Contour Trenching as a Strategy in Watershed Rehabilitation: Application to Nepalese Condition
Abstract
The disastrous soil erosion problems and the uncontrolled movement of water in Nepal's mountains caused by human and livestock activities call for the identification of simple, cheap, and effective rehabilitation techniques. This report analyzes contour trenching as rehabilitation techniques in the United States and examines the applicability and transferability of the techniques to the Nepalese conditions.
The details of contour trench systems as applied by the U. S. Forest Service have been analyzed by reviewing available research papers, handbooks, official records, personal communication, and actual field visits. The results and observations have been delineated for the physical and cultural aspects of the Nepalese watershed system.
Contour trench systems in the United States are designed to hold overland runoff resulting from a high~intensity, short duration rainfall events. The idea is to store overland flow on site and allow it to percolate slowly into the soil. Trenches are an interim measure and are no substitute for rehabilitation measures designed to reduce runoff and erosion for a prolonged period of time, Quantitative evaluation of trenching effects are lacking. The findings of the few quantitative studies report the combined effects of trenching, grazing and fire control. There are examples of tremendous success and disastrous failures. Contour trenches are delicate structures. Evaluation by experienced personnel reveals that contour trenching has a definite role in the rehabilitation of impaired watersheds if the plan is carried out systematically and precisely.
Contour trenches have questionable benefits in areas where large volume, long duration and possibly high intensity rains occur (such as the monsoon areas). Thus, this control method has its limited role in the overall strategy of rehabilitation planning in Nepal. The primary limitation as seen from the analysis is the huge amount of long duration rainfall and direct runoff produced. However, there is some potential for application of contour trenching in the semi-arid parts of Nepal where frequent floodings are caused by short duration, high intensity rainfall. Watershed rehabilitation techniques developed in U. S. can be applied in Nepal in certain cases. However, site specific research support is essential in designing control structures. Nepal needs to develop research projects to identify and apply alternative rehabilitation techniques which can handle large volume of uncontrolled water over the impaired watershed.