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Morphological Variation of the West Rapti River, Nepal

  • August 2022
DOI:10.13140/RG.2.2.16368.12801/1
Authors:
Partha Sarathi Mondal
Partha Sarathi Mondal
Partha Sarathi Mondal
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Abstract

Understanding the morphological variation is helpful to decipher reach scale character and behavior of a river. Further it also helps in conservation and sustainable management of a river system. Hence, the present study aims to understand the morphological variation of the West Rapti River in Nepal. To assess the morphological variation the river is divided into reaches with more or less similar geomorphic unit. The result shows that it has five distinct river class and ten river reaches. It is very important to preserve the river morphology for sustainable development.
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Partha Sarathi Mondal /IJBAS/ 11(3) 2022 ; 62-65
International Journal of Basic and Applied Sciences 62
Full Length Research Paper
Morphological Variation of the West Rapti River, Nepal
Partha Sarathi Mondal
Research Scholar, Department of Geography, Banaras Hindu University, Varanasi-221005, India.
ARTICLE INFORMATION ABSTRACT
Introduction
Rivers not only curve the landscape, but also play an important role in ecological and social system. (Wantzen et al., 2016;
Singh et al., 2017; Anderson et al., 2019). It is important to identify the geomorphic processes of river channel and
floodplain, which construct and sustain riverine ecosystem (Beechie & Sibley, 1997; Gilbert, Macfarlane, & Wheaton, 2016;
Wheaton et al., 2010). Hence geomorphic mapping of variation in channel planform and floodplain morphology for the entire
river determine the phase for reestablishment and management planning (Beechie & Imaki, 2014; Gore, 1985; Heede and
Rinne, 1990; Milner, 1994; Brierley and fryirs, 2005). Therefore, geomorphic diversity and underlain basic process is very
important for river science and sustainable management.
Materials and methods
Study area
The study area lies in the mountainous part of the West Rapti River in the Nepal. The West Rapti River originates from
Dregaunra range (3048 m) of Lesser Himalaya in Nepal. And after flowing through Siwalik Himalaya, Gangetic plain it
meets with Ghaghra River, one of the major left bank tributary of the Ganga River. It has a total catchment area of 26043 km2
out of which 56% (14642 km2) lies in India and 44 % (11401 km2) in Nepal. The length of the Rapti River is 782 km, out of
which about 310 km lies in Nepal and rest in India. In Nepal it is joined by several tributaries such as Jhimruk, Mari, Arun,
Lungri etc. and flows through Rolpa, Pyuthan, Arghakhanchi, Dang, Banke district. From the confluence of the Mari and
Jhimruk, the river is named as West Rapti River in Nepal. The Geographical extension of the study area is from 28°27' 48"N
to 27°49' 8"N and 81°47'14"E to 82°56'37"E (Fig. 1).
Data used and methods
To map the physical heterogeneity a high resolution geomorphic map of the study reach has been prepared using Sentinel
satellite image of 2017 (spatial resolution 10m). The image is typically obtained in pre monsoon season with a cloud cover of
less than 10%. A standard false color composite has been used to identify different geomorphic feature. To demarcate valley
bottom margin or effective valley width a number of cross profile has been drawn across the river at short interval on ALOS
WORLD 3D 30m (AW3D 30m) DSM data. On the basis of degree and break point in slope valley bottom margin has been
identified and marked on the both side of the river.
Vol. 11. No.3. 2022
©Copyright by CRDEEP Journals. All Rights Reserved.
Contents available at:
http://www.crdeepjournal.org
International Journal of Basic and Applied Sciences (ISSN: 2277-1921) (CIF:3.658 ; SJIF: 6.823)
(A Peer Reviewed Quarterly Journal)
DOI:
Corresponding Author:
Partha S. Mondal
Article history:
Received: 09-08-2022
Revised: 12-08-2022
Accepted: 26-08-2022
Published: 27-08-2022
Key words:
River style, reach,
confined, floodplain
Understanding the morphological variation is helpful to decipher reach scale character
and behavior of a river. Further it also helps in conservation and sustainable
management of a river system. Hence, the present study aims to understand the
morphological variation of the West Rapti River in Nepal. To assess the morphological
variation the river is divided into reaches with more or less similar geomorphic unit. The
result shows that it has five distinct river class and ten river reaches. It is very
important to preserve the river morphology for sustainable development.
Partha Sarathi Mondal /IJBAS/ 11(3) 2022 ; 62-65
International Journal of Basic and Applied Sciences 63
Fig. 1 Study area
Finally, on the basis of geomorphic assemblage the study reach divided into 10 reaches on the basis of nested hierarchical
approach of the River Style Framework developed by K. Fyris and Barely, 2000. In this approach basin to reach scale
parameters such as landscape setting, valley setting, Floodplain setting, and channel morphology has been considered to
define a river reach within which operating bio- physical processes are more or less uniform. At first tier landscape setting
has been used which refers to the geologic and geomorphic characteristics of the catchment. Valley setting is incorporated at
second tier, which refers to the ability to lateral adjustment of the channel in terms of confinement. At third tier, floodplain
setting was added, which refers to the presence or absence of the floodplain and its boundary condition to the valley wall
(Bawa et al. 2014). Fourth tier, Channel morphology was integrated, which refers to the planform geometric characteristics of
the river.
Computation of attribute of different channel units of the river
In this stage measurement of attribute of different geomorphic features of the channel is done. For examples, channel width
was measured along the centerline for the entire Rapti River at an interval of 2 km. In addition, channel length, channel area,
sand bar area are also computed. Lastly, statistical analysis is done to infer useful information and draw comparison between
reaches. Average stream power and unit stream of each pixel of different reaches are also computed to infer process-form
relationship.
Extraction of long profile from the DEM
For automated extraction of stream network in ArcGis 10.1, ALOS 30 DEM from JAXA is used. To extract stream network
first DEM data is filled using hydrology tool of ArcGis 10.1. Then flow direction is generated using unidirectional D8
method followed by the generation of flow accumulation raster layer. Subsequently, stream network is produced in raster
format and corrected accordingly. Then main stream raster data of the Rapti River following the thalweg line is converted
into point layer. The flow accumulation and elevation value of each pixel along the entire river is then extracted.
Subsequently, the distance of each pixel from source to downstream is computed using V.Distance tool in QGIS 3.16.10.
Finally, this data was exported in excel format and shorted according to distance from the source of the river.
Results and discussion
Using hierarchical river style classification approach the West Rapti River is divided into 10 reaches and 5 distinct river
classes to understand character and behavior of each reach in a holistic way and are given below (Fig. 2)
Reach 1: Mountainous, confined meandering and no floodplain
In this reach the river is flowing through the mountainous landscape of Lesser Himalaya. The channel is confined by steep
valley wall. As a result no floodplain is developed. The river is bedrock in nature. The river pattern is meandering which is
governed by the bed rock geometry of Himalaya. In channel deposits are Lateral, Mid channel, Point bar, Tributary delta. The
mean width is 60 m. The length of the channel is 12o km with a total area of 7.99 km2. The total sand bar area is 3.45 km2.
Reach 2: Mountainous, partly confined braided and floodplain
In this reach the river is also flowing through the mountainous landscape. The channel is partly confined and floodplain is
also partly confined in nature. The pattern of the channel is braided. In channel deposits are Chute channel, Concave bench,
Lateral, Mid channel, Point bar, Island. The mean width of the river is 260 m with length of 12 km. The total channel area
and sand bar area are 4.10 and 2.98 km2 respectively.
Partha Sarathi Mondal /IJBAS/ 11(3) 2022 ; 62-65
International Journal of Basic and Applied Sciences 64
Reach 3: Mountainous, confined meandering and no floodplain
This reach is similar to reach 1. Here the channel is confined with no floodplain. The meandering geometry is governed by
the bedrock structure of the Himalaya. The average width of the channel is 130 m with a total channel area of 0.98 km2. The
length of the channel is 6.8 km with atotal sand bar area of 0.52 km2.
Reach 4: Intermonate basin, confined braided and unconfined floodplain
In this reach the river is flowing through the intermonate basin landscape of Siwalik Himalaya. The channel is confined by
valley margin in one side and embankment on another side. Whereas the floodplain is unconfined in nature, which indicates
that the floodplain can grow laterally. The river pattern is braided. In channel deposits are Concave bench, Lateral, Mid
channel, Point bar. The mean width of the river is 652 m with a total bar area of 6.8 km2. The length of the channel is 12 km
with area of 8.5 km2.
Reach 5: Intermonate basin, unconfined meandering and floodplain
In this reach the river is also flowing through the intermonate basin. The channel and floodplain is unconfined by
characteristics. So that the channel can migrate laterally. The river pattern is braided with a mean width of 698 m. In channel
deposits are Concave bench, Lateral, Mid channel, Point bar and island with an area of 14.5 km2. The length of the channel is
32 km with an area of 20.4 km2.
Fig. 2 Morphological variation of the West Rapti River
Reach 6: Mountainous, partly confined meandering and floodplain
The channel is again flowing through the mountainous landscape. The channel and floodplain is partly confined by
characteristics. The river planform is meandering in nature with a mean width of 235 m with a total bar area of 1.14 km2. The
total channel area is 2.5 km2 having a length of 11.2 km
Reach 7: Mountainous, confined meandering and no floodplain
In this reach the river is flowing through Himalaya with confined channel and resulting no floodplain. The planform is
meandering controlled by Himalaya. The mean width is 146 m. The channel length is 7.5 km with an area of 1.2 km2. The
total bar area is 0.45 km2.
Reach 8: Mountainous, partly confined meandering and confined floodplain
In this reach the river is flowing through the Siwalik Himalaya. The channel is partly confined by nature. Whereas, the
floodplain of the river is confined. The river pattern is meandering with a mean width of 242 m with an area of 2.14 km2. The
total sand bar area is 1.3 km2.
Reach 9: Mountainous, partly confined meandering and floodplain
The river is flowing through Himalayan landscape. Here the channel and flood plain is partly confined by valley margin. The
planform of the river is meandering with an average width of 356 m. The side bar, point bar, midchannel bar, island dominant
the in channel deposit with area of 1.24 km2. The channel length is 7.5 km with an area of 2.57 km2.
Partha Sarathi Mondal /IJBAS/ 11(3) 2022 ; 62-65
International Journal of Basic and Applied Sciences 65
Reach 10: Mountainous, confined meandering and no floodplain
The river has a meandering planform with confined channel and no floodplain. The mean width is 142 m with a length of 20
km. The total channel and sand bar area are 2.98 and 0.95 km2 respectively.
Conclusion
The result shows that it has five river classes with different combination of hierarchical nested framework. In confined
channel setting the river can’t migrate laterally. Hence, the channel can do either incise or aggradation. Further, in unconfined
settings the floodplain can grow laterally. Whereas, in partly confined channel discontinuous flood plain is present on either
of the channel. For management of the river the pattern must be preserved. In mountainous landscape the channel and
floodplain is confined whereas in intermontane basin the channel is braided and unconfined with floodplain on both sides.
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