Rural earthen roads impact assessment in Phewa watershed, Western region, Nepal

Abstract

Background: This work describes current research being conducted in the Phewa watershed, near Pokhara in Nepal’s Middle hills, a moist sub-tropical zone with the highest amount of annual rainfall in Nepal (4,500–5,000 mm). The main purpose of this study is to focus on the increase and impact of rural earthen road construction in the Phewa watershed as part of land use changes over 30 years in one of Nepal’s most touristic regions. Research methods were interdisciplinary and based on a combination of remote sensing, field observations and discussions with community members. The study compared 30 year-old aerial photos with current high resolution satellite images to detect changes in the watershed road network. Secondly, 42 % of the watershed’s roads (138 km) were surveyed in order to inventory and quantify shallow landslide events. Using an erosion-characteristics grid, their main features were measured (location, size, type and dimensions of damaged areas, etc.) and a Geographic Information System data base was created. We then estimated economic impacts of these events in terms of direct agriculture lands losses and road maintenance. Results: Results of the remote sensing analysis demonstrate that the road network increase is following an exponential trend, which enables us to predict future watershed road network scenarios. Field work investigations have demonstrated that unplanned road excavations are producing mainly embankment shallow planar soil slides and/or gullying which primarily affect roads themselves, wiping them out and blocking vehicle circulation, and secondly, destroying or burying agriculture lands. Along the 138 km surveyed, we calculated an amount of soil material released of about 70,000 cubic meters, which amounted to 99 cubic meters per kilometer annually. Of 179 cases of roadside erosion processes sampled, about 85 % directly impact roads or agricultural lands. Conclusion: The current mode of road construction which is currently occurring in Nepal is largely related with erosion and shallow landslide processes. Considering the exponential growth of rural earthen road networks, we would expect an increase of sediments released by roads and serious consideration must be taken if roads continue to be made without more careful methods. Through simple technologies using low cost and local resources along the lines of ‘green road’ or ‘eco-safe road’ approaches, it may be possible to reduce the impacts of rural road construction.