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547
Bankline Migration of the Jiabharali River,
North Brahmaputra Plain, Assam
P. PHUKON 1 and RATAMALI MACHAHARY
Department of Geological Sciences, Gauhati University, Guwahati – 781014
1Email: p_phukon@rediffmail.com
Abstract
The Jiabharali, a braided north south trending Himalayan river draining central part of the
north Brahmaputra plain, has undergone significant spatio-temporal changes as revealed by
the study of six temporal data (1967-2006). It is observed that there has been a preferential
eastward bankline migration in some segments of this river while maintaining constancy of
spatial position in other segments. The banklines are stable without significant lateral shifting
between Bhalukpung and Potasali while downstream of Potasali upto the confluence with
Brahmaputra near Tezpur town, the river is highly vulnerable to bankline migration excepting
the section around NT Road crossing where the eastward shift is minimal (~200m) during the
last 40 years.The segment downstream of Potasali is characterized by channel straightening
and widening as well as high degree of braid bar development. During the period of study i.e.,
1967-2006, the channel length has decreased by about 6km between Bhalukpung and Tezpur
while a consistent increase in braiding index from 1.2 to ~2.8 is observed for the same period.
It is suggested that an increased bedload, seasonality of discharge and easily erodable bank
material are the controlling variables for the spatiotemporal changes of this large alluvial river.
Key words: Jiabharali river, North Brahmaputra Plain, bankline migration, erosion
INTRODUCTION
The north Brahmaputra plain, N E India is drained by a number of river systems
flowing from Arunachal Himalaya in the north and debouching into the Brahmaputra in the
south. It is part of an actively subsiding foreland basin with river regime bearing signatures
of subtle neotectonic changes and catchment area tectonics. Bank erosion and bankline
migration has been a major cause of concern for north Brahmaputra plain rivers because of
their high degree of lateral channel shifting by way of bank migration and avulsion. However,
apart from a few snapshots (Barman, G., 1981; Goswami, 1985; Goswami, 1991; Goswami,
et al. 1999; Goswami and Das, 2003; Kotoky et al. 2003, 2005; Sarma et al. 2006) and an
integrated small scale geomorphological and hydrological study of some river basins of
Brahmaputra valley completed by Geological Survey of India (1977) detail quantitative study
on the spatiotemporal variability of these large alluvial rivers are very scanty. In view of
extensive areas being affected by the process of channel migration and bank erosion whereby
MEMOIR OF THE GEOLOGICAL SOCIETY OF INDIA
No. 77, 2011, ISBN: 978-81-907636-2-2, pp. 547-557
548
huge chunks of agricultural land have been lost over the years in Brahmaputra valley
(Deccan Herald, 28th May, 2010), it is imperative to map the spatial variability of the river
regime in large scale for developing a predictive model as well as for better floodplain
management.
Fig.1. Location map showing the course of Jiabharali river in North Brahmaputra Plain, Assam
In this study, taking advantage of available high resolution satellite data from Indian
Remote Sensing Satellites (IRS) and Geographic Information System (GIS), an attempt is
made to develop a quantitative baseline data on bank erosion and migration of the Jiabharali,
a north bank tributary of the Brahmaputra River (Fig. 1) flowing from Arunachal Himalaya.
P. PHUKO N AND RATAMALI MACHAHARY
549
The river follows a north-south course for about 64 km from Bhalukpung at Arunachal
foothills to the confluence with the Brahmaputra east of Tezpur with high degree of braiding
and large width/depth ratio. Along this alluvial reach the river is characterized by frequent
avulsion and channel shifting with number of anabranching channels. As such a major part
of its floodplain is occupied by the channel system itself. In its trans-Arunachal Himalayan
segment the Jiabharali, the precursor of which is known as Kameng, is joined by left bank
tributaries viz., Diju, Nameri, the Upper Khari, and Bor Dikrai and right bank tributaries viz.,
Upar, lower and Nam Sonai, Darikati and the Mansiri.
DATABASE AND METHODOLOGY
Quantitative assessment of bank erosion and bankline migration has been made
from six temporal datasets are during the period 1967 to 2006 (Table 2) with Survey of India
topomap of 1967 as base year for computing the spatial changes. The satellite images viz.,
LANDSAT MSS (1973), LANDSAT ETM+(2000), IRS-P6(2005) and Cartosat(2006)
together with topomaps (1967) were brought into same reference system with LCC projection
and WGS84 datum (Table 1) in the GIS (Geographic Information System) environment.
Banklines of the Jiabharali river is extracted from all the five temporal datasets and are
divided into a number of segments of 2 km each starting from the Assam-Arunachal boundary
to the confluence with Brahmaputra river. The amount and direction of bankline shift for
each segments are measured by overlaying the extracted banklines of 1967-1973, 1973-
2000, 2000-2005, 2005-2006 and 1967-2006. Banklines for the year 1967 is taken as a base
line for analysis of the spatio- temporal variability. Area of erosion and deposition on both
the banks of the river were computed for all the segments by overlay and union analysis of
the extracted bank lines.
Table 1. Database details and reprojection parameters
RESULT AND DISCUSSION
Bankline migration
The Jiabharali river show differential migration of both east and west banks in
different reaches (Table 2, Fig. 2-3). Quantitative assessment in GIS environment for the
period 1967-1973 and taking both the east and west banklines of the Jiabharali river extracted
BANKLINE MIGRATION OF THE JIABHARALI RIVER,
NORTH BRAHMAPUTRA PLAIN, ASSAM
550
from Survey of India topomap of the year 1967 as the base, maximum shift of the east bank
towards east by about 2.6 km is observed in the segment 40 near Sonaibasti whereas the
maximum shift towards west being just about 1.4 km found in the segment 18 near Chotaigaon
at the Arunachal foothills. Similarly the west bank shows maximum shift towards east by
Fig.2. Overlay of banklines of Jiabharali river for the period 1967-2006
about 2.4 km around the Borghat area whereas the maximum shift towards west was about
2.1km in segment 30 near Dharikati. For the period 1973-2000, banklines of the year 1973 is
taken as base data and east or westward shifting of the banks are measured. Maximum
shift of east bank towards east was 2.8 km and towards west was 6.2 km whereas maximum
shift on the west bank towards east was 2.8 km and towards west was 1.3 km. During the
period 2000-2005, the maximum eastward shift of the east bank was 2.9 km whereas
westward shift of the bank west bank was 0.6 km. In just one year (2005-2006) the bankline
migration shows maximum shift of east bank towards east by about 0.1km and towards
P. PHUKO N AND RATAMALI MACHAHARY
551
west by about 1.1km. During the same time the west bank shifted towards east by 1.4 km
and towards west was 0.04 km . The maximum net east and westward shifting of the east
bank between 1967-2006 is found to be about 2.9 km near Silanighat and about 2.5 km near
Panchigaon respectively. Simultaneous shifting of the west bank towards east and west
respectively are found to be 2.6 km at segment 40 near Sonaibasti and 1.9 km in segment 30
near Potasali towards west. Overlay analysis of the six temporal datasets (1967-2006)
reveals that the segment near Potasali area shifted its bank towards the west of the channel
whereas segment between Sengilimari, to Mornakuri shows shifting of its channel towards
the east. This period also shows increase in width of the channel. In the segments between
Potasali and Silanighat the river shows straightening of its course as well as higher frequency
of braiding. At the lower segment from Mornakuri down to the debouch point the channel
shows a decreasing trend in width between 1967 to 1973 while reversing to the increasing
trend from 1973 to 2006.
BANKLINE MIGRATION OF THE JIABHARALI RIVER,
NORTH BRAHMAPUTRA PLAIN, ASSAM
2500
2000
1500
1000
500
0
500
1000
1500
2000
2500
3000
Distance in km from Assam-Arunachal boundary along east bank of Jiabharali of 1967
Bankline shift during 1967-2006 (East bank)
Towards East
Towards West
Bankline shift in m
3000
2500
2000
1500
1000
500
0
500
1000
1500
2000
2500
3000
Bankline shift during 1967-2006 (West bank)
Distance in km from Assam-Arunachal boundary along west bank of Jiabharali of 1967
Towards West
Towards East
Fig. 3. Quantum of bankline shifting of Jiabharali River during 1967-2006. A. east bank ; B. west
bank
552
The study shows that within the period of study the Jiabharali banklines have shown
variable amount of both east and westward shifting. A comparison of the temporal datasets
suggests a preferential shifting towards east in recent past. However, there was a major
avulsion through reactivation of the lower segment of earlier Marabharali river downstream
of Panchmail in the year 2005. This has diverted a major part of the flow from Jiabharali and
the channel is now causing active erosion in the eastern part. With this avulsion the west
bank of Jiabharali is also effectively shifted by more than 5 km in this segment. In contrast
to an overall active erosion and depositon between Potasali and the debouch point east of
Tezpur, the segment between Bhalukpung and Chotaigaon (Garo Basti) and around the NT
Road crossing the channel maintains consistency with negligible bank migration.
Table: 2 Bankline shifting of east and west bank of Jiabharali river during 1967-2006
P. PHUKO N AND RATAMALI MACHAHARY
553
Bank erosion
The area of erosion and deposition of each segment and for the east and west bank
of the river are determined from overlay and union analysis for the time periods 1967-1973;
1973-2000; 2000-2005; 2005-2006; 1967-2006. The net deposition during 1967-1973 is about
708 hectares in the east bank and about 1112.0 hectares in the west bank. In the year from
1973-2000 the river shows net result of erosion of 961hectares and 1244.6 hectares for east
and west bank respectively.Considering the whole time spectrum of analysis between 1967
and 2006 the Jiabharali river shows the net effect of deposition of 144 hectare and 17
hectare on the east and west bank respectively (Table 3-4).
It is observed (Table 3 and Fig. 4-5) that while the bank erosion was predominant in
some segments, deposition was overtaking in others. In contrast some other segments showed
both erosion and deposition. Area of erosion and deposition of the period 1967 to 1973 is
measured in different segments at an interval of 2 km each both for east bank and
Fig.4. Area of erosion and deposition along the Jiabharali river between 1967-2006
BANKLINE MIGRATION OF THE JIABHARALI RIVER,
NORTH BRAHMAPUTRA PLAIN, ASSAM
554
west bank between Poatasali-Sengelimari- Dharikati- Sonaibasti and Silanighat - NT road
crossing. Significant deposition on the east bank took place near Chotaigaon (Garobasti)
area on the foothills of Arunachal Hills. Against a total of 1397 hectares lost due to erosion
during this period net gain by deposition was 3216 hectares thus deposition being significantly
Table 3. Bank erosion and deposition of Jiabharali river in individual segments
P. PHUKO N AND RATAMALI MACHAHARY
555
higher than erosion. During 1973-2000, 3954 hectares of area is added to the river by erosion
whereas 1748 hectares of area is filled up by the sediments. As such there was more erosion
Fig. 5. Quantum of erosion and deposition by Jiabharali River during 1967-2006. A. east bank ; B.
west bank
compared to the period 1967-73. Erosion-deposition activities are minimal between
Bhalukpung and Potasali, but further downstream erosion and fill on both the banks of the
river are substantial. From Potasali to Silanighat the river shows erosion on the east bank
around the areas Sonaibasti, Lokra and salanibasti and deposition on the west bank shifting
the bankline both east and west bank towards the east. On an average the area downstream
of the NT Road crossing shows more erosion with minor deposition. Erosion vulnerability is
more pronounced along the west bank from Mansirigaon to Gotlong where a recent avulsion
has shifted the bankline by more than 5 km. The erosion induced bankline shifting between
the NT Road crossing to debouch point east of Tezpur has also resulted in overall increase
of width of the river during 1973 – 2000. During the period 2000-2005 small scale erosion
affected the places near Patgaon, silanighat and east of Mansirigaon, Kholihamari, Kuruwani,
Panchmail gaon whereas deposition of the bank is experienced at few places on the west of
Panchigaon and Panpur Ghat. During this period the total gain from deposition was 420
hecatre whereas erosion loss was about 723 hectare. During 2005-2006 the river shows
negligible erosion with a total eroded area of 192 hectares both on the east and west bank.
Sediment filling was mostly confined to near Potasali on the east bank and Rajghar and
Mansirigaon on the west bank. The total deposition during this period was about 1198 hectare.
From the overlay and union analysis of the banklines for the entire study period (1967 –
2006) it is observed that the river has effected significant erosion and deposition of its banks
BANKLINE MIGRATION OF THE JIABHARALI RIVER,
NORTH BRAHMAPUTRA PLAIN, ASSAM
556
towards east or west. Near Chotaigaon (Garobasti) the east bank of the river experienced
deposition of the sediment whereas the west bank of the river is almost stable. Between
Potasali and Dharikati deposition was more along the east bank whereas the west bank of
the river experienced more erosion towards west and thus shifting both the banklines towards
west. Between the Dharikati to Silanighat, west bank of the river experienced more deposition
while the east bank was subjected erosion, thus shifting the banklines of towards the east of
the river. The segments between NT Road crossing to Mansirigaon experienced deposition
on the west bank and erosion on the east bank shifting the bankline towards the east of the
river. The segments between Mansirigaon to debouch point measured eroded area on the
west bank and alternate deposition and erosion on the east bank of the river. In the segments
near the debouch point the banklines show shifting towards the west bank. The total eroded
area during this time period was 3578 hectare whereas the deposited area is was 3895
hectare. These results suggest that although there has been variable amount of erosion and
deposition along the reaches of Jiabharali river, the 40 years period covered by this study
suggest a rough balance between erosion and deposition. As such it is plausible that there
has been no significant increase in sediment flux from the source region and only reworking
of floodplain sediment is taking place.
Table 4. Net area of erosion and deposition along Jiabharali river during 1967-2006
SUMMARY AND CONCLUSIONS
In its alluvial reach the Jiabharali river has been subjected to significant spatio-
temporal variability resulting changes in morphology and hydraulic geometry as revealed
from study of six temporal datasets between 1967 and 2006. It is observed that there has
been a preferential eastward bankline migration in a few segments of this river while
maintaining constancy of spatial position in other segments. The banklines are stable without
significant lateral shifting between Bhalukpung and Potasali while downstream of Potasali
the river is highly vulnerable to bankline migration excepting the section around NT Road
crossing where the eastward shift is minimal (~200 m) during the last 40 years. In this
segment the river is characterized by channel straightening and widening as well as high
degree of braid bar development which is reflected in changes of braiding index (Brice,1964)
during 1967-2006 from about 1.3 to 2.8. During the period of study the channel length is also
P. PHUKO N AND RATAMALI MACHAHARY
557
found decreased by about 6 km between Bhalukpung and Tezpur. Bankline migration and
erosion-deposition activities are more pronounced in the east bank of the Jiabharali. However,
presently the river shows a tendency of westward shifting as well. The westward migration
however, is expected to be limited by the palaeobank of Marabharali river charcterized by a
well developed stable and partially indurated natural levee.
The Jiabharali river downstream of NT road crossing is highly vulnerable to bank
erosion and bankline migration. Overall erosion and deposition are nearly balanced during
last 40 years plausibly due to sediment generation mainly through floodplain reworking.
However, bankline migration effected through bank erosion and avulsion, is significant in its
entire reach downstream of Potasali. Differential eastward or westward shifting of the
banklines suggest a present tendency of change in planform geometry of the river.
ACKNOWLEDGEMENTS
This write up is part of the work carried out under the DST, Govt. of India sponsored
research project SR/S4/ES-21/Brahmaputra-II/2005 (P-1).
Reference
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BANKLINE MIGRATION OF THE JIABHARALI RIVER,
NORTH BRAHMAPUTRA PLAIN, ASSAM
