Assessment of Bank-Line Shift of Depota River, Assam Using Geospatial Tools

International Journal of Advanced Science and Technology Vol. 28, No. 15, (2019), pp. 942-952

Assessment of Bank-Line Shift of Depota River, Assam using Geospatial Tools

1 2 3 Debashree Borah , Binod Kumar Nath and Ashok Kumar Bora 1Research Scholar, 2Faculty Member, 3Professor 1,3Department of Geography, Gauhati University, Guwahati, India 2Assam Survey and Settlement Training Centre, Guwahati, India Corresponding Email: [email protected]

Abstract

Bank line migration is an intrinsic process of fluvial landscape as river tries to adjust in order to attain some sort of equilibrium state undergoing vertical and lateral mobility forming floodplain. The Trans Himalayan Rivers are highly susceptible to erosion and bank line migration processes owing to a host of natural and human induced factors. As such a study on Depota river, one of the important north bank tributary of Brahmaputra river have been carried out using remote sensing and geographical information system tools and techniques. A period from 1967 - 2017 have been taken in to consideration to examine the bank line migration pattern resulting from both erosion and depositional processes. The study reveals that the river is severely prone to riverbank shift during the peak monsoon season from May to September. The highest shift of 146.62 m along right bank and 134.11 m along left bank in few reaches were recorded during 1967-2017 wiping huge agricultural fields, tea gardens, human properties, livestock’s mainly near Rangapara, Niz-bahbari, Sonajuli, Barjuli T.E., Satai ban gaon etc. Considering nine segments, the total amount of deposition was observed as 99.15 m exceeding the amount of land loss as 48.22 hectares due to the effect of siltation and less cohesiveness among bank materials. Moreover, frequent breaching of embankments has also posed another threat causing massive damages to the floodplain dwellers.

Keywords: channel migration, bank line, erosion, deposition, geospatial tools.

Introduction

Rivers are considered as dynamic entities inducing numerous changes over the landscape of the earth surface. The network of river channels make up the dynamic fluvial system which undergoes continuous changes over time (Gregory, 1977). Rivers self-organize their channel pattern through feedback relationships among bars, channels, floodplain and vegetation, which emerge as a result of the basic spatial sorting process of wash load sediment and bed sediment. The balance between floodplain formation and destruction determines the width and pattern of channel (Kleinhans, 2010). Noteworthy that shifting of channel course is a habitual process leading to one of the major factor in floodplain creation forming an integral part of a river valley mostly in humid tropical regions. Sometimes in order to maintain its natural fluvial regimes, rivers abandon its own course and shifts to a new one. Channel change is largely attributed to the combined effects of both natural and human-induced activities. The type and characteristics of river bank erosion to a certain extent also relies on the cohesivity of the bank materials along with the net force induced primarily by flood water (Knighton, 1984; Morisawa and Hack, 1959). River banks can move away (erosion) or can advance (deposition) which can result in meandering migration, channel avulsion and change in channel width (Bartley et al. 2008).

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The magnitude of channel change plays a vital role in the study of flow dynamics of a river and is also required for proper channel design, river restoration, watershed planning and management (Gardinar,1981).Spatio-temporal shifting of channel caused by bank erosion and flow differentiation coupled with its resultant geomorphic hazards constitute the most important aspect under fluvio-geomorphic study. Studies undertaken by scientists using data available from a variety of traditional sources like toposheets, old maps, etc., through field investigations and availability of Remote Sensing (RS) data along with GIS tools and techniques now have made it possible to identify, monitor and verify shifting courses of rivers across the world in a precise and efficient way. Moreover, researchers have pointed out that the causes responsible can be tectonic, climatic or hydrologic in nature while in some cases it can be the combination of all these (Nongkynrih, 2011). The present paper is an attempt to assess quantitatively the differential rates of channel migration along with changes in land fill and land loss of Depota River, a small tributary of Brahmaputra River. The river course of Depota is found to be highly fluctuated in the flood plains. The excessive amount of water that has been carried during the monsoon season from the upstream exerts a considerable amount of pressure in both the banks of Depota River that made up off loose materials. As a result both erosional and depositional activities have been occurred in concave and convex banks of the river course. The study has been carried out focusing on satellite imageries of different years to extract the bank line shift during the last few decades using geospatial tools and techniques.

Study Area

The present study is confined to the Depota River, a north-bank tributary of Brahmaputra originating from Chariduar Reserve Forest situated at foothill region of Assam and Arunachal Himalaya and flowing through Sonitpur district of Assam. It is bounded by hills of Arunachal Pradesh on the north, the Gabharu sub-basin on the west, the Jia-Bharali sub-basin on the east and Brahmaputra on the south. Located between the longitude 95º35′E and 92º48′E and latitude 26º37′N and 26º55′N, the Depota basin constitutes a total area of 255.20 km2 with a total length of 46 km comprising four tributaries namely Borajuli, Ghagra, Bindukurijan and Bimlajan. The river is known as Manasajuli in the upper course and Depota in the lower course. After originating it flows in straight southerly direction and crosses the N.T road (N.H 15) at latitude about 26º41′ N and finally meets the Brahmaputra at latitude about 26º37′ N.

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Fig.1: Location Map of the Study Area

Database and Methodology

The study is mainly based on the data derived from comparison of the bank lines that has been extracted from the topographical sheet and satellite imageries of different years. Total time spans of 50 years have been taken under consideration for quantitative assessment of amount of erosion and deposition of the banks of Depota River. Survey of India topographical sheet of 1:50000 have been used to extract the bank line for the year 1967 whereas LANDSAT and IRS LISS III imageries have been used to extract the bank line for subsequent years. The bank lines have been manually digitized in GIS environment after necessary geo tagging and rectification of the toposheet and satellite imageries. After digitization, the bank lines of different years have been superimposed and 15 cross sections have been marked and measured randomly at different course of the river based on visual approach. The rate and orientation of riverbank migration have been computed with the help of cross sections. Moreover, the total amount of erosion and deposition have been calculated by generating polygon layers from nine segments of the river with the help of overlay and union analysis technique from 1967 to 2017 in Arc GIS based on the extracted bank lines.

Objectives

The objectives of the present study are 1. Assessment of bank line shift of Depota River 2. To examine the related fluvio geomorphic condition for bank line shifting of Depota River

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Results and Discussion

Channel Migration Pattern

Channel change over spatio-temporal dimension plays a vital role in the study of stream flow dynamics as the change is highly attributed to both vertical and horizontal movement of the channel induced by various factors. Lateral movement and stability of rivers are of particular interest, from geomorphic, engineering and ecological points of view. Such movements may pose threats to human- made structures, while those same movements enhance the diversity that is integral to a healthy riverine habitat. Depota River has been facing similar situation in this case. The river is highly confronted with such movements in few reaches leading to bank line migration, bank widening, cut-off etc. The study confirms that the river have undergone significant changes in few reaches owing to numerous factors. Instability of bank lines are basically found in middle and lower reaches considered for prolonged period of 50 years indicating that it is migrating towards eastern direction accompanied by progressive and regressive changes (figure 2 and 3).

Fig 2: Bankline change along reach 1 (extending Fig 3: Bankline change along reach 3 from Tarajuli T.E to Barjuli T.E) and (Ghogora T.E to Niz-Bahbari gaon and reach 2 (from Barjuli T.E to Ghogora T.E) reach 4 (from Niz-Bahbari gaon to Gelgeli gaon)

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Fig. 4: Riverbank migration due to erosion and accretion in Depota River during 1967-2017

It has been observed from the quantitative analysis that during the period from 1967 to 2003 (Table 1,fig. 2,3 and 4), maximum westward shift along the left bank with a distance of 129.77 meter and 93.41 meter in right bank across C-C′ cross-section was recorded. Similarly high magnitude extension has also been developed along D-D′ cross sections which were attributed to the cause of neck cut-off. The eastward shift of about 392 meter occurred along H-H′ cross-section on right bank and also 283.78 meter eastward in left bank. At this cross-section both the banks have undergone regressive changes leading to deposition. A major shift of the channel took place due to development of chute cut-off near Gorali T.E. The least variation was recognized across A-A′ section with 18.35 meter in left bank and 93.41 meter in right bank. However, it is apparent to notice frequent formation and gradual shifting of meander bends in an alluvial channel in the river course. Different types of meander bends such as extensional, translational and rotational were also developed along the river course of Dipota River.

Noteworthy that the entire channel course have quite irregular shifting pattern during entire period for both banks. From 2003 to 2017 (figure 2 and 3) the banks remained more or less stable across few reaches. The cross-sections at J-J′ near Rangapara with 167 meter shift in right bank and 152.44 meter shift westward in left bank posed vulnerable ones at times. Similar other cross-sections such as N- N′ section near Satai Ban Gaon and Niz-Bahbari and O-O′ section near Majuli Chapori and Gelgeli are the most flood-prone areas experienced heavy flood every year. However, from 1967 to 2017 major bank line changes have been clearly noticed owing to a large time scale of 50 years. The middle course was more prone to subsequent flood and bank line migration. Maximum shifting occurred with 396.26 meter along right bank and 285.62 meter towards west in left bank while the minimum shift took place across E- E′ with 52 meter and 43 meter eastward in right and left bank respectively. The causes responsible for such shifting behaviour might be attributed due to low channel gradient leading to meandering course. Even banks composed of coarse non-cohesive materials lead to instability of bank lines migrating either progressively or regressively. Another vital anthropogenic factor observed was that banks were devoid of vegetations induced by unwise destruction etc. In addition to that many agricultural activities in conjunction with household activities have been also considered as an important factor for bank line migration of Depota River.

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Table 1: Bankline shifting of Depota River during 1967-2017

Cross- 1967-2003 2003-2017 1967-2017 sections Right Bank Left Bank Right Bank Left Bank Right Bank Left Bank (m) (m) (m) (m) (m) (m) A-Aʹ +66.74 E +18.35 E -9.45 E -15.53 E +98.40 E +60.86 E

B-Bʹ +86.28 E +99.70 E +0.43E +5.78 W +89.38 E +93.85 E

C-Cʹ -93.41 W -129.77 W +5.69 W -5.2 W -99.33 W -134.11 W

D-Dʹ +106.72 E +86.61 E -5.57 E +5.28 E +107.76 E +93.06 E

E-Eʹ +42.80 E +32.94 E -9.66 E +10.30 E +52 E +42.9 E

F-Fʹ -110.06 W -117.55 W -13.05 W +20.05 W -93.31 W -97.50 W

G-Gʹ +66.20 E +64.76 E -12.5 E +22.3 E +79.41 E +88.43 E

H-Hʹ +392 E +283.78 E -4.12 E -1.20 E +396.26 E +285.62 E

I-Iʹ -80.42 W -102.13 W +15.38 W -5.14 W -98.95 W -114.32 W

J-Jʹ -167 W -152.44 W -155.09 E +143.35 E -11.15 E -10.7 E

K-Kʹ -+66.20106.65W E -+64.7687.65 W E +37.88-12.5 EW -41.94+22.3 WE -146.62+79.41 WE -134.63+88.43 WE

KHL--KʹLʹHʹ +65.55+392 EW +283.78+61.88 W E +2.9-4.12 W E --2.521.20 W E +68.84+396.26 W E +64.03+285.62 W E

MI--MʹIʹ +108.97-80.42 W W --115.26102.13 WW +15.38+6.97 EW +18.4-5.14 WE +100.96-98.95 W W --114.3296.91 W W

NJ--NʹJʹ -98.28-167 W W --133.25152.44 WW +20.35-155.09 W E +143.35-2.55 W E +115.42-11.15 EW -131.14-10.7 E W

OK--OʹKʹ -106.65W56.45 W +58.05-87.65 W -+37.88123.73 WE +105.32-41.94 W E --146.6271.12 W W -+47.75134.63 WW

L-SourLʹ ce: C+65.55omputed W b y the+61.88 resear Wch er +2.9 W -2.52 W +68.84 W +64.03 W Note: Negative sign denotes progressive and positive sign denotes regressive changes of bank line W= West, E= East M-Mʹ +108.97 W -115.26 W +6.97 E +18.4 E +100.96 W -96.91 W

N-Nʹ -98.28 W -133.25 W +20.35 W -2.55 W +115.42 W -131.14 W ISSN: 2005-423 8 IJAST 947

Erosion and Depositional Pattern

Rumbling through its journey, a river channel carries immense volume of sediments and water induced by its steep gradient in upper and middle section resulting into mass deposition in its lower course. During peak stage, the velocity, volume and shear stress of each river tends to accelerates which leads to scouring of both banks and bed of a channel. However, the three mentioned processes are quite natural but over time due to various anthropogenic activities where encroachment of people in floodplain zone seems to be most crucial one, the river channel behaves unnaturally causing widespread havoc to mankind.

Fig 5: Patterns of erosion and deposition Fig 6: Patterns of erosion and deposition along channel reach 1 and 2 along channel reach 3 and 4

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Table 2: Net area under erosion and deposition of Depota River

1967-2017 Channel Erosion (ha) Deposition Segments (ha) A 1.16 7.43 B 1.70 4.35 C 4.17 5.86 D 3.42 5.16 E 7.13 7.08 F 7.42 5.83 G 12.53 34.39 H 4.09 10.88 I 6.60 18.17 Total 48.22 99.15

Source: Calculated from satellite imageries

A comprehensive analysis of patterns of erosion and deposition has been done for the period 1967 to 2017 for Depota River. Channel segment A and B constitutes minimum progressive change of 1.16 hac and 1.70 hac while regressive change have been recorded as 7.43 hac and 4.35 hac in its upper course respectively (Table 2 and fig.5, 6 ). Gradually entering the middle course, the river encounters both erosion and deposition at a much higher rate. Along the G-segment the erosion is more prominent removing land areas ranging from 7.42 hac to 12.53 hac which may be due to sinuous nature of the river. Every year the high magnitude flood overspills the agricultural croplands and adjoining areas of Satai ban gaon, Rangapara, Bahbari, Barsongaon etc. Similarly, at the same segment highest accretion was observed i.e, 34.39 hac. It can be concluded that the river experiences a total of 48.22 hac of erosion and 99.15 hac of deposition whereby deposition is predominant over erosion along Depota River. It has been found to be due to multiple factors such as low gradient, regressive movement of bank lines and also when sediment load exceeds the transportation capacity of the channel etc.

Flow characteristics of Depota River

The flow of a stream is directly related to the amount of water moving off the watershed into a stream channel whose intensity increases in rainstorm and ceases during dry period. The main factor influencing the stream flow is precipitation. It is possible to relate any one of these factors individually to discharge and at the same time to incorporate indirectly the effects of others. It has been observed that the bank line shifting of most of the north bank tributary of Brahmaputra River is due to the excessive amount of water that has to carry during the rainy season. Therefore it is obvious to note that the amount of rainfall, water level and discharge have definitely some impact on river bank shifting of Depota River.

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Table 3: Average annual rainfall and water level of Depota River (N.T Road Crossing)

oad

R Years Average Annual Average Annual Water .T.

N Rainfall (mm) Level in (m)

- n

i t

a 2011 1087.3 64.30 St 2012 1437.5 64.34

2013 984.5 64.19

2014 1098.2 64.43

2015 1549.5 64.38

2016 1126.1 64.15

2017 1418.7 64.42

Source: Water Resource Department, Govt. of Assam

Table 4: Annual Average Water Discharge during 2011-2017 in Depota River

Average Water Discharge (m3s-1) YEARS ANNUAL MONSOON NON- MONSOON 2011 3.755 5.69 2.374 2012 4.299 5.478 3.457 2013 4.46 5.75 3.538 2014 3.147 4.028 2.6 2015 3.157 4.98 1.838 2016 3.135 4.912 1.865 2017 3.527 5.192 2.338 Monthly 3.63 5.14 2.56 Average

Source: Water Resource Department, Govt. of India NB: Monsoon Months (May to September) and Non-Monsoon Months (October to April)

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Fig 7: Diagrammatic representation of average water discharge in Depota River

It has been estimated that the rainfall pattern over the study area have been highly variable over the years wherein during 2011, the average annual rainfall was sought to be 1087.3 mm which increased to 1549.5 mm in 2015 and 1418.7 mm in 2017. But as compared to the rainfall pattern water level of Depota River has been maintaining a static nature since 2011 to 2017 experiencing 64.30 m in 2011 and 64.42 m during 2017. Thus, the direct relationship between rainfall and water level does not exist in case of Depota River. The reason for static water level over the years in spite of change in rainfall pattern might be the shifting of bank line due to the erosion caused widening of the Depota River as evidenced in various cross sections. Again the annual and seasonal average water discharge (Table 4 and fig.7) indicates remarkable changes during 2011-2017 in Depota River as the average annual discharge was observed highest accounting 4.299 m3s-1 in 2012 that have decreased to 3.527 m3s-1 in 2016. Again the maximum discharge during monsoon period was recorded in 2011 as 5.69 m3s-1 and over time it has remain more or less stable with least fluctuations. It has been observed from the analysis that because of the high amount of discharge during the monsoon season, large amount of sediments have been transported by the Depota River from the upstream and due to tremendous pressure of water and sediments many meandering bends have been developed along the river itself. In addition to that, taking the advantage of loose alluvial soil, the banks have been exposed to severe erosion and as a result the main stream of Depota has been shifted from its original course. However, non-monsoon period records less discharge accompanied by less precipitation and during this period the river especially in middle and lower reaches becomes dry.

Conclusion

The present study elucidates that the geospatial tools and techniques can be considered as the most effective tools in monitoring and assessment of bank line shifting of river and their changing pattern

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over the years. Being a tributary of the gigantic Brahmaputra, Depota River is subjected to severe bank erosion and lateral shifting has been clearly noticed when studied for a longer period from 1967-2017. In few reaches the river has remained more or less stable while some areas such as Gelgeli, Rangapara, Satai Ban gaon, Bahbari experiences severe bank erosion causing acute flood which results in mass destruction of human properties, agricultural fields and tea gardens etc. Moreover, total net area of deposition has exceeded total erosion by 50.93% which can highly be attributed due to siltation. Other controlling variables leading to spatio-temporal change in the river channel are banks composed of non-cohesive materials, gentle slope, rise of bed load etc, artificial embankments etc. Proper and scientific planning along with a study on bed and bank materials will enhance in maintaining the ecology of the riverine habitat reducing its effects in near future.

References

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