Stability drastically degraded, future threats present major challenges
SIKKIM MINES & GEOLOGY DEPT SUBMITS REPORT ON IMPACT OF EARTHQUAKE ON SIKKIM’S LAND AND ROCK MASS
Study of 150 earthquake-induced slides flags 12 for urgent attention and recommends longterm measures for another 32. Disaster preparedness plans should seek out local inputs
GANGTOK, 27 Nov: With more than 150 landslides triggered off all over the State by the 18 September Earthquake, there is now genuine concern over the stability of the soil and rock formations and whether they will be able to withstand the next monsoon. Concern over the safety and security of life and property in the coming seasons has become a top priority with the State government now with the Mines & Geology Department reporting that there are likely to be numerous slides next monsoon as the slopes and hill sides have been destabilized and considerably weakened by the impact of the earthquake. Not just landslides, there is also a threat of glacial lake outbursts in North Sikkim.
According to the report, “The overall stability of land and rock mass has undergone drastic degradation. Keeping in view the changed scenario, the safety of life and properties is of great concern in years to come. The challenging task is to unravel the threat that is likely to be encountered in future.”
The report, which was recently submitted to the government, informs that numerous cracks have developed on the slopes throughout the State; with Sikkim being a terrain where landslide and slope modification process are still active, the risk and probability of earthquake induced landslides are very high.
Not just landslides, there is also the danger of mass mobilization. As the report states: “Due to the impact of recent earthquake, the risk of landslide and mass mobilization has risen to alarming stage and disaster mitigation task will be a challenging task for the future before the onset of next monsoon”.
The preliminary investigation, on which this report is based, identified more than 150 earthquake induced landslides out of which 12 landslides are reported to need urgent attention and another 32 slides identified for long term measures. It was observed by the experts during investigations that the impact of the earthquake was more pronounced on crystalline rocks mainly on its quartzites and variants with adverse slopes and on landmasses adjoining steep slopes and water-ways.
Short term measures recommended by the officials include soil compaction on fissures on landmass to minimize seepage and use of local technologies to retain unstable boulders. Apart from recommending identification of risk areas, it is also proposed that a disaster mitigation cum preparedness plan be put in place.
Importantly, it is also underlined that the involvement of local authorities and public will be vital in minimizing further threats and in coming up with local solutions. Also recommended is the creation of watch-posts or police posts at suitable places for timely rescue in road sections falling in high risk zones.
As part of long term measures, a detailed investigation of high risk zones is proposed with vulnerability of inaccessible areas to be accessed by remote sensing and high resolution aerial photographs. The investigation of these areas should be completed in advance to facilitate preparation of disaster mitigation plans ahead of the ensuing monsoon.
THE REPORT’S APPRAISAL OF NORTH SIKKIM
The Mines and Geology department has just concluded its preliminary assessment of the earthquake affected areas in the state and submitted its report to the government. A copy of the report has been accessed by NOW! The report, though preliminary and involving only accessible areas, consists of an area wise assessment of locations in the State which were affected by the earthquake of 18 September. Accordingly, some of the affected areas and slopes examined and which have been found to be posing a threat have been highlighted as areas needing urgent attention.
The areas falling between Chungthang and Lachung have been found to be among the worst affected, especially the river bank slopes; besides the landmass has also been found to have developed cracks and fissures here. The area between Lema and Lempokhari experienced the worst situation and a large chunk of landmass was washed away by a flash flood which was found to be even greater than the impact caused due to glacial lake out-bursts flood of the previous century. What is of particular concern, as per the report, is the formation of near vertical slope all along the river bank and formation of cracks on the landmass. In this context the report highlights Lema, Lephkha & Bop areas as needing urgent attention.
It was also studied that the rocks here are brittle and susceptible to weathering and jointly forming geologically vulnerable zones with the recent earthquake having led to further aggravating the degradation. The report further states that “…the area being under frost and thaw environment, the chances of rock bursts at higher elevation is high”. This area needs to be further examined in detail.
The major damages at Lachung area were found to have caused due to earthquake induced flash flood with obstruction of streams by the debris from hill slope resulting in landslide in the area. “There is also apprehension of the threat of glacial lake outbursts. The glacial lakes need to be monitored on regular basis to assess the threat,” states the report.
Lachen was found to have remained comparatively safe except for the development of minor cracks on buildings; besides the Chungthang-Lachen section was also found to be subjected to development of instable zones threatening vehicular movement.
There was also a massive landslide between Lachen and Zemu; however the report states that the exact position of future threat is not known at present and that the area needs to be investigated to assess the future threat. The same goes for areas beyond Zema which also reported severe damages.
Upper Dzongu area was among the worst affected by the earthquake with villages there having taken substantial damage and many killed. Here, the report states that the rocks being brittle in nature and there is apprehension that the vulnerable zones have developed which pose threat to life and property of local people in future. However as far as slides are concerned, as most slides were found to have occurred in inhabitable areas any instability due to degradation should not have future impact on life and property, the report states.
For Lower Lingza too, it is reported that landslide and slope failure have not been noticed within the visible zones the formation of unstable landmass and rocks cannot be ruled out and the area needed further investigation.
Comment by Praful Rao
It is coincidental that this statement by Dept of Mines, Govt of Sikkim comes just days after our Workshop on "Hazards and Critical Environmental Issues of Darjeeling-Sikkim Himalaya" on 23Nov2011, where my talk on "5 Months to go for the next monsoons" emphasized on the very same points made above.