Soil erosion and conservation activities on land affected by road drainage: a case study of Nyeri District.
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The existing soil erosion damage caused by water drained from minor roads in the Nyeri District of Kenya was assessed and the soil conservation works needed to stabilize the waterways and gullies along the roads are specified. The study evaluated the land affected by road drains/culverts on 25 minor roads with a total length of 149 km. Of the total of 321 culverts identified, 171 (53%) were found to require channel rehabilitation. 68% of the culverts discharged onto steep slopes (>10%). Erosion was found to be more severe in the coffee-growing zones than in tea-growing areas. In the plateau areas, soil erosion from the culvert outlets was minimal. Due to gentle slopes and more perennial vegetation. 20,346 m of channel excavation was needed to provide artificial waterways for the discharge of water drained from the roads. Channel stabilization with grass cover or installation of scour checks was necessary on very gentle slopes. Steep slopes required stone check-dams and single-row post/stone check-dams. Very steep slopes also required lock-and-spill drains and post/stone/wire check-dams. Gabions (57 crossings) were needed mainly for rehabilitation of large gullies along with double-row post/stone check-dams and post/stone/wire check dams. Cut-off drains were necessary in some cases to divert water from the culvert outlets. In areas where vegetation was easily accessible, brushwood check-dams could be used.