Journal of Nepal Geological Society

Palynological study along the Triyuga River section from the Upper Siwalik sediment and its paleoclimatic implication

Rabin Dhakal — 2022-08-30

The records of past vegetation and climate using palynological study from the Upper Siwalik sediment along the Triyuga River section, eastern Nepal discloses 117 taxa belonging to 36 families. Different palynomorphs from gymnosperm, angiosperm (dicotyledonous and monocotyledonous) and pteridophytes were identified some up to species level like Abies, Picea, Pinus, and Tsuga, Justicia, Strobilanthes, Betula, Terminalia, Artemisia, Corylus, Quercus, Liquidambar, Fraxinus, Salix, Symplocos, Urtica, Typha, Cyathea, Lygodium, Ceratopteris under light microscope. Some palynomorphs were identified up to their family level such as Anacardiaceae, Apocynaceae, Bombacaceae, Leguminosae, Malvaceae, Arecaceae, Liliaceae, and Poaceae. The palynomorphs from this section revealed diversity in vegetation with tropical-subtropical to lower temperate humid and warm climatical condition with plenty of precipitation throughout the time of deposition of the Upper Siwalik. Additionally, the Coexistence Approach (CoA) further highlighted this section experienced at the MAT 16.8-21.7°C, WMT 23.6-26.1°C, CMT 10.6-14.6°C, and MAP 1122-1682 mm, HMP 115-349 mm, LMP 19-73 mm, WMP 82-172 mm.

Durability of sandstones from Sub-Himalaya of central Nepal

Aadesh Budhathoki — 2022-08-30

The Sub-Himalaya of Nepal experiences sub-tropical climate where geologically young and delicate sedimentary rocks are prone to chemical and physical weathering and consequent erosion. The main aim of this study is to identify and evaluate the durability characteristics of some sandstones against slaking, freezing-thawing and abrasion. Lithosomes were characterized in the field. The following laboratory tests included measurements of dry density, specific gravity, water absorption, Slake Durability Index (SDI), Sulphate Soundness (SS), and Los Angles Abrasion (LAA). Sandstones from the Lower Siwalik Subgroup (LSS) are massive to cross-stratified, very fine- to medium-grained, and occasionally calcareous, whereas the sandstone from the Middle Siwalik Subgroup (MSS) are mostly cross-stratified to few massive, medium- to coarse-grained with salt-and-pepper appearance, and uncommonly calcareous. Sandstones have four different types of deterioration and had high to extremely high SDI. The majority of the sandstones have displayed similar slaking tendencies. Under the five-cycle SS test, the majority of sandstones from LSS and MSS have experienced low final weight loss (below 10%). However, two from LSS and three from MSS have experienced greater final weight loss. The LAA values range from 29.66% to 99.14%, and except two sandstones from LSS, rest of the samples have exceeded 45% abrasion showing they were incompetent in terms of abrasion test, and are highly susceptible to abrasion. The uniformity factor ranges from 0.21 to 0.44 indicating that all the sandstones were of non-uniform hardness. Correlation among physical parameter and durability indices was weak to moderate probably because of varied nature of sandstones.

Rock slope deformation analysis of the powerhouse cut-slope at Middle Bhotekoshi Hydroelectric Project in Sindhupalchok District, Central Nepal

Subarna Dhakal — 2022-08-30

This paper elaborates the rock slope deformation analysis in a complex rock mass i.e., metamorphic rocks. At the powerhouse of the Middle Bhotekoshi Hydroelectric Project, Sindhupalchok, Nepal, the rock slope deformation has occurred as a result of excavation of natural rock slope turning into the cut-slope. The deformation yielded by different causes were studied and analyzed by kinematic analysis, inclinometer data combining with numerical modeling. For the study and analysis of the cut-slope, geotechnical data were collected from the existing rock types of the cut-slope. At first, the kinematic analysis was carried out by using the stereonet plot in DIPS. The core logs of investigation time were reviewed and tried to prepare the geological model of this part. The finite element method was used to analyze the stability of the rock cut-slope based on the geotechnical data. Similarly, the inclinometer recorded data were used to compare with the analyzed deformation data and tried to find out the weak zone within the cut-slope. The kinematic analysis showed that the rock slope has the least probability of failure. The result showed that there was tolerable deformation on the slope based on inclinometer data but failure was observed at PSIN-1 where the weak zone is present. The deformation at PSIN-1 was found to be in the thin ductile deformed zones where rock characteristics were different which was considered not operative for the slope stability.

Landslide susceptibility assessment in the Rangun Khola watershed of far western Nepal

Lalit Pathak — 2022-08-30

Considering the serious threat of landslides to life, property, and the environment, this study aimed at exploring past landslides (2005-2020) to evaluate landslide susceptibility. The study is carried out in the Rangun Khola watershed, in western Nepal covering an area of 488 km². The landslide inventory map was prepared, recognizing 494 landslides, among them 70% were used for susceptibility mapping, and the rest 30% for validation purposes. The size of the landslide was found in the range of 103.53 m² to 149120.1 m², with an average of 4677.35 m². Frequency ratio (FR) and logistic regression (LR) models were implemented for landslide susceptibility assessment based on the various intrinsic factors. The validity of the models was assessed by using receiver operating characteristic (ROC) curves. The success rate was 87.6% for the LR model with a prediction rate of 87.3% indicating a good degree of fit. Similarly, with a success rate of 76.4% and a prediction rate of 75.1%, the result obtained from the FR model was a fairly good performance. Thus, both exhibited reasonably good accuracy in predicting the susceptibility of the landslide and are considered to be in land management and hazard mitigation, and policy formulating purposes.

Rock mass characterization and support analysis of pressurized headrace tunnel of the Upper Balephi “A” Hydroelectric Project, Nepal

Kanchan Chaulagai — 2022-08-30

Detailed geological, engineering geological and geotechnical assessment are prerequisites for any design works in underground excavation. Due to complex geology in young Himalayan, it is mandatory. Any misjudgment in support design may results huge losses of cost and time of the project. This paper encompasses detailed analysis carried out for examining the rock mass properties for optimum support along a pressurized headrace tunnel of Upper Balephi “A” Hydroelectric Project, Nepal. Empirical, analytical and numerical methods were applied for safe tunnel design. The rock mass quality and support in these areas were estimated using the rock mass rating (RMR), geological strength index (GSI) and rock mass quality (Q) systems. The detailed rock engineering assessment indicated that there are some critical locations along the headrace tunnel alignment. Rock mass quality values derived from different methods were used for calculating modulus of deformation, Hoek-Brown constants, strength of rock mass, in situ stresses, squeezing and support pressure using available empirical equations. The support determined from the empirical methods were evaluated for the overall stability of the required excavation by using finite element method. The analysis showed that the support pressure and deformation can be predicted very well and magnitude of the displacements and extent of the plastic zones can be reduced significantly by application of the support installation. The numerical modelling reveals that the support suggested by empirical methods are appropriate. Both empirical and numerical approaches are necessary for the confirmation of reliable support design of underground structure.

Marginal fan thick lacustrine sedimentation: Implication for tectonics and/or climatic causes in Kathmandu basin

Mukunda Raj Paudel — 2022-08-30

Very thick gravel deposits are widely distributed from southern marginal to central parts of the Kathmandu basin named as Itaiti Formation. This deposit is more than 120 m thick and deposited around 1 Ma, which is dominated by three types of facies and fans. The first stage fan is widely distributed near the mountain front where sediments aggradation is more active. The third stage fan is the youngest among three fans and is distributed on the present top surface and has covered lacustrine deltaic sequence of the Sunakothi Formation. Seven types of facies elements namely Gms, Gm, Gp, Sp, Sr, Sh, and Fl have been recognized among these three fans. Based on stratigraphic relationship among different geological formations, the first debris flow fans have been spreading over basement rocks and fluvial gravel facies before 1 Ma. This event has played a vital role in the initiation of an ancient lake in the Kathmandu basin. The second and third stage fans originated during the draining stages of the lake. The distal part of the second stage fan and third stage fan is interfingering with the lacustrine delta towards the center of the basin. The first stage debris flow fan was most probably generated by the recent uplift of the Mahabharat range. These gravelly fan deposits and the uppermost Siwalik conglomerate deposits may be synchronous to recent tectonic events throughout the Himalayas. The second and third stage fans have been initiated not only by the tectonics but also by climatic which is indicated by sedimentological and stratigraphic relations among formations.

Qualitative rock-fall hazard mapping around the Siddhababa area along the Siddhartha Highway in western Nepal

Chhabilal Pokhrel — 2022-08-30

The Butwal-Dobhan road section of the Siddhartha Highway is highly hazardous and problematic section due to rock-fall. In this road section, the Siwalik rocks are exposed that contains inter-bedding of hard and soft rocks. The differential weathering pattern of hard rock sandstone and soft rock mudstone exhibit different properties in the presence of water. Mudstone can easily be softened by water and flow down as slurries when saturated, whereas sandstone cannot easily be softened in water resulting overhanging of sandstone and finally dislodges rock blocks as a rock-fall. The main aims of this study are to investigate and understand the causes of rock-fall, to prepare rock-fall hazard map and ultimately to protect people, properties, man-made structures and to spread public awareness against rock-fall problem. For these set objectives, the study area is divided into twenty-three zones based on segment length along the highway, topography and orientation of discontinuities. Two bases are used to collect field data; one is the rock classification basis viz. rock quality designation, rock mass rating, geological strength index, and slope mass rating, and another basis is kinematic analysis viz. planar sliding, planar sliding no limit, wedge sliding, flexural toppling and direct toppling. The collected data are rated and prepared individual susceptible values. Finally, combining the entire individual’s susceptible values along with topographic mosaic, final hazard map was prepared where five different hazard zones have been identified and are categorized into very low, low, medium, high and very high hazard zones. Results from the study could be used as the preliminary study for hazard assessment and land use planning, and to disseminate public awareness.

Lithostratigraphy and metamorphism of the Khudi-Tal area, west central Nepal

Niraj Singh Thakuri — 2022-08-30

The study is mainly focused on lithostratigraphy, petrography and geological structures to decipher the metamorphic facies and metamorphic history of the Khudi-Tal area along the Marsyangdi Valley, west central Nepal. Geological mapping was carried out covering 142 sq.km, on the scale of 1:50000. The Main Central Thrust (MCT) separates lithological succession of the Lesser Himalaya and Higher Himalaya around Bahundanda, Probhi and Naiche of the Khudi-Tal area. Lithological succession is dominated by metapelitic to psammatic schist, siliceous to dolomitic marble, quartzites, graphitic to garnet schist, kyanite garnet gneiss and banded gneiss from south to north. The kyanite isograd follow the MCT separating Kyanite zone with mineral assemblages Ky+Grt+Qtz+Ms+Bt+Pl of the Higher Himalaya and Garnet zone as Grt+Bt+Ms+Chl+K-Fel+Pl+Qtz in pelitic rocks, Bt+Ms+Chl+Plag+Qtz in Psammatic rocks and Cal/ Dol+Qtz+Pl+Bt+Ms in calcareous rock of the Lesser Himalaya. Based on the mineral assemblages, the Higher Himalaya belongs to Amphibolite Facies whereas the Lesser Himalaya belongs to Epidote-Amphibolite Facies. The lithological succession of the Higher and the Lesser Himalaya has undergone poly-phase metamorphism. In the Lesser Himalayan zone rocks, the regional burial metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type garnet grade inverted metamorphism M2 and later post-MCT retrogressive metamorphism M3. In the Higher Himalaya rocks, the early phase kyanite bearing medium temperature/ high pressure regional prograde metamorphism M1 (pre-MCT/ eo-Himalayan) is followed by barrovian type inverted metamorphism M3 (syn-MCT/ neo-Himalayan) and over printed by later phase retrogressive metamorphism M3(post-MCT).

Landslide susceptibility mapping of the Main Boundary Thrust (MBT) region in Tinau-Mathagadhi Section of Palpa District, Lumbini Province

Kabi Raj Paudyal — 2022-08-30

The study was carried out in Tinau-Mathagadhi section of Palpa District, Lumbini Province. The output of the landslide susceptibility analysis using frequency ratio (FR) is evaluated. Google Earth (CNES/Airbus and Maxar Technologies) Imagery of 50 cm spatial resolution was used to detect landslides. Data training and testing sampling was created using the landslide inventory. Eight causative factors were derived from topographic, geological, and land-use maps. The causative factors and training events were used to determine the FR ratings. The objective of present study is to address the effect of the Main Boundary Thrust (MBT) along with other important parameters in landslide susceptibility mapping. The integration of causative factors that assigned FR scores yielded the landslide susceptibility map. The validation of 79.6% was obtained using the ROC-AUC curve. Among eight causative factors, it was found that distance from the Main Boundary Thrust (MBT), aspect, land-use, and geology are dominating factors in the occurrence of the landslide. The FR method is based on a quantitative relationship between landslide inventory and landslide affecting factors. The method is valid for Tinau-Mathagadhi section and MBT is one of the causes of landslides in this area.

Characterization of large-scale landslides and their susceptibility evaluation in central Nepal Himalaya

Bikash Phuyal — 2022-08-30

Large-scale landslides (LSL) are characterized by complex nature of failure mechanism, which depend on geological setting and associated factors of the area. The aim of this research is to identify the distribution pattern of LSL and all landslides in the central Nepal Himalaya and evaluation of their controlling factors. 7239 landslides were extracted from the study area by the interpretation of satellite imageries and field surveyed information. 28 landslides were classified as LSL and descriptive statistics were calculated. A comparative susceptibility assessment between all landslides and LSL was performed by frequency ratio model (FRM). Landslide susceptibility assessed from FRM was classified into five categories using the natural breaks method and adjustment from field evidences: very low, low, medium, high, and very high. The very high, high and medium susceptibility classes comprised of 38.91%, 33.29%, 18.76% for all landslides, and 39.51%, 29.65%, 20.98% for LSL. The result clearly indicated that the role of controlling factors varies differently depending upon the size of distributed landslides. To understand the significance of controlling factors for LSL, different potential cases were validated by success rates with area under the curve (AUC). The computed AUC in success rates for LSL is 65% and for overall landslides with similar controlling factors is 75%. The AUC values in different potential cases showed that the prime factors to control the LSL are geomorphology, rainfall, and geological structures.

Hydro-geochemical characterization and suitability analysis of spring water of the Mai Khola Watershed, Ilam, eastern Nepal

Champak Babu Silwal — 2022-08-30

The interaction of different physical and chemical factors as well as anthropogenic activities influences the chemistry of groundwater. The hydro-geochemical assessment of the groundwater is the basic part of the water resources management and its assessment in regards to its usability is considered vital. The study involves the measurement of physicochemical parameters (pH, EC, EH, DO, TDS) of the observed 147 springs and the ionic concentration of the representative 32 springs along the Mai Khola Watershed using standard analytical procedure. The results of the physicochemical parameters and ionic concentration were analysed and interpreted using different indices and graphical methods. The physicochemical parameter suggests that spring water is weakly acidic to alkaline and the measured EC and TDS suggest low interaction of rocks and water therein. The piper diagram indicates Ca²⁺ and Mg²⁺ together (alkaline earth metals) dominate over combined Na⁺ and K⁺ (alkali metals) and Cl^- and SO₄^2- (strong acids) predominate over HCO₃^-(weak ones). The Gibbs plot also satisfies the movement of groundwater from the precipitation domain to the rock-water interaction domain. The groundwater in this area is derived from a shallow aquifer with low rock-water interaction and the water is good for drinking purpose and excellent to good for irrigation. The spring water in the study area possesses no threat to quality deterioration, however proper management and conservation plans are required to maintain the quality of the water.