V. Dangol and P. D. Ulak
Department of Geology, Tri-Chandra Campus, Tribhuvan University, Kathmandu, Nepal
The high intensity rainfall of 19 and 20 July 1993 triggered off a large number of mass movements in the Lothar Khola watershed of Central Nepal. Most of the slides were reactivated on highly fractured and weathered rocks during the downpour. Among more than 40 landslides encountered in the watershed, the large rockslides were found at Purbangkhani, Karse, and Loling. A detailed study of landslide distribution in the watershed revealed that most of the landslides (about 65 % of the total landslide area) occurred on slopes ranging from 26 to 40°. These slopes were nearly equal to or a little steeper than the internal friction angle of constituting soil or rock mass. According to land use pattern, more than two thirds of the landslide area fell in the forestland and there were no landslides on the grassland. Similarly, rockslides were concentrated (about 53% of the total rockslide area) on slopes covered by slates and phyllites. On the other hand, an overwhelming majority of soil slides (more than 85% of the total soil slide area) occurred on residual soils. Debris flows were also very common in the Lothar Khola watershed. During the debris flow, from 1 to 3 m deepening of the riverbed was observed in many erosional zones whereas the sediment accumulation reached up to 4 m in the depositional zones.