Evolutional Characteristics of Debris Flow in the Siwalik Hills of Nepal

As a youngest Mountain of the world, Siwalik region of Nepal is facing sever-al mass movements like landslide and debris flow in monsoon period every year. Debris flow is very common in the steep slope of weak and fragile sedimentary rocks. This paper used remote sensing data and GIS to evaluate evolutional characteristics of debris flow hazard in the Siwalik hill of Babai watershed based on the geological, topographical and hydrological factors. All together 101 debris flow polygons were made by using Google Earth and by field verification. Digital Elevation Model (DEM) was used to analyze debris flow distribution and topographical features. Lithostratigraphy was studied to evaluate geological characters and rainfall data was used to evaluate hydrological character. Average slope for debris flow evolution in Lower, Middle and Upper Siwalik is 34˚, 50˚ and 30˚ respectively. The average 24 hours rainfall to occur debris flow is found 160.67 mm. The area and length of debris flow channel are significantly different in three geological formations. The temporal distribution of debris flow from 2001 to 2018 shows that the trend of debris flow generation is higher in Middle Siwalik rocks with slope greater than 30˚. Three major sources of debris flows were evaluated namely slide induced debris flow, fall induced debris flow and erosion induced debris flow where the slide induced debris flow is predominant. This study can be better source to understand the general mechanism of debris flow generation to the policy makers for reducing the future impact of debris flow in the overall Siwalik zone of Nepal.

In addition, there is a specified curvature in topography in debris flow source area. Due to those regions, the debris flow erodes the heavy materials from the hill slope or gulley and deposits a huge amount of sediments. During that mechanism, it destroys properties and life every year in the mountainous region of Nepal during rainy season. The primary triggering factor for debris flow generation is intense rainfall which leads to reduced effective stress and thus slope instability. Every recorded debris flow event in Nepal has coincided with intense or long duration rainfall.
The Himalaya is geologically very young and tectonically most unstable mountain landscapes of the world [9]. These landscapes are vulnerable to various natural hazards, including landslides, debris flows and soil erosion primarily triggered by extensive rainfall of monsoon [10]. Debris flows are major landslide problems in Nepal which are initiated at the gullies of hill tops in small catchments and flow down with the high velocity [11]. The Siwalik region of Nepal has been suffering by landslides and debris flows since the past. The Siwalik consists of loose and fragile sedimentary rocks and quaternary fluvial deposit erodes out with a large amount of sediments and transported in the time of intense rainfall. Most of the Siwalik hill consists of steep slope morphology and mobilizes a lot of loose sediments under the influence of surface runoff [12]. The debris flows triggered by heavy rainfall in 2016 and 2018 have received much attention in the west Siwalik zone of Nepal. The large scale debris flow in different events of 2016 and 2018 caused large flooding and damming in the western Terai region of Nepal. In Nepal, many people are living under the hills and mountain with agricultural practices. The peoples are unknown about the potential area for landslide and debris flow. Due to sudden and unexpectedly generated mechanism, the debris flow often causes life and properties loss in the hilly and Himalayan region of Nepal. To identify the mechanism and generation of debris flow in the hilly and mountainous region is very important task for sustainable life and development in the country like Nepal. Finding the topographical, geological and hydrological factors for the evolution of debris flow in the Siwalik hills is the main purpose of this study.
The sediments for the debris flow may be generated from landslide and erosion on the slope [13]. Scientific investigation and qualitative discussion of the definition of debris flow is very important task for present. [14] studied about the role of debris flow on the change of 10Be concentration in the Seti River of Nepal. [15] studied the landslide and debris flow of the Madi River of Nepal and they reported that heavy rainfall is the cause of large landslide and debris flow in the Nepal Himalaya. [16] studied the debris flow of Bhotekoshi River valley of Nepal. [17] studied about the debris flood of Seti River of Nepal and elaborated International Journal of Geosciences the mechanism of glacial lake outburst flood (GLOF) and resulting debris flood in the glacial lake originated River. [13] reported that the probability of debris flow to occur increases with increase of topographic factors in the thrust zone at weak sedimentary terrain. They concluded that the topographical and geological factors are mainly responsible for gully-type debris flow in the Siwalik region. There is limitation of study of debris flow in the study area mostly in Siwalik region. This study aims to fulfill the data gap of study area related to debris flow in terms of topographical, geological and hydrological factors.
Based on the above considerations, the paper aims to identify the status and mechanism of debris flow evolution in the catchment scale of Siwalik hill of Nepal. The results will give the characteristics of debris flow and information of debris flow evolution in the Babai River watershed of Siwalik hills of Nepal. This work is very important for present and future to understand the generation and mechanism of debris flow in Siwalik hills though it is in catchment scale but it will be the reference and basement for the further study about landslide and debris flow. This study may be useful to compare the result with other global sedimentary terrains.
The Siwalik in the study area has altitudes ranging from 300 m to 1500 m and exhibits a very rugged topography with highly dissected gullies and steep slopes. Most of the mountain ridges in the study area extend in the east-west direction, parallel to the main geological structures. The landforms of the study area are mainly controlled by the tectonic processes and subordinately by mass wasting and weathering. The erosional landforms predominate over the depositional ones. There are rugged hills, numerous deep gorges, steep slopes, cliffs, and active gullies representing the erosional landforms, whereas river terraces, alluvial The climate of study area is sub tropical. The winter is very cold and summer is very hot. The rainfall is heavy and occasionally intense during rainy season.
The average annual rainfall over the period 1986-2017 is in the range of 1400 -3000 mm. More than 80 percent of rainfall occurs in rainy season.

Geological Setting
The bed rock geology of study area is dominated by younger (Neogene) sedimentary rocks. Rocks of study area are divided into three formations [18]. Mudstone bed is medium to thickly bedded, highly fractured and highly weathered. Sandstone bed is of thinly bedded and spheroidically weathered [18]. The  minerals present in sandstone are quartz, muscovite, biotite and feldspar. Lower Siwalik composed of siltstone, red shale facies with minor sandstone and pseudo conglomerate [19]. Middle Siwalik consists of medium to coarse grained thickly bedded salt and pepper as well as pebbly sandstone and variegated mudstone. Sandstone beds are massive, less compact and weathered due to poor intergranular bonding [18]. The sandstone of Middle Siwalik consists of quartz, feldspar, muscovite and biotite. The bed of sandstone is thicker than mudstone. The thickness of sandstone bed varies from 30 cm to 3.4 m whereas thickness of mudstone bed varies from 7 cm to 52 cm. Upper Siwalik consists of cobble and pebble bearing conglomerate interbedded with yellow to brown mudstone. The cementing material of conglomerate is calcite and mostly clay. The size of clast in conglomerate varies from 5 mm to 10 cm. Most of the flat lying area is covered by superficial quaternary deposits. Study area consists of four major thrusts namely Main Central Thrust (MBT), Babai Thrust (BT), Bheri Thrust (BhT) and Malai Thrust (MT). Main Boundary Thrust separates the study area with Lesser Himalaya. Babai Thrust and Bheri Thrust are considered as active thrust of the study area.
In hilly terrain, colluviums including debris-flow and other slope debris deposits, mostly of late-quaternary period. Colluviums occur as relatively thick deposits filling drainage courses. However, there are deposits which are considerably thinner and of greater areal extent on some hill slopes in the study area. The colluviums derived from erosion and landslide typically consists of sub angular cobbles and boulders of sandstone and mudstone with some conglomeratic fragments. Huge amount of alluvial deposits occur in river terrace and valley fill.

Debris flow inventory
The debris flow inventory map was prepared by using polygon based method in the satellite images. The time series Google earth images and images of Alaska International Journal of Geosciences Satellite facility were used for inventory. The polygons were drawn from scar to the fan by following flow channel. The KML file of inventory was obtained the Q-GIS, freely available software. The total area covered by flow, length of flow and perimeter of flow were identified from GIS. The prepared inventory was finalized by intense field study. The major sources of debris flow, nature of flow, types of movements were identified in the field study. The spatial and temporal inventory was prepared from 2001 to 2018.
Geological mapping Geological mapping was carried out in the field. Topo-sheets of 1:25,000 prepared by department of survey, Nepal were used to prepared geological map. The strike, dip and dip amount of bed rock was measured in the field and plotted in the Topo map. The study area is divided in the three formations on the basis of Lithology. After draft mapping, the topo map was scanned and digitized in the GIS. The digitized map was re-drawn and finally prepared the Geological map.
Topographical factors Topographic map at scale 1:25,000 provided was obtained from the department of survey, government of Nepal. The digital elevation model (DEM) with 30 m of spatial resolution was obtained from contour map provided by department of survey, Government of Nepal. The slope, aspect and elevation were identified from DEM.
Hydrological factor Rainfall data of 2001 to 2018 was taken from Department of Hydrology and Meteorology, Government of Nepal. Maximum 24 hours rainfall and total annual rainfall was obtained. Field work In this work, researcher spent most of the time in the field and did evaluation about different causative factors for debris flow evolution. Elevation, slope angle and average depth of gulley were measured in the field. The debris materials were observed and assumptions were made. The types of flow, origin of flow and evolution mechanism were studied in the field.
Data management The data were arranged and evaluated in the excel sheet. R software was used to analyze the data. The area and length of debris flow were evaluated with geological formations. The frequency and cumulative frequency of debris flow in different geological units were evaluated in MS Excel and R software. The different of area and length of debris flow and slope of debris flow origin were identified in different geological formations. One way analysis (One way ANOVA) is used to test the differences of debris flow among the three geological formations; box plots are prepared to visualize the information.
On the basis of field data, three types of debris flow sources are categorized namely slide, flow and fall. On the basis of source, three types of debris flow are categorized namely slide induced debris flow, fall induced debris flow and erosion induced debris flow.

Inventory of Debris Flow
All together 101 debris flow events were studied in the field (Figure 3). The total area occupied by debris flow is 19.249 km 2 among 1157 km 2 total study area. The debris flows are found to be evolved after the material deposited by landslide at the hill slope and continuous erosion of slope materials by rainy water. The origin of debris flows are classified into three groups namely fall, slide and erosion.
The slide is found higher in number among the other types of movement. Slides are found to be the main source of debris flow (68.3%) followed by erosion (22.7%) and fall (9%).    The result shows that the temporal distribution of debris flow is higher in

Source of Debris Flow
The

Topographical Distribution
More than 40% area of Middle Siwalik has steep slope having inclination greater than 30˚ whereas only 15% area of Lower and Upper Siwalik is greater than 30˚. There are topographical differences in different Siwalik hills. Due to that the rock falls and rock slides are common in Middle Siwalik, and the slide/fall induced debris flows are greater in number. Slope of hill is found one of the major causes for debris flow. Figure 9 shows that the landslides are originated in the steep slope and deposit the sediment at the concave slope and valley of different geological unit of Siwalik zone. The deposited materials flow down with the running water during rainfall. The mixture of water and plastic deposits flow down with high speed, and travels long distance to the downhill (Figure 9    categorized the slope into four categories. The minimum slope is less than 15˚ and maximum is greater than 45˚. More than 95% landslides are found above the slope 30˚. The trend of debris flow from 2000 to 2018 is shown in Figure 14.
Fall and slide induced debris flows are shown in Figure 15 and Figure 16.

Rainfall and Debris Flow
The    From the above result, the rainfall, geology and topography are found major responsible factors for debris flow generation in the study area. As many previous researchers, the Siwalik sediments and bed rocks are found very weak, weathered and fragile. Most of the slope of Siwalik zone are steep and having thick bed rock. In many places, the bed rocks are found covered by colluviums and Quaternary deposit. The weak sedimentary rocks like sandstone, mudstone, shale and conglomerate are easily erodible and most potential for landslides and debris flow. [20] concluded that 20˚ slope is enough to generate debris flow from the colluviums deposits. Our result showed that 30˚ slope is the critical slope whereas the higher number of debris flow events has occurred in the slope greater than 30˚. [3] studied about the debris flow evolution in Japan and concluded that the landslide block transformation is major cause of debris flow. Our result shows that most of debris flows are generated from landslide mass in the slope greater than 30˚. [10] studied about the rainfall induced landslide and de-

Conclusion
This paper analyzed the topographical, geological and hydrological information for the generation of debris flow in Siwalik of Babai River watershed, Nepal. The qualitative and quantitative characteristics revealed by rainfall, geology and topography are described. The rock characteristics, slope, topographic features and rainfall have played to generate the debris flow from landslide, erosion and flood in the study area. We found that the slope greater than 30˚ is prone to landslide,

Data Availability
The Google images were found freely from the free online Google earth image.
The references were found from Google Scholar. Topographical maps of study area were found from the Department of Survey, Government of Nepal.