Slope denudation and the supply of debris to cones in Langtang Himal, Central Nepal Himalaya

The onset and termination of the formation of debris cones and the rate of denudation of source areas were studied in the Langtang Valley, central Nepal Himalaya. A stereo photogrammetric analysis was employed to map a detailed topography of debris cones, through which the volume of 19 cones was calculated. Debris cones were classified into four types (types 1–4) based on the criteria, such as incision of a toe, sizes of a cone and surface boulders, altitudinal distributions and vegetation cover. Cones of types 1–3 had been formed after deglaciation following the Langtang stage (3310 years BP). Type 1 cones are characterized by incision of toes by glaciers of the Lirung and Yala stages. The formation of the type 1 cones terminated well before 2000 years BP. Radiocarbon dates of charcoal fragments and buried soils show the major period of formation of the type 2 cones from 3300 to 2000 years BP. The rate of sedimentation calculated near the toe of cone 3 has been reduced in the last 5 centuries from 30.0 cm/100 years to 5.5 cm/100 years. The large amount of denudation of source areas (116.9–132.5 m) indicates that slope failures initiated the formation of type 3 cones. The development of type 4 cones made less of a contribution to landscape evolution in the valley. The rates of denudation for source areas were calculated except for those above type 3 cones. The average rates are 15.6–6.2 mm/year (above type 1 cones), 3.2 mm/year (type 2 cones) and 3.4 mm/year (type 4 cones). Debris cones of types 2 and 3 are located on the valley floors which are completely deglaciated. Those of types 1 and 4, on the other hand, are located by the large valley glaciers with debris cover. The formation of type 1 cones is important especially in terms of debris supply to moraines and supra-glacial debris in the upper valley.