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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)

zone as most of the

zone as most of the vegetation has been destroyed and well developed drainage (important indication of old slide zone) has been replaced by haphazard drainage lines. Furthermore, the large boulders of earlier sliding activity have mostly been buried under the huge amount of debris and boulders of presently active Horticulture Colony – Masjid Mohalla slide zones. Cutting and digging of slope at the toe portion of this Old Slide Zone-II for the construction of new houses and shops in Horticulture colony and Masjid Mohalla localities has also added to slope instability (Plate 5). According to locals the forest fire is an annual unfailing event in the Varunavat Parvat and surrounding area. During the present course of field investigations in August, 2003 many burnt out tree trunks and shrubs were observed in the area of old slide zone-I of Varunavat Parvat (Plate 6f). According to locals this destruction of vegetation was due to forest fire in this area during the summers of 2003. It is also observed that many a times forest fire over Varunavat Parvat is ignited by dangerous anthropogenic interference. In the area of pine forest very little grasses and shrubs can grow and most of this sparse vegetation is destroyed by forest fires during summers as the dry leaves of pine trees easily catch fire during this season. Repeated destruction of grasses, shrubs and trees in Varunavat Parvat area has resulted in an increase in the impact of rain splashes during the incessant rains and cloud burst events of rainy season. This in turn has increased the soil erosion. Another example of anthropogenic intervention in Varunavat Parvat area is blasting of hill slope for time to time widening and clearing (removal of boulders and debris) of Rishikesh-Gangotri National Highway, particularly in the area where Tambakhani slide intersects the National Highway. Furthermore the road link to Sangrali and Pata villages via Tekhala in the east of Varunavat Parvat has also been established after blasting the hill slope at a number of places. What adds, further to this dangerous anthropogenic interference in the fragile terrain is heavy vehicular traffic (during the pilgrimage season of May to October) on Rishikesh-Gangotri National Highway, that passes through the lower slope of Varunavat Parvat. Valdiya (1987) explained that the vibrations generated by the passage of heavy vehicles create oscillations of different frequency in the rocks and thus change the stress pattern reducing shear strength and inducing mass movement. Uttarkashi township is an important halt on the way to famous Hindu shrines of Gangotri and Gaumukh (the place of origin of river Ganga). Vibrations of hundreds of vehicles (buses, trucks and jeeps) which get their daily passage through Rishikesh-Gangotri National Highway in the lower reaches of Varunavat Parvat and blasting of hill slope for time to time widening and clearing of National Highway may also be considered as contributing factors for slope instability. It appears that major contributing factor for triggering large scale landslides from Varunavat Parvat was excessive pore water pressure caused due to high monsoon rainfall. Uttarkashi town experienced rainfall of the order of 652.02 mm and 602.20 mm in the months of July and August, 2003 respectively. This was followed by 200.40 mm precipitation in the month of September, 2003. Analysis of the available rainfall data has illustrated that the actual monthly rainfall of 652.02 mm during July, 2003 was second highest for this rainy month in particular (after 655 mm in July, 2000) during the nine years period of 1995-2003. Furthermore, the actual monthly rainfall of (602.20 mm) during August, 2003 was maximum for this rainy month during 1996-2003 period. Interestingly the months of July and August, 2003 have together experienced the Prognosis and mitigation strategy 639

DPM 17,5 640 rainfall of the order of 1,254.22 mm which is maximum for these rainy months during last nine years i.e. during 1995 to 2003 period (Figure 8). Slope instability in Varunavat Parvat area was further enhanced by various natural and man made pathways for infiltration of rainwater. Detailed field investigations prior to large scale sliding activity of September, 2003 have revealed few major cracks in the crown portion of the present day active slides (in the vicinity of Tambakhani slide) on Varunawat Parvat. One such major crack trending E-W with width varying from few centimeter to about a meter was observed in the body portion of the Old Slide Zone-I of Varunawat Parvat. In the mid August, 2003 this crack was extending from the large and active slide scarp of Tambakhani slide to the small slide zones of Ramlila Ground and Masjid Mohalla. Few major cracks in the Old Slide Zone-I in the close vicinity of the active scarp of Tambakhani slide were trending NW-SE with varying width 10 to 20 cm. Another set of cracks observed in this area was more or less trending ENE to WSW with a width of the order of 15 to 20 cm and these cracks were developed immediately below the main scarp of presently active slide zones. In addition to the E-W, NW-SE and ENE-WSW trending cracks the other major avenues for infiltration of water were three to four sets of joints trending NW-SE, NE-SW and E-W with varying joint aperture from 1 to 8 cm (Plate 6e). Ill conceived unlined drains immediately above the active scarp of Tambakhani slide within the scarp and crown portion of the Old Slide Zone-I have further enhanced the slope instability (Plate 6g). Though these drains were constructed to flash out the rainwater away from the slide area, yet they served as easy pathways for infiltration of rain water because they were unlined and abruptly terminating (on the flanks of active scarp of Tambakhani slide) with no connecting vertical drain. Infiltration of rain water from these horizontal drains into the active scarp of Tambakhani slide and small scarps of Ramlila Ground and Masjid Mohalla slides has further increased pore water pressure and hence enhanced the slope instability. It can be deduced from the foregoing discussion that prolonged sessions of incessant rains in the months of July and August, 2003 and excessive infiltration from a number of surface cracks, joints and unlined drains have altogether altered the hydrology and increased the pore water pressure, this in turn has caused rapid head-ward extension of small zones of Ramlila Ground and Masjid Mohalla slides during July to September, 2003 and by September 23rd, 2003 the small slide zone of present day Ramlila Ground slide completely merged with the scarp of large active zone of Tambakhani slide. Interestingly, all these active slide zones were developed within the large Old Slide Zone-1 of Varunavat Parvat wherein, the soil was not so competent, number of openings were developed for infiltration of rain water and rocks of the area were highly sheared and weak. Furthermore, the major discontinuities (faults, anticlinal and synclinal axis) in the vicinity of Varunavat Parvat, active tectonics, pronounced seismic activity (Uttarkashi earthquake of 1991) and anthropogenic intervention in the form of blasting for road cutting, movement of large number of vehicles through the fragile zone and alteration of hydrology due to construction of unlined drains in slide areas have been the equally contributing factors for large scale sliding activity. Above case study of Varunavat Parvat landslide takes into account the monitoring and assessment of sequence of landslide events and entire episode of landscape obliteration and also the analysis of causative factors. It is not only Uttarkashi

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