Landslide hazard assessment in NE India along the Gangtok-Tsomgo / Changu Lake and Gangtok / Chungthang-Lachen corridors

Informations

Funding country

Norway

Acronym

-

URL

-

Start date

1/1/2016

End date

12/31/2022

Budget

640,584 EUR

Fundings

Abstract

The project area of this international collaboration (NGU, NGI, WIHG, IITKgp) was located along the main roads connecting the capital of Sikkim (India), Gangtok, with China towards the north and east. This area receives higher precipitation as compared to the precipitation in the north-western Himalaya and is seismically active as indicated by the Mw 6.9 2011 Sikkim earthquake. Therefore, the occurrence of the large variety of landslide types and the high frequency in the area owe its origin to the tectonics/earthquake and to the heavy rainfall. In order to assist communities in effective aerial planning, the project provided multiple products. The main product is a landslide susceptibility map that is built on an inventory of landslides, including an inventory of large rock slope failures and rock slope instabilities, as well as a seismic hazard map elaborated by the project. The inventory of large rock slope failures and rock slope instabilities was carried out within the Norwegian data base for unstable rock slopes and translated into English for this project. As this database also includes secondary effects, the susceptibility map also includes the effects of landslide damming. Three large rock slope failures (RSF) occurred within the past 40 years along the 80 km long trans-Himalayan highway between Gangtok and Yumthang in the state of Sikkim in India. The first RSF occurred on September 10th, 1983. It was triggered by exceptional rainfall and impacted the settlement of Manul with a life loss of ca. 200 persons. The second RSF occurred close to the village of Yumthang on March 11th, 2015. The rock avalanche jumped over a 300-m-high cliff causing an airblast, that entirely flattened 1.4 square kilometres of mountain forest. The deposit overlies two generations of prehistoric rock-avalanche deposits. The last RSF occurred on August 13th, 2016, after at least 10 years of pre-failure rock-slope deformation. The deposit formed a dam in the Dzongu valley. The formed lake cut off the road access of the valley behind the dam for more than two years. Back-analysis of stability conditions indicate that rock structures caused slope deformation, while the collapse was triggered by high water pressure following monsoonal rain. Systematic mapping of deposits along the highway reveals 24 RSF deposits on the valley flanks, indicating further prehistorical events. Most settlements are located on these deposits because they represent relatively flat areas in the steep sloped valleys. Seven of those deposits could be dated by terrestrial cosmogenic nuclide dating revealing ages younger than 10 kyr (0-10 kyr). Results indicate that most of these deposits are composed of several events separated in time by several thousand years. Results indicate that there is a strong control on rock slope failure distribution by rock properties. The large RSF that from steep slopes within competent gneissic rocks to the north of the Main Central Thrust are in general younger. The slides to south of this fault, where rocks are relatively soft and slopes more moderate, are significantly older. 22 unstable rock slopes that are deforming today have been discovered by satellite image analysis and field reconnaissance from road. Their distribution corresponds regionally with the boulder deposits and minor landslides triggered by the magnitude MW 6.9 earthquake on 18.09.2011 in W Sikkim. In contrast to the Norwegian experience large rock slope failures in the Himalaya cause destructive air blasts reaching far beyond of the impact of the landslide itself and a method was developed to quantitatively assess this secondary effect. Based on ground-based remote sensing techniques (LIDAR, SFM of photogrammetric models) and satellite-based InSAR data analysis, as well as Pleiades 1A tri- stereo images and intensive ground surveys, a number of vulnerable spots (Yumthang, Lanta Khola, Dzongu, Chandmari, and Sichey) have been selected for in-depth studies, including stability and run-out analysis and effects of secondary effects as appropriate. Mitigation measures including rerouting, tunneling and slope stabilization by draining of the slope or installation of supporting mitigation measures were presented for some of those and other landslides in the Gangtok area. The sites were selected based on communication with local authorities.