Laws of erosion in Majuli: A statistical approach on GIS based data

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S.K. Lahiri / South East Asian Journal of Sedimentary Basin Research 1 (2013) 80-89

80

Laws of erosion in Majuli: A statistical approach on GIS based data Siddhartha Kumar Lahiri

a,b*

aDepartment of Applied Geology, Dibrugarh University, Dibrugarh 786004, India bEngineering Geosciences Group, Indian Institute of Technology, Kanpur 208016, India

_________________________________________________________________________________________________________________________

ABSTRACT

Majuli, the largest riverine island in the world having human habitation (Population: 0.16 million), located in the upper reach of the Brahmaputra valley

of Assam and the abode of Vaishnavite spiritual centres called ‘Satras’(around 64) has eroded as per our measurement at an average rate of 3.1 km2/yr

during the period 1915-2005. In 1915, the area of the island was 787.9 km2 which became 508.2 km2 in 2005. This amounts to 35.5% planform

reduction. Unlike other braid bar deposits, Majuli is not a direct outcome of the river sedimentology. The Majuli type situations are very often a

response of structurally controlled geomorphic highs in the valleys to the lateral bank migration of the rivers. However, after it comes inside a channel

belt, the river processes as well as the captured highland, both influence each other. The highland posses as an impediment, reducing thereby the stream

power of the channel. The river on the other hand, erodes the land steadily but unevenly. This mutual interaction influences the braid bars, channel

geometry and some related geomorphic parameters of the river regime. Majuli is fast eroding, but is it eroding uniformly? Is there any relationship

between the erosion of the Majuli and the adjoining braid bars? What is the nature of sediment load distribution orchestrated by the river regime? Is

there any change in the aggradation-degradation trend in the neighbourhood of the Majuli Island? There is a knowledge gap relating the nature of

unevenness in the erosion of the Majuli Island in planform. The present work deals with three major issues. First, quantification of the plano-temporal

changes of a few geomorphic parameters like areas of channel belt (CHB),braid bar (BB), channel (CH), average width of the channel (W), right bank

(RB) and left bank (LB) line migration in 13 number of reaches covering the channel belt bearing the Majuli Island between two consecutive periods

1915-1975 and 1975-2005. Secondly, assuming relative change in erosion of the Majuli Island (ME) is correlatable with changes in different

geomorphic parameters mentioned above, we have used the ‘method of least squares’ to find out the empirical equations which are highly constrained

by place and time. It is observed that a set of second order parabola equations which we have argued as ‘Laws of erosion in Majuli’ can relate the

percentage rate of erosion of the Majuli Island with the above mentioned six parameters. Thirdly, erosion of Majuli might influence the aggradationary

tendency, if any, in the neighbourhood of the Majuli Island. Accordingly, we have investigated the influence of Majuli erosion (ME) on Braid bar

density variation in the planform (BB) as well as on BB/CH. We find that increasing erosion of Majuli witnesses side by side a growing tendency in the

braid bar formation, more so in the lower part of the island. Moreover, this also shows that the eroded material is not getting transported in the further

downstream side rather the aggradationary tendency has sharply increased in the adjacent areas of the lower Majuli over the last four decades compared

to the earlier period.

Keywords: Geomorphic high, Aggradation, Plano-temporal change, Method of least squares

_________________________________________________________________________________________________________________________

1. Introduction

Three mountain fed rivers, the Siang, the Dibang and the

Lohit confluence in the upper tip of a narrow valley to form

another river, one of the large tropical rivers of the world

(Hovius, 1998; Latrubesse et al., 2005; Tandon and Sinha,

2007), the Brahmaputra which has drawn the attention of

some of the finest scientists (Bristow, 1987; Goswami, 1985;

Curray, 1994; Goodbred and Kuehl, 1998, 2000; Richardson

and Thorne, 2001; Goodbred et al.,2003; Sarma and Phukan,

2004,2006; Kotoky et al., 2005; Sarma, 2005) ever since

Coleman (1969) published his seminal work on the channel

processes and sedimentation of the river. Studies related to

fluvial dynamics and landform evolution attained a heightened

interest as the role of tectonics was increasingly appreciated

(Holbrook and Schumm, 1999) to influence the morphology

and migration characteristics of a river. In the upper Assam

valley, where Brahmaputra originates and subsequently acts as

a valley divider, the channel belt of the river includes a

number of islands of different shapes and sizes. Among these,

some of the islands are constituted of much older flood plain

deposits and that means older than the river at the place of

study and rest are the islands which came into being as a

direct consequence of the river architecture where the

alluvium is constituted of very recent flood plain deposits

(ages in decadal or century scale) and of course these are

much younger than the age of the river at its present location.

The first one is Majuli-type and the second one is Char type.

Majuli type situations come into existence due to the river

dynamics and Char type situations arise due to hydro

dynamics and sediment transportation characteristics of the

river. Thus the history of human habitation for a Majuli type

land area might be older than the river flowing around it

whereas the habitation history of the Char type areas must

have started much recently. The old Majuli, which is presently

located between Jorhat town in the south bank and North

Lakhimpur town in the north bank of the Brahmaputra River

has witnessed a surge in erosion rate from 2.46 km2/yr

during1915-1975 to 4.40 km2/yr during1975-2005.

________________________________________________________________

* Corresponding author at: Department of Applied Geology, Dibrugarh University,

Dibrugarh 786004, India. Tel.: +91 373 2370247; fax: +91 373 2370323.

E-mail address: [email protected] (S.K. Lahiri).


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As the Majuli Island acts as the cultural and religious head

quarter of a sizable population of the valley, the fear of its

extinction has gripped recently the minds of the inhabitants as

well as those who are psychologically linked to the place.

There is a central question-will Majuli erode completely in the

near future? But before that question is addressed this is good

to ask how well do we know about the trend of erosion in

Majuli? Does Majuli erode uniformly? Does erosion of Majuli

mean areal expansion of the channel bearing zone of the

Brahmaputra channel belt? Is there any connection between

the formation of new braid bars and the erosion of the Majuli?

What is the status of the Brahmaputra river in the

neighbourhood of the Majuli island in terms of the variation in

the braid bar and channel area ratio? In the present work, first

of all, we have documented in detail the measurements of

plano-temporal variations in some of the geomorphic

parameters like average width (W), magnitudes of left bank

(LB) and right bank (RB) migration of the river over the

period 1915-2005, relative changes in the areas of the channel

belt (CHB), channel (CH), braid bar (BB) and then BB/CH

ratios for different sub units of the Majuli bearing zone of the

Brahmaputra channel belt. Secondly, we have related

mathematically the erosion of the Majuli Island with all the

aforesaid parameters making a simple assumption that the rate

of erosion of Majuli is expressible as a function of different

geomorphic parameters associated with it and expressed them

in terms of “Laws of erosion in Majuli”. Thirdly, we have also

tried to investigate the trend of variation in the braid bar areas

and the BB/CH ratio as a function of the erosion of the Majuli

Island. Thus, in the present work, instead of isolating Majuli,

we have nurtured the concept of “Riverscape-landscape

duality” to understand the dynamics of Majuli and its

neighbourhood.

2. The study area

The present study area that includes the Majuli of 1915, is

situated (Fig.1) between the longitudes 93024/-950 and

latitudes 270-27015/. If we put Majuli at the centre of our

observation, the N-NW side is bordered by the frontal thrust

belt (HFT) of the eastern Himalayas and S-SE side is bordered

by the Naga Thrust of the Naga Patkai Thrust belt (NPT). In

the extreme south west corner there is Mikir hills. The

Brahmaputra River, after travelling a more or less straight

path along NE-SW, suddenly takes a westward turn as if to

Fig. 1. Location map of the Majuli Island shown (A) inside India and (B) upper reach of the Brahmaputra valley. (C) the study area is

divided into thirteen number of smaller second order reaches. Place abbreviations are, Jo-Jorhat; Si-Sibsagar; NL-North Lakhimpur; Dh-

Dhemaji; Di-Dibrugarh; Ti-Tinsukia.


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bypass the Mikir hills. The geo-tectonic setting of the Majuli

Island shows that the place is situated between the Bouguer

gravity anomaly contours 220-240 mGal and first order

basement depth contours 3.6-5.0 km (Narula et al., 2000). It

clearly shows a prominent ‘Low’ in the NW part of the Majuli

Island. A first order tectonogeomorphic zonation of the

intermontane valley reach that divides the area into three types

respectively, uplift, slope and depression, puts the Majuli

Island on the ‘Lower central uplift’(Lahiri and Sinha, 2012).

3. Method & approach

To understand the nature of the recent morphological changes

around the Majuli Island area, the 23.5m resolution IRS-P6-

LISS-3 image, taken on 15 December 2005, was compared to

the topographic maps prepared during 1912-1926 seasons

having scale 1: 253,440 and 1977 (scale: 1:250,000)

topographic map of the Survey of India. The standard methods

of digitization, image to image registration by selecting proper

Ground Control Points (GCPs) and ground verification were

followed using ERDAS software for co-registration of all

data. We have divided the Majuli bearing channel belt of the

Brahmaputra River into 13 smaller reaches chosen arbitrarily

varying in reach lengths 4.5-8.5km. The arbitrariness in the

choice of the reaches does not matter because we are

interested in the percentage change in unit areas. The

advantage is, we can assign local names to different reaches

which are easily recognisable for common usage. In the

present work, we are principally interested to understand the

connections among different planform geomorphic parameters

so that possibilities of engineering interventions can be

explored on a more rational approach. It is in this context,

three things need better clarification. These are about natural

laws and planform fluvial dynamics, method of least squares

and physical significance of curve fitting.

3.1. Natural laws and planform fluvial dynamics

Fluvial dynamics obey certain natural laws. Nobody contests

that. The problem is how to find out that law. In the common

philosophical understanding, carried forward by the

philosophers particularly the stoics like Cicero, the natural

laws are supposed to be immutable, eternal and universal and

free from divinity or any other kind of ethical connotation

which was prevalent during the days of Aristotle (Shellens,

1959). Ever since the development of quantum theory, laws of

nature were recognised as merely probable, not universal,

linking different physical quantities. The element of ‘eternity’

in natural laws assumes an overall causal regularity which

enables the observer to predict accurately. The element of

‘eternity’ fails to explain the abrupt changes like accidents as

well as the process of initiation. The understanding that the

ascent of relationships among different physical quantities

from the plane of hypothesis → theory →law, as if something

unalterable, is questioned by Karl Popper (1959). According

to Popper, if a theory cannot be falsified, it is not scientific.

He proposes the growth in scientific knowledge as

PS1→TT1→EE1→PS2. Thus, in response to a problem

situation (PS), a number of competing conjectures or tentative

theories (TT) are proposed which are systematically subjected

to rigorous attempts of falsification that result into error

elimination (EE) which in turn, result into more interesting

problem situations (PS1 to PS2). Thus, Popper rejects classical

empiricism which was responsible for the classical

observationalist-inductivist account of scientific methodology.

Popper described his philosophy as ‘critical rationalism’. In

our present discussion, we have adopted the Popperian sense

of understanding to define the “Laws of planform fluvial

dynamics”. These ‘laws’ are valid only within strictly defined

boundary conditions, temporal as well as spatial.

3.2. Method of least squares

Between two sets of correlatable variables, if we know which

one is independent variable, then the respective changes can

be plotted which will give us a distribution pattern. Next, we

can have a straight line through this maze of points to assign a

simple ‘character’ to this distribution. There are many

possibilities of slope variations as well as the magnitudes of

intercepts. The distance by which any arbitrary point lies

away from the line is the amount of ‘misfit’. If we square all

these misfits and sum up, the situation for which we get the

least value, is taken as the best fit solution for the problem.

This is known as the Method of least squares, the credit for

which goes jointly to Legendre (1805) and Gauss

(1809).There can be ordinary least squares (linear) or non

linear least squares. In the present work, we have applied

linear as well as non linear method of least squares and tried

to fit polynomial equations of the type,

Y = A + BX + CX2

If the contribution of the last coefficient is too small, we have

neglected it. Obviously, for the linear curve fit, the equation

considered is of the type, Y = A + BX.

We have considered two types of situations. In the first type,

the rate of erosion of the Majuli Island in different reaches

[that is, {(old area – new area)/old area}×100] was treated as a

dependent variable and the geomorphic parameters like CHB,

BB,CH, W, LB, RB were taken as independent variables. In

the second type, we have tested the percentage variations in

BB and BB/CH as the dependent variable whereas the rate of

erosion of the Majuli Island as the independent variable.

3.3. Physical significance of curve fitting

Any multi variable phenomenon, as a time series or planform

projection of variability, when subjected to forcings of

different orders, we get curves which are commonly known as

‘wiggle traces’. The curve fitting technique helps us to

identify as well as isolate different orders of events expressed

in simple sinusoidal mode of expression. Secondly, curves

representing phenomenon can be extrapolated as well as

interpolated. Thus, if the sampling interval is large, we can

know what happened between two points of observation.


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Extrapolation helps to predict the future if we are dealing with

a time series data.

4. Results and interpretation

4.1. Reach scale erosion of the Majuli Island

In this section, we would like to describe a few specific

observations during 1915-2005 relating the nature of plano-

temporal changes observed for 13 different reaches of the

Brahmaputra channel belt (Fig. 2) which include Majuli, char

areas (BB) and the water bearing channels (CH). The

elevation difference between Reach-1 (88m above the mean

sea level or msl) and Reach-13 (73m above the msl) is 15m.

4.1.1. Reach 1: Tekeliphuta

This reach having a length of 4.7 km shows that it reduced

from 14.2 km2 in 1915 to 6.8 km2 in 1975 and in 2005 it was

simply nonexistent. During 1915-2005, the average width of

the channel belt increased by 3.1%. New braid bar formation

increased tremendously. From about 10 km2 in 1915, it

became 23.6 km2 in 2005 and that amounts to 134% increase.

BB/CH during the above said duration of ninety years shows a

remarkable rise of about 76%. Thus in spite of the erosion of

the Majuli Island, the reach in general was showing massive

aggradation.

4.1.2. Reach 2: Ratanpur-Sonoal Kachari gaon

Reach location: 27003′54″ - 27005′45.6″ lat and 94026′52.8″ -

94030′28.8″ long. Reach length: 8.36 km. Average elevation :

86m above the msl. During 1915-1975, the Majuli Island

reduced by 25% whereas with respect to 1975, the area

reduced by about 54% as measured in 2005. The erosion was

mainly in the southern part of the island where Sonoal

Kachari gaon is located. For the whole period 1915-2005, the

reduction was about 65%. The average width of the

channelbelt during the same period increased from 11.57 km

to 12.21 km (5.5% growth) and mainly the left bank of the

river was migrating. Braid bar area increased by about 131%.

Channel bearing area also increased by about 96%. Thus,

effectively, in spite of unabated erosion of the Majuli Island,

BB/CH ratio increased within the channel belt indicating

effective aggradation.

4.1.3. Reach 3: Maj-Deurigaon-Dighaligaon

Reach location: 27001′37.2″-27003′54″ lat and 94022′30″-

94026′52.8″ long. Reach length: 8.4km. Average elevation:

85m above the msl. Average width of the channel belt

increased from 15.88km in 1915 to 16.81km in 2005 and that

means about 5.86% growth. Majuli reduced by about 31%

during 1915-2005. Channel area increased by 96% whereas

the braid bar area increased by 45%. Thus, the BB/CH area

reduced considerably (-26%), indicating degradation.

4.1.4. Reach 4: Kumarbari- Jengraimukh-Gayangaon

Reach location: 26059′31.2″- 27001′37.2″ lat and 94018′25.2″-

94022′30″ long. Reach length: 6.37km. Average elevation:

85m above the msl. Interestingly, in this reach, average width

of the channel belt has decreased over time from 23.11km in

1915 to 21.93km in 1975 and then again a slight increase to

22.54km in 2005. Thus for the overall period 1915-2005,

there is a narrowing of about 2.5%. The magnitude of erosion

also shows a declining trend from about 11% during 1915-

1975 to 3.38% during 1975-2005. BB/CH ratio shows a

highly fluctuating trend from 1.72 in 1915 to 0.61 in 1975 and

then again shooting sharply upward to 2.0 in 2005. This

shows an overall aggradation.

Fig. 2. Reach scale plano temporal variation in the Majuli

bearing Brahmaputra during (A) 1915, (B) 1975 and (C) 2005.


84

4.1.5. Reach 5: Baruigaon-Domgaon



85m above the msl. In this reach too, the average width shows

an overall declining trend during 1915-2005 of about 3.2%.

For the same period, the erosion of the Majuli is also least and

is about 0.32%. BB/CH ratio shot up from 3.7% during 1915-

1975 to 54.8% during 1975-2005 indicating recent increase in

aggradation.

4.1.6. Reach 6: Garmur Bar Sattra-Kamalabari

Reach location: 26056′31.2″- 26057′57.6″ lat and 94012′36″-


84m above the msl. This is the singular reach which is not

only free from erosion rather the area of Majuli Island

expanded by about 9.2% during the period of ninety years

(1915-2005). Average width of the channel belt has increased

by about 1.4%.The BB/CH has also decreased during the

same period by about 6.9% showing degradation. The reverse

trend observed in this reach is probably due to the anti erosion

measures taken up by the government to protect the most

prominent of the ‘Satras’.

4.1.7. Reach 7: Kalakhowagaon

Reach location: 26055′1.2″-26056′31.2″ lat and 94009′46.8″-

94012′36″ long. Reach length: 5.99km. Average elevation:

82m above the msl. Rate of erosion of the island is 15.8%

during 1915-2005. Average width of the stretch is more or

less constant. BB/CH shows highest fall of about 39.3%

which is an indication that this is the site of degradation.

4.1.8. Reach 8: Goalgaon

Reach location: 26053′16.8″-26055′1.2″ lat and 94006′28.8″-


80m above the msl. During 1915-1975, the rate of erosion was

quite severe (38.4%). However, subsequently, the situation

has drastically improved. During 1975-2005, this reach shows

areal expansion from 33.15 km2 to about 51 km2 (about 54%

growth). This is also unique in comparison with the general

trend of development and is probably due to the

anthropogenic intervention in the form of raising

embankments, a very common means adopted by the state

agencies to arrest erosion. There was about 46% growth in the

BB/CH ratio during 1915-2005, showing aggradation.

4.1.9. Reach 9: Bebejia Kaibartagaon

Reach location: 26051′54″-26053′16.8″ lat and 94003′14.4″-


80m above the msl. This reach also follows similar trend to

that of Reach-8. During 1915-1975 it shows a trend of severe

erosion of about 35% which was simply reversed during the

later phase showing not only cessation of erosion, rather it

expanded by 32%. This is probably due to the planning

measures related to controlling the erosion by the state

agency. However, in the overall consideration for the period

1915-2005, there was a net erosion of about 15%. The average

width of the channel belt was also showing a decreasing trend

during 1915-1975 (about 14%), but during 1975-2005, it

increased by 18% and for the entire period 1915-2005, the

overall situation was just a slight increase of 1.25%. The

channel area increased more (by 25.4%) than the braid bar

area (about 12%). Thus, BB/CH reduced by about 11%

showing degradation.

4.1.10. Reach 10: Michhamara

Reach location: 26050′42″- 26051′54″ lat and 93059′52.8″-


73m above the msl. This indicates a fall in elevation of 7m

from the previous reach. The rate of erosion is very high-25%

during 1915-1975 and 68.6% during 1975-2005.The average

width of the channel belt decreased by 3.1%. BB/Ch increased

by 27.5%, indicating aggradation.

4.1.11. Reach 11: Karaiguri

Reach location: 26049′26.4″- 26050′42″ lat and 93056′24″-


73m above the msl. The reach suffered tremendous erosion.

During 1915-1975, 58% of the area eroded and subsequently

99.7% of the area washed away; leaving behind a very thin

lining. The average width of the channel belt had remarkable

shrinkage (15%) during 1915-2005. During the same period,

braid bars had 39% growth, channel areas had about 80%

growth and as a follow up, BB/CH had a fall of about 23%,

indicating a zone of degradation.

4.1.12 Reach 12: Ahatguri


93056′24″ long. Reach length: 5.94km. During 1915-1975,

about 28% of the Majuli Island washed away and

subsequently, whole of the area succumbed. The width of the

channel belt decreased by about 14% during 1915-2005;

BB/CH shows 29% increase indicating thereby aggradation.

4.1.13 Reach 13: Subansirimukh


93052′40.8″ long. Reach length: 6.09km. About 61% of Majuli

reduced during 1915-1975 and the remaining part collapsed

during 1975-2005. The width of the channel belt reduced

slightly (about 1%) and BB/CH increased by about 13%

during 1915-2005, showing an overall tendency of

aggradation.

4.2. General trend of erosion of the Majuli Island

The rate and pattern of erosion observed (Fig. 3A-B) in and

around the Majuli Island in the last ninety years is very high

and diverse. Between 1915 & 1975, the surface area of the

island reduced from 787.87 km2 to 640.5km2 (18.7%

reduction) and then to 508.2km2 by 2005 (35.5% reduction as


85

compared to the 1915). The average rate of erosion in the last

thirty plus years has increased considerably from 2.46 km2/yr

(1915- 1975) to 4.40 km2/yr (1975-2005). The length of this

spindle shaped island has also reduced greatly from 79.7 km

in 1915 to 75.16 km in 1975 and then 63.33 km in 2005(about

20.5% reduction compared to 1915). If we compare changing

geomorphic parameters 1915-1975 (Fig. 3A) with 1975-2005

(Fig. 3B), we have a few interesting observations (Table 1): :

a) Based on the rate of erosion, Majuli Island can

distinctly be divided into three parts - upper, middle

and lower Majuli. Middle Majuli is fairly stable

compared to the upper and the lower.

b) As expected, the trend of variation in the channel

belt areas (CHB) and the average widths (W) show

a close similarity and the reduction of the Majuli

Island for the period 1915-2005 can be correlated

with the second order approximation of the

polynomial fit as (Fig. 4A & 4D):

ME=36.84 – 0.7604 CHB + 0.154 CHB2

ME= 33.45 + 0.668 W + 0.3538 W2

It simply shows that the narrower stretches of the

channelbelt are more erosion prone.

c) Braid bars formed due to the river processes show

different types of changes over time like emerging

new ones, waning older ones and some of the braid

bars remain stable for much longer period. The

cumulative braid bar areas (BB) in different reaches

(excluding the area covered by the Majuli Island)

show an overall decreasing trend of about 9.3%

during 1915-1975. However, during 1975-2005, the

trend was not only reverse, there seems to be a

vigorously progressive tendency of braid bar

formation with overall growth rate of 53% and

simultaneous increase in severity of erosion of the

Majuli Island. Thus, a growth rate of 38.8%

observed for BB during the period 1915-2005 is not

a continuous growth story rather it includes a

complete cycle of high-low-high. The second order

polynomial fit law of reduction of the Majuli Island

with respect to the changing braid bar areas can be

expressed as following (Fig. 4B):

ME = 9.777 + 1.405 BB + 0.006463 BB2

d) There is a remarkable shift in the comparative trend

of the changing channel area (CH) and the braid bar

area (BB). During 1915-1975, they show a more or

less opposite trend whereas the overall channel belt

area (CHB) was following the similar trend of BB.

Also, the average amplitude of CH was higher than

both the BB and the CHB. This shows that wherever

braid bar deposits increased, the actual channel

areas decreased. Moreover, increase in channel belt

area was principally due to the increasing area of

braid bars. However, during 1975-2005, CH and BB

were showing similar trend and BB was maintaining

consistently much higher amplitude all along the

Majuli Island. As we move downstream, the

amplitude difference between BB and CH keeps on

increasing, showing a rising trend in aggradation.

The BB/CH ratio reduces drastically from 2.34 in

1915 to 1.24 in 1975 and then again 2.49 in 2005.

Thus, the overall trend of aggradation has definitely

increased during 1915-2005. The second order

polynomial fit law of reduction of the Majuli Island

with respect to the changing channel areas (CH) can

be expressed as following (Fig. 4C):

ME = 37.24 + 1.006 CH - 0.008772 CH2

e) The bankline shifts, right and the left with respect to

the downstream direction of flow can be correlated

with the erosion of the Majuli Island as

following(Fig. 4E & 4F):

ME 45.42 + 17.16 RB + 0.4711 RB2

ME 24.7 1.99 LB + 6.189 LB2

Fig. 3. Quantitative analysis of the geomorphic parameters

observed in relation to the temporal variability of planform

erosion of the Majuli Island. (A)Absolute changes in the

channel belt areas, average widths of different smaller

units, cumulative areas of braid bars (Majuli excluded),

cumulative channel areas and amount of erosion of the

Majuli Island during 1915-1975, (B) same parameters

plotted during 1975-2005.


86

Fig. 4. Application of ‘method of least squares’ and polynomial curve fitting for the erosion of the Majuli Island during

1915-2005 as a function of changes in the (A) Channel belt area, (B) Cumulative braid bar area change excluding the

area of the Majuli Island, (C) Exclusively channel area change, (D) Reach wise changes in the average width of the

channel belts, (E) Left bank line shift, (F) Right bank line shift. A scatter plot was prepared in ascending order for the

magnitudes of different parameters and second order polynomial curve fitting was done.

Fig. 5. (A) Linear minimum least square relation between the erosion of the Majuli Island and changes in the net

area of the braid bars in the immediate neighbourhood. (B) Linear minimum least square relation between the

erosion of the Majuli Island and the percentage change in the ration of the areas of braid bars and channels in

different reaches.

A

B


87

The effect of right bankline shift (Fig. 4E) and the left

bankline shift (Fig. 4F) show an opposite trend on erosion of

Majuli. Right bankline is very close to the western border of

Majuli. Over the years, due to the human intervention and

migration of the Subansiri River in the further downstream

direction, the right bank channel flow has practically

stagnated and erosion has also decreased rapidly. Thus, higher

discharge accelerates erosion in the right bank and vice versa.

On the other hand, when the left bankline moves closer to the

Majuli Island, force per unit area of discharge cross section

increases and that accelerates erosion of the island. Thus,

there is no doubt about it that the erosion of the Majuli Island

is principally done by the river laterally.

4.3. Effect of erosion of the Majuli Island on the braid bar

formation

This is a general expectation that the erosion of the Majuli

Island is influencing the nature of braid bar formation in the

adjoining channel belt of the Brahmaputra. Accordingly, when

we investigate the relation in the form BB = f (ME) and try to

fit a straight line, for different time durations, we get straight

line equations as following (Fig. 5A):

17.78 + 0.7167 ME (1915-1975)

BB = 48.07 + 0.3975 ME (1975-2005)

BB = 14.38 + 0.6631 ME (1915-2005)

Here, we observe a jump in the distribution pattern of the

points from the status during 1915-1975 and 1975-2005. The

broader temporal change during 1915-2005 is found to be

lying somewhere in the middle. All these curves show a near

parallel and increasing general trend showing erosion and new

braid bar formation has a positive correlation, more erosion

results into increase in the cumulative areas of the braid bars.

However, at zero erosion of the Majuli, during 1915-1975, we

observe a decay in the changing areas of the braid bars and

during 1975-2005, for the identical condition, there was

growth. Thus, there must be other factors responsible for the

braid bar formations.

4.4. Erosion of the Majuli Island and the aggradationary

tendency

When the percentage changes in the BB/CH ratio is plotted as

a function of the percentage erosion rate of the Majuli Island

per unit area, the linear curve fit method gives us the

following equations:

( ) 48.47 + 0.576 ME (1915-1975)

(BB/CH) = 115.9 + 0.286 ME (1975-2005)

(BB/CH) = 7.137 + 0.142 ME (1915-2005)

We observe a remarkable jump in the aggradationary tendency

of the Majuli bearing channel belt of the Brahmaputra

(Fig.5B) from 1915-1975 to 1975-2005. Secondly, the slope

of the curves indicates that there is a positive correlation

between the increase in the rate of erosion of the Majuli Island

and the aggradationary tendency in the corresponding

channelbelt reaches. However, the BB/CH over the period

1915-2005 shows a minimal change, albeit with an upward

tendency, with increasing rate of erosion. Thirdly, the places

where there is no erosion of the Majuli Island; aggradation

continues. This shows that there are other causes of

aggradation. Fourthly, low slope angle of the straight line

indicates even a very high increase in the rate of erosion

brings just an incremental change in the BB/CH. Thus, the

aggradation is influenced only marginally by the erosion of

the Majuli Island.

5. Discussion

5.1 Role of geomorphic parameters in the evolution of the

Majuli Island

We have related different fluvio-geomorphic parameters like

CHB, W, BB, CH, BB/CH, LB, and RB with the rate of

erosion of the Majuli Island in different reaches by measuring

their temporal variations in the planform. The laws evolved

thereby are useful to understand and describe the gross

changes in the geometrical relationships of associative

features. For example, we have observed clearly that,

(a)The Majuli Island as a whole, is not behaving uniformly-

upper, middle and lower Majuli is having significant

differences.

(b) We have also observed that the changes in the channel belt

area (CHB) and the average width of different reaches (W) are

related to the erosion of Majuli in a way that wherever the

river shows a widening tendency, the rate of erosion of the

Majuli Island is considerably lesser. Thus, there is a

competing relationship between the erosion of the banks and

the island. The reaches where the banks are constituted of

loose materials (compared to the banks of the island), the

island remains protected at the cost of increased erosion of the

river bank. However, wherever, the river is forced to flow

through a narrower stretch (stable bank material), the rate of

erosion of the island is greater.

(c) For both the braid bar (BB) and channel (CH) areas, with

the percentage rise of these two parameters, rate of erosion

kept on increasing. However, after reaching a threshold value

of 75%, further rise do not intensify the erosion rate of the

island (Fig. 4B-C).

(d) Also, the trend of BB (Fig. 3A-B) shows clearly that in the

lower Majuli part, aggradation has increased vigorously

during 1975-2005 than 1915-1975. This also indicates that the

variations in the sediment budgeting in different stretches are

quite different.

(e) If we assume that the erosion of Majuli Island is also

acting as a ‘forcing’ to determine the new braid bar formation,

then the statistical analysis shows the result in affirmative.


88

Moreover, the erosion of Majuli is also influencing the

BB/CH ratio and the aggradationary tendency observed more

prominently in the lower Majuli.

5.2. Will Majuli erode completely in the near future?

We have seen from our measurements and comparison of

different geomorphic parameters over the last ninety years that

contrary to the usual belief, every part of Majuli Island is not

eroding uniformly; rather some of the reaches have grown in

area. During 1915-2005, the average rate of erosion of the

Majuli Island was 3.1km2/yr; however, during 1975-2005, the

rate of erosion became 4.4km2/yr. During the same period

(1975-2005), the erosion rates of upper, middle and lower

Majuli were respectively, 16.3%, 5.2% and 32.3%. This

shows clearly the unevenness of erosion and the lower Majuli

is eroding much more severely compared to the rest of the

island. Reach-6, 8 and 9 were showing an overall growth

during the period 1915-2005.

The erosion of the Majuli Island seems to have taken place

both due to the head on impact as well as lateral impact (Fig.

6) but presently left lateral impact seems to be the more

dominating cause. The nature of thalweg migration is an

important parameter that needs to be understood. The

direction along which the Brahmaputra is migrating presently

does not indicate the possibility of complete erosion of the

island in the near future. However, lower Majuli is highly

vulnerable.

6. Conclusion

The overall erosion of the Majuli Island has

increased over time but the erosion is highly uneven

and the lower Majuli is eroding much more severely

compared to the rest of the island.

The statistical method of least squares shows clearly

that the geomorphic parameters like variability in

the channel belt area (CHB), braid bar area (BB),

channel area (CH), average width (W), right bank

line (RB) and left bankline (LB) can be treated as

‘geomorphic forcings’ influencing the erosion of the

Majuli Island. There seems to be a competing

relation between the bank erosion and the erosion of

the island. Reaches where the banks are stable

witness more erosion of the island.

Conversely, the erosion of the Majuli Island can also

be treated as a forcing which is influencing the braid

bars as well as the aggradation-degradation

mechanism of the Brahmaputra channel belt.

Increasing erosion seems to favour aggradation.

Acknowledgement

The author is thankful to IIT Kanpur and Dibrugarh

University, Assam, for providing the institutional support to

conduct this study. Thanks to the India Office Library and

Records, London, UK, for providing the topographic map of

the study area prepared during 1912-1926 seasons. The author

also acknowledges the SAP grant of UGC that helped partly

to do this research.

Fig. 6.The cartoons show some of the possibilities of erosion of the Majuli Island. Each of the possibilities demands independent

redress measures if the relic island is to be saved from complete extinction.


89

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Time period Upper Majuli (28km wide)

Middle Majuli (10km wide)

Lower Majuli (45km wide)

19

15

-19

75

About 44 km2 area eroded which was about

11.4% of the landmass during 1915.The

Brahmaputra Channel belt area (CHB) and

obviously, the average width (W) of the

channel belt were showing a crest and trough

with decreasing trend towards the downstream

direction. Braid bar (BB) area was showing the

same trend with a mild increase of about

6.37%. Channel area (CH) on the other hand

was showing a sharp increase of 59.8%

indicating thereby an overall degradation trend.

Formation of new braid bars much lesser than

the erosion rate of the older flood plain areas

of the Majuli Island.

This portion does not only show

absence of erosion; about 9.6 km2

area (about 15% more than 1915)

was added. This was probably a

direct fall out of the massive

embankment projects undertaken.

CHB, W and CH were showing

increasing trend. BB was showing a

reducing trend of about 31%. Thus,

in spite of the expansion of the

island in this unit, the degradation

trend was common to the upper

Majuli. Check in erosion and

reduction in new braid bar

formation.

A severe erosion of about 113 km2 area;

i.e., about 1.74 km2/y (>34% of the 1915

landmass). CHB, W were showing a

contraction trend. BB decreased by

about 12%; CH on the other hand

increased by 86.7%. This again shows

degradation. Severe erosion of the

Majuli Island as well as reduced

tendency of new braid bar formation.

19

75

-20

05

Erosion aggravated in absolute terms (about 56

km2) as well as in percentage (about 16% of

the landmass of 1975). CHB, W increased. BB

was showing a tremendous increase of about

56.6%. With a crest and a trough, CH

decreased by 13.4%. A clear case of

aggradation. Severe erosion of the Majuli

Island and the simultaneous increase in the

new bed bar formation.

Small but positive erosion; about

5% of the 1975 landmass. CHB, W

decreased showing contraction of

the Brahmaputra in this patch. CH

decreased by about 31% and BB

increased by 32.6%. Aggradation.

Mild erosion of the Majuli Island

and substantial increase in the new

braid bars.

Unabated massive erosion - about

32.3% of the land mass existing during

1975 was eroded; at the increased rate of

2.4 km2/y. Frontal and end part of this

zone shows expansion of the channel

belt with most of the middle region

unchanged. CH decreased by 28.4%. BB

increased massively (53.5%) showing

vigorous aggradation. Severe erosion of

the Majuli Island and vigorous

formation of the new braid bars.

Appendix

Table 1

Temporal and planar trend of erosion of the Majuli Island for the period 1915-2005

Documents

Laws of erosion in Majuli: A statistical approach on GIS based data