Narendra Raj Khanal

Geomorphic and geologic controls of geohazards induced by Nepal’s 2015 Gorkha earthquake

Nepals quake-driven landslide hazards Large earthquakes can trigger dangerous landslides across a wide geographic region. The 2015 Mw 7.8 Gorhka earthquake near Kathmandu, Nepal, was no exception. Kargal et al. used remote observations to compile a massive catalog of triggered debris flows. The satellite-based observations came from a rapid response team assisting the disaster relief effort. Schwanghart et al. show that Kathmandu escaped the historically catastrophic landslides associated with earthquakes in 1100, 1255, and 1344 C.E. near Nepals second largest city, Pokhara. These two studies underscore the importance of determining slope stability in mountainous, earthquake-prone regions. Science, this issue p. 10.1126/science.aac8353; see also p. 147 Satellite imaging isolated hazard potential for earthquake-triggered landslides after the 2015 Gorkha earthquake in Nepal. INTRODUCTION On 25 April 2015, the Gorkha earthquake [magnitude (M) 7.8] struck Nepal, followed by five aftershocks of ≥M 6.0 until 10 June 2015. The earthquakes killed ~9000 people and severely damaged a 550 by 200 km region in Nepal and neighboring countries. Some mountain villages were completely destroyed, and the remote locations, blocked roads, and landslide-dammed rivers prevented ground access to many areas. RATIONALE Our “Volunteer Group” of scientists from nine nations, motivated by humanitarian needs, focused on satellite-based systematic mapping and analysis of earthquake-induced geohazards. We provided information to relief and recovery officials as emergency operations were occurring, while supported by one of the largest-ever NASA-led campaigns of responsive satellite data acquisitions over a vast disaster zone. Our analysis of geohazards distribution allowed evaluation of geomorphic, tectonic, and lithologic controls on earthquake-induced landsliding, process mechanisms, and hazard process chains, particularly where they affected local populations. RESULTS We mapped 4312 coseismic and postseismic landslides. Their distribution shows positive associations with slope and shaking intensity. The highest areal densities of landslides are developed on the downdropped northern tectonic block, which is likely explained by momentary reduction of the normal stress along planes of weakness during downward acceleration. The two largest shocks bracket the high-density landslide distribution, the largest magnitudes of the surface displacement field, and highest peak ground accelerations (PGAs). Landslides are heavily concentrated where PGA was >0.6g and slope is >30°. Additional controls on landslide occurrence are indicated by their clustering near earthquake epicenters and within specific lithologic units. The product of PGA and the sine of surface slope (defined as the landslide susceptibility index) is a good indicator of where most landslides occurred. A tail of the statistical distributions of landslides extends to low values of the landslide susceptibility index. Slight earthquake shaking affected vulnerable materials hanging on steep slopes—such as ice, snow, and glacial debris—and moderate to strong shaking affected poorly consolidated sediments deposited in low-sloping river valleys, which were already poised near a failure threshold. In the remote Langtang Valley, some of the most concentrated destruction and losses of life outside the Kathmandu Valley were directly due to earthquake-induced landslides and air blasts. Complex seismic wave interactions and wave focusing may have caused ridgetop shattering and landslides near Langtang but reduced direct shaking damage on valley floors and at glacial lakes. CONCLUSION The Gorkha earthquake took a tremendous, tragic toll on human lives and culture. However, fortunately no damaging earthquake-caused glacier lake outburst floods were observed by our satellite analysis. The total number of landslides was far fewer than those generated by comparable earthquakes elsewhere, probably because of a lack of surface ruptures, the concentration of deformation along the subsurface thrust fault at 10 to 15 km depth, and the regional dominance of competent high-grade metamorphic and intrusive igneous rock types. Landslide distribution and effects of a huge landslide. (A) Landslides (purple dots) are concentrated mostly north of the tectonic hinge-line. Also shown are the epicenters of the main shock and largest aftershock. Displacements are from the JAXA ALOS-2 ScanSAR interferogram (21 Feb and 2 May 2015 acquisitions). (B and C) Before-and-after photographs obtained by D. Breashears in Langtang Valley showing complete destruction of a large part of Langtang village by a huge landslide. The Gorkha earthquake (magnitude 7.8) on 25 April 2015 and later aftershocks struck South Asia, killing ~9000 people and damaging a large region. Supported by a large campaign of responsive satellite data acquisitions over the earthquake disaster zone, our team undertook a satellite image survey of the earthquakes’ induced geohazards in Nepal and China and an assessment of the geomorphic, tectonic, and lithologic controls on quake-induced landslides. Timely analysis and communication aided response and recovery and informed decision-makers. We mapped 4312 coseismic and postseismic landslides. We also surveyed 491 glacier lakes for earthquake damage but found only nine landslide-impacted lakes and no visible satellite evidence of outbursts. Landslide densities correlate with slope, peak ground acceleration, surface downdrop, and specific metamorphic lithologies and large plutonic intrusions.

Abandonment of Agricultural Land and Its Consequences: A Case Study in the Sikles Area, Gandaki Basin, Nepal Himalaya

Abstract This paper examines the extent, causes, and consequences of abandonment of agricultural land near the village of Sikles in the Nepal Himalaya. Socioeconomic information was collected in a household survey. Abandoned agricultural land and geomorphic damage were mapped at plot level for an area of 149.6 ha. Plot-level analysis showed that nearly 49% of all khet land and 37% of all bari land had been abandoned. About 10% of all khet land had been completely damaged by landslides and floods. Nearly 41% of all abandoned plots were subjected to different forms of geomorphic damage. The amount of geomorphic damage on plots abandoned earlier is greater than that on plots abandoned recently. Abandonment of agricultural land does not automatically lead to plant colonization because geomorphic damage is intensified prior to colonization. Abandoned land requires further management for plant colonization as well as for reducing the risk of geomorphic hazards. Prevailing government policies and acts are not effective in managing abandoned land. The phenomenon of abandoned agricultural land observed in the Nepal Himalaya is not unique: it is common in many mountain areas in the world. However, this phenomenon has recently led to pronounced socioeconomic and environmental problems in Nepal.

Glacial lake outburst flood risk assessment of Sun Koshi basin, Nepal

The ongoing retreat of glaciers in the Hindu Kush-Himalaya (HKH) is associated with climate change. While deglaciation can cause a suite of impacts, one of the most visible and tangible impacts is the formation of glacial lakes. Some of these lakes can burst out causing large flash floods with the potential to cause significant damage to property, lives and livelihoods. At the moment, knowledge of the current glacial lake outburst flood (GLOF) risk in the HKH is incomplete, and a proper risk assessment is often circumvented. There is a need for a comprehensive GLOF risk assessment in order to support proper planning of mitigation and adaptation strategies in this context. In this paper we present a methodological approach for the GLOF risk assessment. The major part of the risk assessment is GLOF simulation and downstream impact assessment. The methodology was applied to the Sun Koshi river basin, a trans-boundary river basin between Tibet (China) and Nepal. A glacial lake outburst hydrograph was simulated using a dambreak model. The outburst flood was routed along the river using a hydrodynamic model to estimate the potential impact areas. A field survey was conducted to assess the potential damage caused by the GLOF. The peak outburst flood could be in the order of 7900 m3 s−1. The analysis shows that about 950 ha of land and a large amount of infrastructure are exposed to the GLOF. The economic risk due to the direct impact of a GLOF is estimated to be about US

Review of studies on land use and land cover change in Nepal

197 million.

A comprehensive approach and methods for glacial lake outburst flood risk assessment, with examples from Nepal and the transboundary area

Land use and land cover (LULC) in Nepal has undergone constant change over the past few decades due to major changes caused by anthropogenic and natural factors and their impacts on the national and regional environment and climate. This comprehensive review of past and present studies of land use and land cover change (LUCC) in Nepal concentrates on cropland, grassland, forest, snow/glacier cover and urban areas. While most small area studies have gathered data from different sources and research over a short period, across large areas most historical studies have been based on aerial photographs such as the Land Resource Mapping Project in 1986. The recent trend in studies in Nepal is to focus on new concepts and techniques to analyze LULC status on the basis of satellite imagery, with the help of geographic information system and remote sensing tools. Studies based on historical documents, and historical and recent spatial data on LULC, have clearly shown an increase in cropland areas in Nepal, and present results indicating different rates and magnitudes. A decrease in forest and snow/glacier coverage is reported in most studies. Little information is available on grassland and urban areas from past research. The unprecedented rate of urbanization in Nepal has led to significant urban land changes over the past 30 years. Meanwhile, long term historical LUCC research in Nepal is required for extensive work on spatially explicit reconstructions on the basis of historical and primary data collection, including LULC archives and drivers for future change.

Climate change on the southern slope of Mt. Qomolangma (Everest) Region in Nepal since 1971

Like other mountainous areas, Nepal is highly vulnerable to glacial lake outburst floods (GLOFs), and this vulnerability has increased due to climate change. Risk reduction strategies must be based on a comprehensive risk assessment. A comprehensive methodological approach for GLOF risk assessment is described and illustrated in case studies of the potential GLOF risk posed in Nepal by four glacial lakes, one located in China. People, property and public infrastructure (including hydropower plants, roads and bridges) are vulnerable, and there is a need to integrate GLOF risk reduction strategies into national policies and programmes.

Glacial Lake Outburst Flood Risk in the Poiqu/Bhote Koshi/Sun Koshi River Basin in the Central Himalayas

Based on monthly mean, maximum, and minimum air temperature and monthly mean precipitation data from 10 meteorological stations on the southern slope of the Mt. Qomolangma region in Nepal between 1971 and 2009, the spatial and temporal characteristics of climatic change in this region were analyzed using climatic linear trend, Sen’s Slope Estimates and Mann-Kendall Test analysis methods. This paper focuses only on the southern slope and attempts to compare the results with those from the northern slope to clarify the characteristics and trends of climatic change in the Mt. Qomolangma region. The results showed that: (1) between 1971 and 2009, the annual mean temperature in the study area was 20.0°C, the rising rate of annual mean temperature was 0.25°C/10a, and the temperature increases were highly influenced by the maximum temperature in this region. On the other hand, the temperature increases on the northern slope of Mt. Qomolangma region were highly influenced by the minimum temperature. In 1974 and 1992, the temperature rose noticeably in February and September in the southern region when the increment passed 0.9°C. (2) Precipitation had an asymmetric distribution; between 1971 and 2009, the annual precipitation was 1729.01 mm. In this region, precipitation showed an increasing trend of 4.27 mm/a, but this was not statistically significant. In addition, the increase in rainfall was mainly concentrated in the period from April to October, including the entire monsoon period (from June to September) when precipitation accounts for about 78.9% of the annual total. (3) The influence of altitude on climate warming was not clear in the southern region, whereas the trend of climate warming was obvious on the northern slope of Mt. Qomolangma. The annual mean precipitation in the southern region was much higher than that of the northern slope of the Mt. Qomolangma region. This shows the barrier effect of the Himalayas as a whole and Mt. Qomolangma in particular.

Characteristics of landslide in Koshi River Basin, Central Himalaya

The Himalayas have experienced several glacial lake outburst floods (GLOFs), and the risk of GLOFs is now increasing in the context of global warming. Poiqu watershed in the Tibet Autonomous Region, China, also known as the Bhote Koshi and Sun Koshi downstream in Nepal, has been identified as highly prone to GLOFs. This study explored the distribution of and changes in glacial lakes, past GLOFs and the resulting losses, risk from potential future GLOFs, and risk reduction initiatives within the watershed. A relationship was established between lake area and volume of lake water based on data from 33 lakes surveyed within the Hindu Kush Himalayan region, and the maximum possible discharge was estimated using this and other previously developed empirical equations. We recommend different strategies to reduce GLOF risk and highlight the need for a glacial lake monitoring and early-warning system. We also recommend strong regional cooperation, especially on issues related to transboundary rivers.

Lemthang Tsho glacial Lake outburst flood (GLOF) in Bhutan: cause and impact

Koshi River basin, which lies in the Central Himalayas with an area of 71,500 km2, is an important trans-boundary river basin shared by China, Nepal and India. Yet, landslide-prone areas are all located in China and Nepal, imposing alarming risks of widespread damages to property and loss of human life in both countries. Against this backdrop, this research, by utilizing remote sensing images and topographic maps, has identified a total number of 6877 landslides for the past 23 years and further examined their distribution, characteristics and causes. Analysis shows that the two-step topography in the Himalayan region has a considerable effect on the distribution of landslides in this area. Dense distribution of landslides falls into two regions: the Lesser Himalaya (mostly small and medium size landslides in east-west direction) and the Transition Belt (mostly large and medium size landslides along the river in north-south direction). Landslides decrease against the elevation while the southern slopes of the Himalayas have more landslides than its northern side. Change analysis was carried out by comparing landslide distribution data of 1992, 2010 and 2015 in the Koshi River basin. The rainfall-induced landslides, usually small and shallow and occurring more frequently in regions with an elevation lower than 1000m, are common in the south and south-east slopes due to heavy precipitation in the region, and are more prone to the slope gradient of 20°~30°. Most of them are distributed in Proterozoic stratum (Pt3ε, Pt3 and Pt2-3) and Quaternary stratum. While for earthquake-induced landslides, they are more prone to higher elevations (2000~3000 m) and steeper slopes (40°~50°).

Investigation and Analysis of Geohazards Induced by the 2015 Nepal Earthquake Based on Remote Sensing Method

BackgroundThe Hindu Kush Himalayan (HKH) region being seismically active and sensitive to climate change is prone to glacial lake outburst flood (GLOF). The Lemthang Tsho GLOF breached in the evening of 28 July 2015 innorth-western Bhutan is reminds of the looming threat, and stresses the need to have good risk management plan. The need to understand the physical processes in generating GLOF to is therefore imperative in order to effectively manage the associated risk. The paper therefore assesses the cause and impact of the Lemthang Tsho GLOF event using field and remote sensing data.ResultsThe collapse of near vertical wall of supraglacial lake triggered by 2 days of incessant rainfall, opened up the englacial conduit resulting in emptying of interconnected supraglacial lakes into Lemthang Tsho. The5.1 magnitude earthquake epicentered 187 km to southeast in the Indian state of Assam in the morning (7:10 am Bhutan Standard Time) of the same day is unlikely to have played any role in triggering the event. The estimated volume of water unleased is 0.37 million m3, with peak discharge estimated to be ranging from 1253 to 1562 m3/s, and velocity of 7.14–7.57 m/s. The impact was minimal and confined up to 30 km downstream from the lake. The flood took lives of 4 horses, washed away 4 timber cantilever bridges, 148 pieces of timber, damaged 1 acre of land, and washed away 1 km of trail. The team also monitored 3 out of 25 identified critical glacial lakes and downgraded the risk of all 3 critical glacial lakes based on the finding. This brings the number of critical glacial lakes in Bhutan to 22.ConclusionThe threat of GLOF still looms large in the Himalaya, particularly in view of impact of climate change and frequent seismic activities. There is a need for good risk management practices which starts fromidentification of critical glacial lakes, to prioritize in-depth investigation. As the present list of critical glacial lakes are largely based on inventory done over a decade based on topographic maps some of which datedback to 1960s, we need to revisit the critical glacial lakes and assess the risk considering recent changes. The new assessment needs to consider supraglacial lakes as one of the criteria in evaluating the GLOF risk, as highlighted by the Lemthang Tsho GLOF.