Erosion Scars Caused by Earth Flows - A Case Study from Central Kenya

摘要

Earth-flow features are described on a small-holder farm in Nyandarua District (Central Kenya), where over 900 earth flows occurred with most of the farms (> 50 %) in the region being affected. Individual flows affected areas from a few square metres to two hectares. In the area, the earth-flow features occurred exclusively on grazing land and on convex slopes, and are the result of extremely slow processes. The first visible signs are usually shallow scars along the contour, breaking the grass vegetation. Subsequent mass movement is very slow and gradual. According to old area residents, some of these earth-flow features have been observed as long as 60 years ago. Because of the extremely slow development, it was not possible for the local residents to date the earthflows; these earth flows "had always been there". Therefore, these features could not be attributed to recent ( < 30 years) land-use changes. One earth flow studied in more detail revealed that soil stratification, impeded drainage leading to oversaturation of the top soil layers, and seasonal sub-surface flow seemed to be mainly responsible for soil movement. No obvious triggering effect could be identified.