LANDSLIDE INDUCED RIVER BED UPLIFT IN THE TAL VALLEY OF GARHWAL HIMALAYA, INDIA

A large scale landslide occurred in the Tal valley of Garhwal Himalaya in late January, 1990. The most interesting feature associated with this slide was the 4 m uplift of a 200 m long and 25 m wide part of the river bed (at an average rate of 13.8 cm/ day) between 28 January and 25 February 1990. Studies revealed that the slide occurred because of complex slope processes consisting of rotation acid slump movement attributable to the accumulation of pore water pressure in extremely shattered and pulverized rocks during prolonged surface moistening. The process of movement was triggered by the gravitational force exerted upon the sheared material which dislocated a block at the upper part of the slide along a slip plane which terminated below the river bed. Counter resistance was provided by the rocks below the river bed to subdue the further movement of the slide. This indicates that uplift of the river bed and formation of terraces is also related to the nature of the slide and the volume of slumped material.