The Effect of Road Design and Land Use on Gully Erosion: A Case Study of the Rural Access Roads in Embu District, Kenya

Abstract

The study of soil erosion in Kenya is largely limited to agricultural and pastoral land. Little attention has been given to the effects of road design on soil erosion, although they cause more inconvenience than any other form of soil erosion. The contribution of road design to gully erosion was investigated in Mbeti North and Municipality locations of Central Division of Embu District. The study was conducted on 2 minor roads (road A classified as E. No. 632 and Road B classified as D. No. 467) with a total length of 800 m. Methods used included interviews and discussions with stakeholders, visual observations, surveying, and measuring changes on sediment deposition. A total of six culverts were identified of which 4 (66%) were found to require rehabilitation and 83 % of them discharged onto steep slopes (> 10%). Visual observations further revealed that the roads were designed to drain runoff at several points through mitre drains and culverts. Most of the mitre drains were blocked upstream leading to massive amounts of runoff flowing through the culverts and onto farm land causing gully formation. Gullies A and B were controlled using checkdams of brushwood and stone gabions respectively. By the end of the second season soil had accumulated substantially in both gullies. Gullies A and B had accumulated about 50 and 70 mm depth respectively. At this rate and all factors constant it may take approximately 20 years (considering current average depth of 1.4 m) for the two gullies to fill and heal completely.