Geological hazards in Pokhara Valley, western Nepal

Abstract

The Pokhara Valley is mainly occupied by coarse grained sediments weakly to strongly cemented by calcareous cement. These sediments are the result of at least 3 major episodes of debris flow events along the Seti River originating from the Annapurna Region. They are named as the Tallakot, Ghachok and Pokhara formations (Yamanaka et al., 1982). The debris flow had dammed the tributary valleys (Gurung, 1970) during the corresponding period contributing the development of lacustrine environment (Fort, 1986), of which several examples can be cited. The material of the Tallakot debris flow episode, which is the oldest one, was also found about 3 km upstream of the Mardi Khola (a major tributary of the Seti river) diverting about 900 from its main flow course where it dammed the stream with the creation of lacustrine environment behind. Although the age of the different debris flow deposits giving rise to the corresponding formations in the Pokhara Valley has yet to be determined, the age dated for the latest event i.e., for the Pokhara glacio-fluvial event as 600-1100 years before present (Yamanaka et al., 1982, and Fort, 1987), is debatable because of lack of any historical evidence.

In all the debris flow events the material is dominated by calcareous sediments both in clasts and in matrix. Because of their high content of calcareous material the calc-rudites so deposited are suffering from karstification of different intensity producing solution channels, chimneys and pinnacles forming underground caves and cavities. Although the material deposited by the Tallakot debris flow event is also suffering from the karstification but the material deposited by the Ghachok debris flow event has been found to be most susceptible for karstification. The importance of the karstification in the Ghachok Formation is also increased because the city core area of Pokhara is underlain by this formation.

Geologic hazards in the Pokhara Valley can be divided into debris flow hazard, karst hazard and landslide hazard.