GLOBALBÆREKRAFT-Forskning for global bærekraft

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.

Project outcomes and impacts are closely related to the project work, the outputs and the communication with stakeholders. This project used techniques such as satellite based InSAR, ground-based LIDAR scanning, UAV photogrammetry, and cosmogenic nuclide dating that are strongly used in Norway in landslide hazard mapping. These techniques are not in the same way introduced in the Indian Himalaya for landslide mapping and have not been used by the Indian project partners before. Hence did the project led to knowledge transfer from the Norwegian partners to the Indian partners. Due to collaboration with the local university in Sikkim this knowledge transfer reached institutions beyond the project partners and the next generation of geoscientists. In addition, did this collaborative project in Sikkim showcased the advantage of tackling complex geologic problems with a multi-institutional approach in which each partner provided it main expertise. The project produced multiple outputs with high relevance to natural hazards, such as a landslide inventory, an inventory of large rock slope deformations that is based on the Norwegian mapping approach and data base for unstable rock slopes, a landslide susceptibility map, and a detailed earthquake hazard map. These products can be used in regional and urban planning, road planning and road construction as well as planning of hydropower usage. Based on communication with public stakeholder these results are not only scientifically documented and available but also awareness to natural hazard issues was risen in Sikkim and stakeholder can implement gained knowledge in future development projects. Another output are deformation maps calculated with InSAR data over Sikkim. These show both deformation in urban environments but also in high mountain regions such as on moraine dammed lakes in the high catchment with sometimes very difficult access. Case studies of landslide in Gangtok showed the relation between meteorological conditioning and slope deformation. This show cases that this tool is ideal to map out pre landslide deformation in an urban environment and that moraine dam stability might be assessed based on deformation rates. As InSAR data are today often free of charge does this experience invite public institutions to take more benefit out of such public data. The project focussed efforts on selected key landslides and showed that computer-based slope stability modelling helps to assess stability on local scales and run-out modelling to assess the hazard zone of potential slope failures. Such methods can in future be implemented at other sites. Furthermore, did the project proposed mitigation measures for those key sites that expand over traditional slope stabilization in form of draining of slopes and/or anchoring and includes experiences from Norway such as re-routing and tunnelling. This might lead to a change of paradigm from structural landslide mitigation to passive mitigation measures.

In high mountain settings such as the Himalayas, losses of life and infrastructure are closely linked to natural events (e.g. earthquakes/rain-triggered landslides, floods), at times exacerbated by human interventions, and poor land-use planning and development. Landslides and their secondary effects such as dam formation, dam collapse, and related downstream flooding are one of the highest risks to society. In India, landslides triggered by the moderate 18th September 2011 Sikkim earthquake, have caused several casualties and significant economic losses, but much stronger earthquakes are possible in this area. This project proposal focuses on susceptibility ranking natural rock slopes that can lead to long run-out landslides with possible river blocking in NE India. This work will be based primarily on remote sensing data (optical and microwave satellite imagery) and supported by selected field investigations. The aim is to develop a multi-approach procedure to assess unstable slopes, that will facilitate effective land-use planning taking into account the primary and secondary consequences of landslides. The study area extends along two transects where important communities (e.g. Mangan and Chungthang) are settled and where two strategic corridors, one connecting Gangtok (capital of Sikkim) with Silliguri (NH 31A), and the Sikkim Highway India to China, develop. Considering the landslide prone context, the extension of the mountain chain in India and the strategic relevance of main corridors connecting India to neighbouring countries, points out the need of establishing a systematic mapping approach in India. The approach and outcomes of this project could be applied to other regions.