Ranjan Kumar Dahal

A study of local amplification effect of soil layers on ground motion in the Kathmandu Valley using microtremor analysis

Past researchers have anticipated the occurrence of a great earthquake in the central Himalayas in the near future. This may cause serious damage in the Kathmandu Valley, which sits on an ancient lake bed zone, with lacustrine sediments of more than 500 m depth. In this study, the predominant frequency of ground motion is evaluated using the Horizontal-to-Vertical (H/V) spectral ratio technique and recordings of ambient noise. The results of the H/V ratio show two peaks in about 20 percent of the locations, which are distributed mainly in and around the center and northern part of the Kathmandu Valley. The predominant frequencies vary from 0.5 Hz to 8.9 Hz in the study area, whereas the second resonance frequency varies from 4 Hz to 6 Hz in the center and northern part of the valley. This indicates that the center and northern part of the valley have a wide range of resonance frequency due to two levels of impedance contrast — one may be from the surface layer and the other may be from the layer underneath. These two levels of resonance indicate the importance of considering the effects of surface and lower layers during the planning and designing of infrastructures in the Kathmandu Valley.

A replication of landslide hazard mapping at catchment scale

Landslide hazard assessment is a primary tool to understand the basic characteristics of slopes that are prone to landslides, especially during extreme rainfall. In this study, weights-of-evidence modelling a bivariate statistical method, and a logistic regression model, a multivariate statistical method, were used for landslide hazard mapping in two catchments of the Siwaliks in the Nepal Himalaya. Two typical catchments, Charnath and Jalad of the Siwaliks in easternNepal, were selected for the landslide hazard mapping. Both modelling approaches were applied by considering 10 intrinsic factors and one extrinsic factor. Mainly digital elevation model-based causative factors and field data were used to prepare data layers of landslide causative factors. In many approaches formodelling of landslide hazard in GIS, the model validation process is always dependent, and landslide data, which are used to calculate a landslide hazard index (LHI), are applied for verification. However, in this study, the LHI was calculated in one catchment (Jalad) and the same index for a different class of causative factors was applied for another catchment (Charnath), and the LHI wasverified. The verification results were very promising, with an independent prediction rate of about 75%. This validates weights-of-evidence and logistic regression models for landslide hazard assessment in the Siwaliks Range of Nepal.

Areal distribution of large-scale landslides along highway corridors in central Nepal

Landslides are the most frequent natural disaster in Nepal. As such, the scientific study of the Nepal landslides has been in progress for several years, but no significant achievement has been made in preventing landslides and mitigating disaster damage yet. As one important aspect of dealing with landslides is understanding their distribution pattern based on geological and geomorphological formations, this paper addresses these issues, and attempts to present a scenario of large-scale landslide distribution along the road corridors of major highways in central Nepal. As a result of landslide mapping using aerial photographs, topographical maps and field verification, the following points were understood: (1) the distribution of large-scale landslides is relatively dense over the area close to tectonic thrusts; (2) slate and phyllite zones have a greater ratio of landslide distribution; and (3) topographies with a mean slope angle of about 27–36 degrees have denser distribution of large-scale landslides.

Initiatives for earthquake disaster risk management in the Kathmandu Valley

Situated over the Himalayan tectonic zone, Kathmandu Valley as a lake in geological past has a long history of destructive earthquakes. In recent years, the earthquake risk of the valley has significantly increased due mainly to uncontrolled development, poor construction practices with no earthquake safety consideration, and lack of awareness among the general public and government authorities. Implementation of land use plan and building codes, strengthening of design and construction regulations, relocation of communities in risky areas, and conduction of public awareness programs are suitable means of earthquake disaster risk management practice. Kathmandu, the capital of Nepal, is still lacking earthquake disaster risk management plans. So, this paper highlights some initiatives adopted by both governmental and nongovernmental organizations of Nepal to manage earthquake disaster risk in the Kathmandu Valley. It provides some comprehensive information on recent initiatives of earthquake disaster risk management in the valley and also highlights the outcomes and challenges.

Topo-stress based probabilistic model for shallow landslide susceptibility zonation in the Nepal Himalaya

While dealing with slope stability issues, determining the state of stress and the relation between driving force and resisting force are the fundamental deterministic steps. Gravitational stresses affect geologic processes and engineering operations in slopes. Considering this fact, a concept of topo-stress evaluation is developed in this research and used to produce a shallow landslide susceptibility map in a model area. The topo-stress introduced in this research refers to the shear stress induced by the gravitational forces on the planes parallel to the ground surface. Weight of the material on a slope and friction angle of the jointed rock mass are the two fundamental parameters that are considered to govern topo-stress in this study. Considering topo-stress as a main factor for initiating shallow landslides, a GIS-based probabilistic model is developed for shallow landslide susceptibility zonation. An ideal terrain in central Nepal is selected as the study area for this purpose. Two event-based shallow landslide inventories are used to predict accuracy of the model, which is found to be more than 78xa0% for the first event-landslides and more than 76xa0% for the second event-landslides. It is evident from these prediction rates that the probabilistic topo-stress model proposed in this work is quite acceptable when regional scale shallow landslide susceptibility mapping is practiced, such as in the Himalayan rocky slopes.

Rainfall event-based landslide susceptibility zonation mapping

Landslide susceptibility assessment is a major research topic in geo-disaster management. In recent days, various landslide susceptibility and landslide hazard assessment methodologies have been introduced with diverse thoughts of assessment and validation method. Fundamentally, in landslide susceptibility zonation mapping, the susceptibility predictions are generally made in terms of likelihoods and probabilities. An overview of landslide susceptibility zoning practices in the last few years reveals that susceptibility maps have been prepared to have different accuracies and reliabilities. To address this issue, the work in this paper focuses on extreme event-based landslide susceptibility zonation mapping and its evaluation. An ideal terrain of northern Shikoku, Japan, was selected in this study for modeling and event-based landslide susceptibility mapping. Both bivariate and multivariate approaches were considered for the zonation mapping. Two event-based landslide databases were used for the susceptibility analysis, while a relatively new third event landslide database was used in validation. Different event-based susceptibility zonation maps were merged and rectified to prepare a final susceptibility zonation map, which was found to have an accuracy of more than 77xa0%. The multivariate approach was ascertained to yield a better prediction rate. From this study, it is understood that rectification of susceptibility zonation map is appropriate and reliable when multiple event-based landslide database is available for the same area. The analytical results lead to a significant understanding of improvement in bivariate and multivariate approaches as well as the success rate and prediction rate of the susceptibility maps.

Knowledge of disaster risk reduction among school students in Nepal

Because of extreme vulnerability to natural disasters, Nepal is considered a disaster hotspot in the world. For a small country with just a little less than 30-million population, the disaster statistics are always frightening. School students of Nepal are also in extreme risk of natural disasters, especially when they are in schools. In this context, a few education programmes for disaster risk reduction (DRR) have already been initiated and the results have also been already documented. However, an evaluation of the real scenario with the help of an independent research is still lacking. Therefore, this research aims to explore benefits of existing education programmes of DRR in Nepal. Altogether, 124 students from 17 districts were interviewed and various questions related to disaster information, disaster knowledge, disaster readiness, disaster awareness, disaster adaptation, and disaster risk perception were asked. Statistical analysis such as histogram analysis, distribution analysis, bivariate correlations, and independent sample t-tests were conducted to examine the relationship between students in disaster education-related programmes and the key DRR issues-related dependent variables. Findings of this independent research confirmed that initiatives taken for disaster education in Nepal are not enough and a major challenge for DRR in a school community for a country like Nepal is implementing methods, especially at the individual level. Likewise, the disaster education should not only be confined within the school students, but it must also be promoted to families and communities, which is very essential to elaborate knowledge of DRR and to contribute to a disaster safe society in the country.

Geo-Disaster and Its Mitigation in Nepal

Geologically young and tectonically active Himalayan Range is characterized by highly elevated mountains and deep river valleys. The mountainous terrains of Nepal Himalaya are characterized by dynamic physical processes, and therefore, mitigation of geo-disaster is a big challenge. A better understanding of the geological nature of the terrain and the interaction of various triggering factors of geo-disaster will greatly help in the development of safer infrastructures, mitigation of geo-disaster, and control of land degradation in the Himalaya. Over the years, Nepal has gained a significant amount of experiences in geo-disaster studies, especially in design and survey of geo-disaster mitigation programs, in the fields of hazard and risk assessment, in low cost rural road engineering; in community based river training work and in slope maintenance incorporating indigenous techniques. This chapter provides an overview of geo-disasters in Nepal and their mitigation efforts. In this chapter, all geological zones of Nepal are evaluated from the perspective of geo-disaster occurrences. Geo-disaster mitigation efforts of both governmental and non-governmental sectors are also evaluated. People’s perception for geo-disaster mitigations efforts are also discussed in this chapter.

Deterministic slope failure hazard assessment in a model catchment and its replication in neighbourhood terrain

In this work, we prepare and replicate a deterministic slope failure hazard model in small-scale catchments of tertiary sedimentary terrain of Niihama city in western Japan. It is generally difficult to replicate a deterministic model from one catchment to another due to lack of exactly similar geo-mechanical and hydrological parameters. To overcome this problem, discriminant function modelling was done with the deterministic slope failure hazard model and the DEM-based causal factors of slope failure, which yielded an empirical parametric relationship or a discriminant function equation. This parametric relationship was used to predict the slope failure hazard index in a total of 40 target catchments in the study area. From ROC plots, the prediction rate between 0.719–0.814 and 0.704–0.805 was obtained with inventories of September and October slope failures, respectively. This means September slope failures were better predicted than October slope failures by approximately 1%. The results show that the prediction of the slope failure hazard index is possible, even in a small catchment scale, in similar geophysical settings. Moreover, the replication of the deterministic model through discriminant function modelling was found to be successful in predicting typhoon rainfall-induced slope failures with moderate to good accuracy without any use of geo-mechanical and hydrological parameters.

Earthquake-Induced Landslides in the Roadside Slopes of East Nepal After Recent September 18, 2011 Earthquake

Being located in central part of the Himalayan ranges, Nepal is regarded as one of the earthquake-prone countries in the region. Earthquake is a major concern of Nepal because of rapid population growth, poor land use planning, precarious settlement patterns, and poorly implemented building code. Earthquakes in Nepal have been reported since 1255 while major earthquakes were recorded in 1408, 1681, 1810, 1833, and 1866, 1934, 1980 and 1988. Recent earthquake of September 18, 2011 measuring 6.9 in Richter scale killed 6 people and injured 30 people in Nepal. There were many roadside slope damages near the epicenter area. To assess the roadside slope damages after this earthquake, a field visit was conducted and a landslide inventory map along the roadside slope was prepared for the most problematic area. For this study, Koshi and Mechi highways of eastern Nepal were selected as study area and earthquake-induced landslides on the roadside slopes were evaluated in terms of characteristics and mode of occurrences.