they become rapidly outdated because a new road attracts settlement. Deterioration of surface drainage and erosion start at unforeseeable points where people choose to settle. Copyright © 2002 John Wiley & Sons, Ltd.