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Prognosis and mitigation strategy for major landslide-prone areas Varunavat Parvat landslide in Uttarkashi

Prognosis and mitigation strategy for major landslide-prone areas A case study of Varunavat Parvat landslide in Uttarkashi township of Uttarakhand (India)

hazard zone (Valdiya,

hazard zone (Valdiya, 1991). Bhagirathi river at Uttarkashi follows an anticlinal axis and fault. NE-SW trending Sangrali-Pata fault and the synclinal axis of E-W trending Kot syncline are only a few kilometers north of the presently active slide zones of Varunavat Parvat. Furthermore, Ujeli-Khand lineament is also in the close vicinity of Prognosis and mitigation strategy 637 Figure 7. Occurrence of small earthquakes (of M3 or less during 1980-1988 period) before the main impending event (on 10 October 1991) of Uttarkashi earthquake of M b6.6 (after Valdiya, 2004) Figure 8. Graph showing rainfall in the study area

DPM 17,5 638 Varunavat Parvat and towards the western extremity of Uttarkashi township another lineament referred to as Gyansu lineament is disposed from NE to SW. In the area under investigation four major sets of joints are observed. The joint set trending NW-SE (N30 o W to S30 o E) is dipping at an angle of 80 0 towards NE and has varying joint spacing of 30 cm to 60 cm with joint aperture of the order of 1-3 cm. Other near vertical joint set is trending NE-SW (N45 o E-S45 o W) and has joint spacing of about 30 cm with joint aperture varying between 2 to 8 cm. Another set of joints is trending NNE-SSW and dipping due east at an angle of 80 0 with joint spacing varying between 30 to 60 cm and aperture of the order of 3 cm. Yet another set of joints is dipping at an angle of 20 o due NE and has joint spacing of the order of 2 to 5 cm with joint aperture or opening of 1 to 5 mm. Unconsolidated colluvial deposits developed extensively over Varunavat Parvat are the major source of debris material of present day active slides of Ramlila Ground, Horticulture Colony and Masjid Mohalla localities. Large old slide zones on Varunavat Parvat were reported by DMMC (2003); Uniyal et al. (2004) and NRSA (2001, 2004). During the course of present investigations an attempt has been made to delineate the extent of old slide zones on Varunavat Parvat. It has been observed that two prominent old slide zones exist on Varunavat Parvat. One such old slide zone is developed in the crown portion of presently active slide zones of Tambakhani, Ramlila Ground, Masjid Mohalla and Horticulture Colony (Figures 5 and 6). This old slide zones is observed on the basis of concavo-convex slope morphology between 1,690 and 1,660 meter contour lines. It is between 1,675 and 1,690 meter contour lines that the scarp of this old slide is disposed. However, its body portion is heaping out valley ward and has angular fragments and boulders with thin soil cover. Distinct change in the shape of tree trunks with downward and outward bending of lower trunks about one to two feet above the sloping and undulating ground is also observed in this area (Plate 6c). This bending of the chirpine tree trunks is in a symmetrical pattern that might have developed due to ground movement in the early stage of their growth. Contrary to this, the chirpine trees of the surrounding area are not having this kind of symmetrical bending of their trunks. On the basis of the convergence of above discussed evidences viz. concavo-convex slope, step like morphology, transported rock fragments (by gravity transport) and unique resemblance in curvatures of the tree trunks the existence of a large old slide zone (Old Slide Zone-I) can well be established within and in the immediate upslope area of the presently active slide zones (of Varunavat Parvat). It is from within this Old Slide Zone-I that the active slide scarp of Tambakhani slide might have emerged more than a decade back and it is this very old slide zone-I which has again witnessed the triggering of large scale slides (Ramlila Ground and Horticulture Colony- Masjid Mohalla slides) in September/October, 2003 (Figure 5; Plates 6b and 6c). Another major old slide zone identified in Varunavat Parvat area is developed in Horticulture Colony locality and immediate upslope area. The scarp of this slide zone can be observed in the middle reaches of of Varunavat Parvat in the immediate upslope of Horticulture Colony area. During the course of present investigations this slide zone has been termed as Old Slide Zone-II (Figure 5). The existence of this Old Slide Zone-II is evident by the presence of large boulders with lichen growth and weathered surfaces, concavo-convex slope morphology, step like topography and vegetation anomaly (in the form of dense trees as compared to surroundings) in this area. The recent sliding activity of (September/October, 2003) has however obliterated many evidences of this old slide

Disaster management strategy for potential slide zones of Kumarkhera in Narendra Nagar township of Tehri Garhwal district, Uttarakhand, India