The retreat of two roadside cut slopes in weathered gneiss in the Middle Himalaya of east Nepal has been assessed by engineering geological mapping, sequential photography and slope profiling over a 40 year period, from 1979 to 2018, allowing approximate degradation rates to be calculated. The rate of removal of completely weathered gneiss and residual soil was about twice that of highly weathered material, primarily reflecting increased micro-cracking and breakdown of feldspars and micas in the weathered fabric. Surface water erosion by rilling and gullying in the summer monsoon wet season was the dominant process affecting the slopes, with landsliding (mainly by soil falls and debris slides) playing a secondary role. Drainage and bio-engineering protection measures applied to the slopes within the first 3–12 years after excavation were effective in reducing the degradation rate by between one and two orders of magnitude once the slopes had developed a moderate vegetation cover. The total volume of material removed from the slopes over four decades is equivalent to average rates of ground lowering in the range of 150–320 and 2–4 mm a−1 before and after protection works, respectively. The latter range is similar to long-term denudation rates measured in drainage catchments in the region with similar settings.
Supplementary material: One figure in three parts showing slope retreat over time for a complete set of 28 measured cross-sections and two tables giving details of calculated cross-sectional area losses and approximate intersectional volumetric losses of slope material are available at https://doi.org/10.6084/m9.figshare.c.7005885