Geomorphologic and Hydrogeologic Characteristics of Populated Rockslide Deposits (Sagarmatha National Park, Khumbu Himal, Nepal)

Abstract

Surface morphology and internal composition of large rockslide deposits have been frequently studied, but their hydrogeologic conditions and consequences for (drinking) water quality and quantity on such deposits are largely unknown. In this study we provide first information on this critical relationship for two large rockslide deposits in the Khumbu Himal (Nepal), which are at the same time the main settlement areas in the region. In the first step, we investigated the Lukla and the Namche-Khumjung rockslides with respect to their dimensions and internal composition based on orthophotos and digital elevation models, geomorphologic field mapping, and the analysis of rockslide outcrops. Secondly, we studied their hydrogeologic characteristics by means of spring water mapping, sampling and analyses. As a consequence of the fragmented and highly shattered rockslide material, both deposits are characterized by 1) effective infiltration, 2) short residence times of percolating water and 3) by only small amounts of available spring water and surface runoff at all. Human activity on the studied rockslide deposits can therefore be described as an ambivalent relationship: On the one hand, the rockslide deposits provide a gentle topography and the only available areas for extensive settlements and agriculture in the steep upper DudhKosi catchment; On the other hand, their internal composition accounts for water scarcity—a critical issue for the local population demanding for adaptation strategies, especially in the light of the ever—increasing trekking and expedition tourism in the region.