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LANDSLIDE RISK ALONG THE ARNIKO HIGHWAY:

(Emma Askew, 2018)


WHY IS THE ARNIKO HIGHWAY AN IMPORTANT AREA TO RESEARCH?


The Arniko Highway, northeast of the Kathmanhu valley, Nepal, is predominately known for being the vital trade link between Kathmandu and China. It was recognised as the most adversely affected highway in Nepal after the 7.8Mw Gorkha earthquake that occurred on the 25thApril 2015, damaging a total 11.3km of the highway which had large socio-economic implications due to the stopping of trade. It is a vital to understand how landslide risk has changed along this highway since the earthquake as there is high social vulnerability due to the increased density of surrounding settlements since road development. By identifying the locations most at risk from future hazard, effective mitigation can be implemented to minimise or prevent large social, economic and environmental damage. Previously, research has identified the stretch of highway from Sakhuwa to Chaku as particularly hazardous to slope failure. Yet overall, there has been a significant lack of research conducted in this area, making the development of future hazard largely unknown and urgent to study to prevent the exacerbation of hazard impacts.


POST-EARTHQUAKE LANDSLIDE HAZARD:


After an earthquake, landslide hazards are increased and can persist for a long period of time due to the increased susceptibility of slope failure in the coseismic-landslide areas. Globally it is understood that large earthquake events can initiate a chain of following hazards, with modern research emphasising the importance in understanding how existing landslides can influence future landslides through path-dependency effects. Research from the 1999 Chi-Chi earthquake (7.3 Mw), Taiwan, and the 2008 Wenchuan earthquake (8 Mw), China, shows that that increased post-earthquake hazard is largely due to the abundance of unconsolidated sediment deposited the slopes from coseismic landslides. With this, the studied sediment budgets after Chi-Chi emphasised the importance of the earthquake-landslide sediment supply on the frequency of post-earthquake hazards, with estimations showing that hillslope erosion rates were increased by an order of 5, and mass-wasting to have remained elevated 4 years after the earthquake event. Moreover, research indicated the importance of revegetation in controlling the post-earthquake hazard response. In Wenchuan, it was found that landslide hazard decreased after 2009 as the landslide effected area began to revegetate at a rate of 12% a year.


However, it must be considered that although the fundamental understanding behind post-earthquake landslide behaviour and control factors can be useful for future hazard assessment, the findings of post-earthquake landslide hazard are site-specific and cannot be directly applied at a global scale.

THE FIELD INVESTIGATION:


Landslide hazard research commonly uses remote-sensing techniques in combination with fieldwork to verify satellite interpretations. In order to assess the Arniko Highway, field investigations were undertaken during March 2018 for two weeks, in which GIS techniques were used pre-fieldwork to identify the important sites to investigate. The field investigations were used to collect data on local-scale slope factors, such as rock fracture characteristics, in which seismically-induced fracturing could decrease material shear strength (or rock strength) which would give rise to future failure. Other factors assessed included rock weathering and rock bedding plane dip. Techniques including geomorphological mapping were used to ensure a detailed understanding of each landslide scar. It is important to recognise that the fieldwork was largely limited by challenges of site access with equipment and safety implications.


Overall, to compare our data between each landslide site we used decision trees and risk rating systems; finding that the factor of developed channels within the landslide scar was the most significant in causing high persistence of the landslide hazard. As a result, the presence of channels determined a debris-flow morphology, and where the channels were deeply incised and active the debris across the landslide scar could be efficiently moved with a low threshold to triggering factors (such as rainfall). With this, we could identify the landslide scars with the highest risk rating and the areas that required the highest attention for management. However, our work was limited as we could not accurately upscale our findings to a regional level due to overriding site-specific factors, such as rock type determining the strength of the slopes. Although, it would not be realistic to undertake field investigations across every landslide-scar along the Arniko Highway, which suggests a management framework needs to be developed with consideration for this.


INTERDISCIPLINARY APPROACH: NSET


Due to the investigation limitations and complications with upscaling local findings, it is important to recognise that landslide management should be developed with an interdisciplinary approach; considering both the physical risk and social vulnerability to the risk.


During the visit to Nepal, we got involved with NSET (National Society for Earthquake Technology) which is an organisation that educates Nepalese communities in how to become earthquake-safe, as well as acting a coordinator between the Nepelese government and wider society. NSET reduces landslide risk through encouraging long-term societal resilience, amongst low cost slope stabilizing measures. Examples of their projects include the ‘Baliyo Ghar’ programme (2015-2020), translated as ‘Strong House’, and their School Earthquake Safety programme (1999) to reinforce buildings (such as with doors opening outwards and bookcases screwed into the walls). These actions ensure buildings are resilient to earthquakes, although it is impossible to make them completely earthquake-proof. Overall, it is crucial to understand the importance that the magnitude of landslide hazard goes beyond environmental understandings, and needs to consider the direct risk reduction to society to achieve the most effective forms of landslide hazard management.


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