Disaster management strategy for potential slide zones of Kumarkhera in Narendra Nagar township of Tehri Garhwal district, Uttarakhand, India
Himalayan landslides, have been explained by Uniyal (n.d., in press), as the prolonged sessions of incessant rain during monsoon season, infiltration of water, and alteration of hydrology of an area, which has fragile rocks pronounced seismic, and tectonic activity, and in some cases anthropogenic intervention as well. A brief account of natural factors and anthropogenic intervention leading to slope instability in the study area is given in the following: . Many evidences of old sliding were observed in Kumarkhera and upslope area. Vegetation anomaly in the form of growth of dense trees (as compared to surroundings) in and around Kumarkhera area on a concavo-convex slope indicates an old slide zone. Concave slope in the immediate upslope of Kumnarkhera is representing the lower scarp portion of an old slide. However the head scarp of the old slide zone is represented by steep slope in the further upslope in upper reaches of the hill. The drainage in the area is well developed over a landscape that gives step like appearance (see Plates 1-5). Boulders with weathering surfaces are also noticed in the immediate upslope of Kumarkhera. On the basis of the convergence of the previously discussed evidences, namely vegetation anomaly, concavo-convex slope, step like morphology, transported rock fragments (by gravity transport) the existence of a large old slide zone can well be established in Kumarkhera area. It is from within this old slide zone that the small active debris slide, debris flow and subsidence zones are emerging. The scarp of this slide zone can be observed in the upper reaches of hill in the immediate upslope of Kumarkhera area. . Swollen with monsoon rains and attendant deluge the small seasonal drains (nalas) passing through Kumarkhera have been causing severe bank erosion and increasing the vulnerability of houses and shops constructed on their banks. . The cutting of unstable slope for widening of Rishikesh-Gangotri National Highway has to some extent aggravated the slope failure. Valdiya (1987) and Ives (1987) referred to unplanned slope cutting and excavation for road construction as one of the main triggering factor for landslide incidences in Himalaya. Furthermore, the unplanned and rigorous cutting of slope at Kumarkhera (for commercial and commercial cum residential purpose) on both the sides of Rishikesh-Gangotri National Highway has fostered the slope instability. Consequently the angle of repose of old slide material has been altered at many places and this in turn is enhancing slope instability in the form of small active slide and subsidence zones in the upslope area of Kumarkhera (see Plate 7). . The ill conceived construction of two to three storied houses in this area in the recent years has not only overloaded the fragile slope but has also obstructed or to some extent blocked the flow of surface runoff of the upslope area at many places. . Increase in the moisture content and pore water pressure due to congested drainage is decreasing the shear strength of old slide material over which Kumarkhera rests, this is also evident by bulging of ground at Kumarkhera. . Carbonate rocks of the area are also susceptible to chemical weathering and this problem is aggravated particularly when there is congested drainage, surface Disaster management strategy 365
DPM 19,3 366 Plate 7. A full view of Kumarkhera area situated over an old slide zone. Some small active mass wasting zones are seen in the northern and southeastern part of this habitation runoff and household wastewater is not drained out properly and allowed to percolate. . The seismic shocks are biggest triggering factor as they modify the system of forces in such a manner that driving forces get the upper hand (Valdiya, 1987). The area under investigation lies in Earthquake High Damage Risk Zone IV and seismic shocks of a future earthquake may easily transform the potential slide zones of Kumarkhera into active slide zones. The vibrations generated by the passage of heavy vehicles create oscillations of different frequency in the rock and thus change stress pattern, reducing shear strength and inducing mass movement. Since Kumarkhera is located on Rishikesh-Gangotri-Yamnotri route, hence it also witnesses vibrations of hundred of vehicles that pass through this area during pilgrimage season. Recommendations Diversion channels should be constructed in the upslope area of Kumarkhera in order to divert the flow of surface runoff particularly during rainy season and minimize percolation of water into the fragile slope of the area. These Diversion channels should be properly lined and be draining away from this area. A series of wire mesh retention walls are required to be erected along valley walls of small seasonal streams (nala) passing through Kumarkhera. Construction of multi-storied houses should be banned in this area, as it would cause overloading of the subsidence prone hill slope. In order to minimize the sliding forces only lightweight houses should be allowed in this area.