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

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developed having huge accumulation of debris and the toe of the slide is steep. Occasional falling of boulders and debris material has also been reported during the successive monsoon of the year 2004 and 2005 (Figures 4-6; Plates 1-4 and Plate 6a). Horticulture Colony-Masjid Mohalla-Jal Sansthan Colony slide During the cloud burst events of July/August, 2003 another small slide zone (with dimensions more or less similar to that of Ramlila ground slide zone) developed on Varunavat Parvat in the upslope of Horticulture Colony and Masjid Mohalla localities of Uttarkashi township. In mid-August, 2003 this active slide zone was disposed about 80 meters east of Tambakhani slide and merely 30 meters east of Ramlila Ground slide zone and was experiencing continuous head-ward expansion. It was on 1 October 2003 (i.e. eight days after the initiation of large scale sliding event of Ramlila Ground slide) that the active slide zone of Horticulture Colony-Masjid Mohalla slide merged with the head scarp of Tambakhani and Ramlila ground slide zones, consequently another major slide was triggered from Varunavat Parvat and sliding continued throughout the month of October, 2003. The approximate length of this slide zone is 580 meters and it bifurcates in the immediate upslope of Horticulture Colony and Masjid Mohalla slide localities (Figures 4-6; Plate 5). Gyansu slide Gyansu slide is a debris flow slide and is developed towards the western extremity of Uttarkashi township along a seasonal stream called Gyansu Nala. This slide zone has a long historyof devastation. About two and ahalf decades back on the nights of 24/25 June, 1980, 24 lives were lost and a number of houses, huts and cattle yards were destroyed by large scale debris flow along Gyansu Nala. Incessant rains in Uttarkashi township in July and August, 2003 have once again activated this debris flow slide zone and Rishikesh-Gangotri National Highway (NH-108) was blocked a number of times in July and August, 2003. This debris flow slide has a total length of about 1.3 km and its width varies from 10 to 20 meters and the slide material comprises boulders of phyllite and quartzite along with debris material. The scarp of this flow slide is active and lies on the west bank valley wall of Gyansu Nala and has few seepage zones, furthermore a number oftrees are uprooted inthis portion ofthe slide. The body ofGyansu flow slide is elongated and flow lobe is well developed with a width of about 16 meters at its intersection with Rishikesh-Gangotri National Highway. It is in this area that the habitation of Gyansu is developed and a number of shops and houses along both the flanks of this flow slide are at very high risk during the rainy season (Figure 4, Plates 9 and 10). Discussion Cumulative effects of natural factors and anthropogenic activities have been cited as the causative factors for some of the potential slide zones of Uttaranchal (Jayan, 2004; Uniyal, 2004; Uniyal and Rautela, 2005). The phenomenon of landslides in Uttarkashi township has been explained by weaving together the different threads of evidences adduced from seismicity, geology, morphotectonics, landuse pattern, landslide history, hydrology, rainfall pattern and anthropogenic activities (Figures 2, 4, 5, 7 and 8; and Plate 6). Triggering of Tambankhani landslide in early 1990s (according to locals) can be attributed to frequent ground accelerations and heavy precipitation in an area of Prognosis and mitigation strategy 635
DPM 17,5 636 Plate 9. View of the Gyansu flow slide Plate 10. Close view of the flow lobe of Gyansu flow slide at Rishikesh – Gangotri National Highway complex tectonics and long history of mass wasting. According to Valdiya (2004) the Uttarkashi earthquake (Mb 6.6) in October 1991 was preceded by pronounced microseismicity for about eight years (Figure 7). Ground accelerations and movements may have most severe effects in the areas previously affected by landslides along or near significant faults (Northmore et al., 1988). According to Valdiya (1987) seismic shocks are the biggest triggering factor. The ground vibrations of the intensity VIII or more on Modified Mercalli (mm) Scale have caused landslides on instable hill slopes (Shah and Nagarajan, 2004). Complex tectonics has significantly contributed to the slope instability in the area. The township of Uttarkashi is in the close vicinity of a number of major structural discontinuities. Main Central Thrust (MCT) is merely 9 km NE of Uttarkashi township. The zone of MCT is highly unstable domain nearly 5-20 km. wide and has highly sheared and pulverized rocks (Valdiya, 1985) and whole MCT zone lies in high seismic
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Prognosis and mitigation strategy for major landslide-prone areas Varunavat Parvat landslide in Uttarkashi

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