Paper ID #35033

Landslide mobilized debris ow at Kalli village in Achham, : A case study

Mr. Diwakar K C, The University of Toledo

I am Diwakar K C born in Nepal. I completed my bachelor’s degree in Civil Engineering from Trib- huwan Univerity, Nepal in 2014. I completed my Master’s degree in Bridge and Tunnel Engineering from Huazhong University of Science and Technology, China in 2017. I have 3 years of experience working on Hydropower projects. Currently, I am pursuing a Ph.D. at The University of Toledo, Ohio in Civil Engineering under the supervision of Dr. Liang-Bo Hu. Mr. Harish Dangi, NEA Engineering Company Ltd

My name is Harish Dangi from , Nepal. I did my B.Sc. in Geology and M.Sc. in Engineering Geology from Tri-Chandra Campus, Tribhuvan University, Nepal. Currently, I work at NEA engineering company limited as an engineering geologist. Mr. M. Wasif Naqvi, Michigan State University Dr. Liangbo Hu, University of Toledo

c American Society for Engineering Education, 2021 Landslide mobilized debris flow at Kalli village in Achham, Nepal: A case study

Diwakar K C1, Harish Dangi2, Mohammad Wasif Naqvi3, Liang-Bo Hu4

1Graduate Research Assistant, Dept. of Civil and Environmental Engineering, University of Toledo, Toledo, OH 43606, USA. E-mail: [email protected]. 2Geologist, NEA Engineering Company Limited, Trade Tower, Thapathali, Kathmandu, Nepal. E-mail: [email protected] 3Graduate Research Assistant, Dept. of Civil and Environmental Engineering, University of Toledo, Toledo, OH 43606, USA. E-mail: [email protected] 4Associate Professor, Dept. of Civil and Environmental Engineering, University of Toledo, Toledo, OH 43606, USA. E-mail: [email protected]

Abstract: Debris flows are fast moving gravity-driven mass flows that carry significant momentum and pose a considerable threat to human lives, infrastructure and environments. With its complex geomorphology, diverse landforms, harsh climates and fragile geologic conditions, the Himalayas, one of the youngest mountain ranges with some of the highest peaks of the world, often experiences extreme mass movements along the downslopes. The present study is focused on debris flow events that are still active in Kalli village of in the lesser Himalayas of Nepal. The runout distance of large boulders in this event reached nearly 3 kilometers indicate that the area is highly vulnerable to debris flow damage. A field study was conducted to assess the causes of the landslide which mobilized the debris flow. The key material parameters, debris material distribution and rainfall patterns were analyzed. The field study shows that the major factor for the repeated landslides was the local geology which consists of inter-bedding of shale and sandstone with different deformability, competency and permeability along with the orientation of joints. Subsequently, numerical simulations were performed to evaluate the extremity of potential debris flows in this region. A multi-phase mass flow model was employed in the numerical modeling to calibrate the relevant parameters in the studied debris flow. The simulation results fairly match the field observations in terms of run out distance and deposition pattern, and demonstrate that the multi-phase mass flow model can be of promising potential for assessing and predicting future debris flow hazards in this region.

Keywords: landslide/debris flow; pore pressure; runout distance; deposition pattern; multiphase numerical model; basal friction.

Introduction

The collision of Indian plate and Eurasian plate has formed Himalayas in Asia. Active plate tectonics causes the instability to the mountains of Himalayas. Rugged topography, weak and complex geological structures, fragile soil, seismic activities, high-intensity rain render these mountains susceptible to landslide/debris flows [1]. In Nepal, which is in the central Himalayas, natural disasters and mass wasting phenomenon like landslides/ debris flows, avalanches, Glacier lake outburst flood (GLOF), mudflows, earthquake and earthquake-triggered landslides are common [2-4].

Naturally, landslides/debris flows can be triggered by water, volcanoes, earthquakes and their causes depend on underlying geology, soil type, and land use pattern [5]. Very few studies have been conducted on the mass flow phenomena in Nepal. Hasegawa et al. [6] studied the causes of large-scale landslide in the Lesser Himalayas of central Nepal. Ghimire [7] investigated landslide occurrence and its relationship with terrain in the Siwaliks, Nepal. Dhital [8] studied the debris flow in Kulekhani area and reported that the debris flow was triggered by heavy rainfall; and the other causes were geology, geomorphology, slopes, and soil type. Paudel et al. [9] examined the relationship of debris flow with rainfall in Kulekhani area. Bhandari and Dhakal [10] evaluated the debris flow initiation criteria mainly based on slope and rainfall in the Babai watershed.

In the present study a case study at Kalli village in the Achham district of Nepal is examined. The geological background and development of debris flows are discussed in detail. The field study is followed by numerical modeling to assess the extremity of potential debris flows in this region. Recent development in various numerical models [11~16] have given rise to the possibility of quantifying the phyiscal and mechanical process of debris flows. In the present study a multi-phase model developed by Pudasaini and Mergili [16] is employed.

Study area

The study area is in the Achham district of Nepal (Figure 1), which is in the Lesser Himalayas. The coordinates of landslide crown which is at the lower part of Kalli village, are 28.93° N and 81.34° E. The annual mean rainfall in the area is around 1400 mm of which major portion (around 80%) occurs between June to September. Due to high-relief topography, after heavy rain the ephemeral channels contribute to the Karnali river with flood water and sometimes with debris, which is the main drainage that passes through the area. Highly rugged topography, deep ravines, mountain peaks, deep narrow river valley are the typical topographical features of the area. The elevation of the Kalli village is around 950 meters from the sea level. The elevation of Karnali river at the location where the debris moves from the study area to Karnali river is around 370 meters from mean sea level.

Figure 1. Study area in the map of Nepal

Methodology

The present study is mainly focused on field study and numerical simulation to evaluate the potential debris flows in this region.

Field study

After the collection of the required data and desk study, field study was performed with reconnaissance survey to acquire the overall concept on landslide and debris flow of the study area. The field traverse around the landslide and debris runout area helped to visualize the present status of the debris flow and landslide along with the engineering geological and geotechnical condition of the study area. Field work included collection of data on lithological variation, details of exposed discontinuities and geotechnical property of rock around the landslide area.

Numerical simulation

Numerical simulation is conducted in r.avaflow2.1 software which is based on the multiphase mass flow model proposed by Pudasaini and Mergili [16]. It considers three phases, i.e., solid, fine solid and fluid. The solid is modeled with shear-rate-independent Mohr-Coulomb plastic rheology; the fine solid with a shear and pressure-dependent Coulomb-viscoelastic rheology and the fluid with shear-rate-dependent Herschel-Bulkley rheology.

For numerical simulation digital elevation model of the terrain is necessary and is downloaded from the website of Alaska satellite facility [17]. The background image is taken from google map and is georeferenced using GIS. There is no precise data available on the initial volume of debris, it is roughly estimated from the previous landslide in the area where the local residents provided some relevant information. The volume of solid, fine solid and fluid are considered 64,872m3, 27,540 m3 and 92,412 m3, respectively.

The density of solid, fine solid and fluid are assigned as 2650 kgm-3, 2000 kgm-3 and 1000 kgm-3. The internal and basal angle of friction for solid is assigned 25 and 8; for fine solid is 10 and 4. The kinematic viscosity for fine solid and fluid phase are assigned 102 m2/s and 10-3 m2/s, respectively.

Results

Findings of field study

The studied Kalli landslide and debris flow areas lie within the Suntar Formation of the lesser Himalaya comprising fine to medium grained green gray sandstone alternatively with purple shale. Several micro fold with differential weathering of Shale and sandstone is observed around the crown and body part of the landslide. These two major rocks forming the interbedding at landslide area have different physical and mechanical properties.

The bedding of the rock gently dips about 20-25 degrees toward South-West with two major joints. These joints are almost vertical, joint dipping 70-80 degree towards North (J2), and joint dipping 80-85 degrees towards East (J2). The kinematic analysis with the acquired data shows minimal slope failure which is shown in Figure 2.

Figure 2. Kinematic analysis of rock joints

This mass wasting included several ground movement phenomena. The upper part, with almost vertical slope, represents rock fall with deep slope failure. Other smaller landslides were on the wings of the major landslides. From the middle part of the landslide to the toe, there was a shallow debris deposit. The debris flow alignment from the toe of the landslide to the Karnali River showed the rhythmic material distribution with fine to coarse material deposition in sequence as shown in Figure 3.

Figure 3. Debris deposits close to the toe of the landslide (left) and two kilometers away from the landslide toe. The boulder encircled by the circle at the lower right corner of the left image is around 150 mm. The red circle on the right picture encloses a handheld GPS. Tension cracks were present at the crown part the landslide (Figure 4) which shows that the landslide was an active landslide and further suggest its susceptibility to landslide hazard during monsoon season.

Figure 4. Cracks around the crown of the landslide. The red circle on the left photograph encloses a cellphone and the red circle on the right encloses a man as a scale. Water springs were found at the toe of landslide which represented the accumulation and seepage of the water from the flat land above the crown part. Basically, with the property of variable permeability and persistency of the shale and sandstone, such seepage was a promising factor for increase in pore pressure and weathering which ultimately resulted in slope failure.

Simulation results

In the current study the major objective of numerical simulation is to examine the runout distance and deposition pattern. The map showing the run-out distance and deposition obtained from the numerical simulation is presented in Figure 5.

Figure 5. Final deposition map The deposition obtained from the simulation closely resembles the actual field observation. It shows that large amount of debris is deposited in the river and along the bank. The deposition depth can be estimated from the contours in Figure 5. The outermost contour represents 0.001 m depth of deposition while the innermost represents the maximum, i.e. 2.38 m depth of deposition as shown in the legends on the right under “Change of topography”. The negative signs in the legends means deposition.

The wide debris flow-fan at O3 (shown by dashed line) in Figure 5 used to be paddy field before the 1983 debris flow event. The Alluvial fan seen in the numerical simulation fairly matches the field condition; the fan obtained from the numerical simulation is slightly narrower than the actual fan. This may be due to the change in elevation and roughness of the area before and after debris flow event. The deposition along the debris flow channel (Dogade stream) and along the bank of Karnali river are plotted in Figure 6 and Figure 7 respectively.

650 Elevation 600 Elevation + Deposition 550

500

Elevation (m) Elevation 450

400

350 0 500 1000 1500 2000 2500 3000 Flow distance (m) Figure 6. Elevation profile and deposition along the debris flow channel (Dogade stream)

374 Elevation 372 Elevation +Deposition

370

368 Elevation Elevation (m) 366

364 0 50 100 150 200 250 300 350 Distance (m) Figure 7. Elevation and deposition along the Karnali river bank The debris deposition depth along the Dogade stream seems negligible as shown in Figure 6, because of relatively very high elevation of the stream bed compared to the depositional depth.

Conclusions

The major cause of landslide which mobilize debris flow is poor geological conditions along with the topography. As the landslide area is dominated by inter-bedding of Sandstone and Shale, difference in persistency, permeability is expected. This cause instability and ultimately results deep slope failure along with the tension cracks. The seepage above the crown part of the landslide also plays a significant role to slope failure in these rock types. In addition, it is also found that the debris flow is particularly active during rainy season; therefore, rainfall is one of the major triggering factors of landslide/debris flows in the region. There can be many influencing and triggering agents that make the Himalayas in Nepal prone to landslides and debris flows.

Further study is needed for the determination of physical properties of rocks and soil in the area. The strength, porosity, hydraulic conductivity, including petrographic analysis can be useful to determine the types of rocks or soil which are susceptible to landslide/debris flows. The area is in active seismic zone, the seismicity also should have played role in landslides/debris flows in the area.

The deposition and runout distance obtained from the numerical simulation matches the field conditions at lower value of internal angle friction, 25 and 8 for solid and fine solid respectively and very low basal friction angle, 10 and 4 for solid and fine solid respectively, where the debris flows to Karnali river in single stage. In the real field the debris flow has occurred in two major stages. In the first stage the landslide occurs. The debris from the landslide deposit very close to the landslide toe which has limited fluidity. In the next stage, with addition of water the debris flows down to Karnali river. Apart from these two major stages, debris is also transported in very diluted form with the water current. The change in the internal angle of friction and basal friction angles in these three stages, and mechanism involved shall be analyzed in further study.

Acknowledgment

The authors wish to thank Mr. Prithivi Bir Thapa and Ms. Sushma Kadel for their assistance during the field study.

References

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