C H A PTER 7
CHANGES OF LANDFORM AND THEIR IMPACT ON CHANNEL
AND BASIN MORPHOLOGY
7.1 Changes of Landform
The flood events different magnitudes and durations clearly indicate their
differential impacts on the landform developments (taking flood as geomorphie
agent) along and across the Disang river basin.The change of landforms of the
basin is related to the energy of the rivers. The energy of river, on the other
hand, is controlled by a number of factors. Such factors can change the local
base level, discharge, runoff, etc. An important contribution to the development
of landform as a function of time and space was made by Schumm and Lichty,
1965. The change in development of landforms also depends upon the length of
the time-span in the midst of factorial influences. If there is no major uplift, the
land is slowly worn down and the surface is continually lowered towards the
base level. Over a long period of time the landform slowly loses its energy and
mass which finally reflects the lowering of altitudes. If the streams in a drainage
network remain at grade, the slope of the river bed and the landform attains
dynamic equilbrium. During the stage of grade minor fluctuations may occur.
As a result, cyclic variation in rainfall may cause consequent cyclic changes in
stream flow and sediment discharge and hence minor alteration in the long
profile gradients may occur within a short period of time. A steady state may
then occur, wherein erosion, transportation and deposition have their balance.
The landscape would be slowly changed but due to self-regulating mechanisms
the process involved in the drainage network may be attaining the state of
equilibrium. The basin under study has also undergone changes which can be
explained by considering the above mechanism of landform development, well
evidenced by the profiles (Fig 4.5,a & b) and hypsometric curve (Fig 5.11). In
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the following few paragraphs sheet erosion, shifting of channels, river bank
erosion, sedimentation and lastly the human interference have been discussed
to evaluate and interpret the changes of landform in the Disang river basin.
7.1.1 Changes due to Sheet Erosion
Sheet erosion in the basin is associated with laminar flow of water on the
surface which is smooth or slightly rough. It has been observed that a large
volume o f fine particles are being concentrated along and across the channels of
the Disang river basin during flood times. Setting of more and more sediments
along the channels use to contribute to more and more overland flows during
flood storms. The overland flow creates sheet erosion in the basin. Sheet
erosion, though apparenly simple, actually involves complex process of
sedimentation on land as it is affected by a large number of interrelated factors.
There are various forms of sheet erosion mainly involved in soil. Soil erosion
due to soil creep and sheet wash phenomena occurs on the convex divides and
interfluve areas. Sheet wash, on the other hand, is mainly confined to slopes. It
attains higher intensity on the steeper and steepest slope segments. Sheet
erosion involves two dynamic processes like - (i) the action of rainwater on
weathered rock debris or soil and (ii) the transport of sediment by running
water, etc.
The distribution of sheet erosion depends on the average rainfall, its
impact in response to geomorphic environment of the basin and the condition of
its adjacent areas. Sheet erosion is very high in the hilly areas where the slope
ranges above 25°. The lower reach of a basin comprising plains including the
active floodplain of the basin have comparatively less erosion. Here the
peripheries of marches, swamps, beels and other depressed pockets and areas
have been under constant sheet wash and bank-line erosion. Sedimentation has
been a regular phenomenon on the beels or wetland areas of the Disang river
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basin. Thus most of the lowlying areas o f the basin have gone eliminated which
have now converted into rich agricultural fields. Such type of agricultural fields
is found in Mahmara, Lakwa, Betbari and Disangmukh (middle and lower
reach) areas of the Sivasagar district of Assam.
7.2.2 Changes due to Shifting of Channels
The shifting of channels has caused change of landform characteristics.
A clear picture of channel shifting in the basin has been given in chapter 5. The
channel shifting has changed the fhivio-geomorphic situation and environment
o f the basin. This has also altered the flow dynamics of the river. It is observed
in the field that channel geometry, channel morphology and the overall
hydraulic situation particularly in the middle and lower parts of the Disang river
basin have been changed largely due to frequent channel shifting. The overbank
flow has been a regular phenomenon in the Disang river basin. So, there has
been deposited a huge amount of sand, clay, pebbles, etc. along the beds and
banks of the channels. The river bed becomes loaded with alluvial deposits
during monsoon periods and makes itself high enough above the nearby
agricultural fields.
7.1.3 Changes due to River Bank Erosion
Bank erosion not only along the Disang river but also along the major
tributaries of the Disang river basin has added an important dimension in the
fluvio-geomorphic characteristics of the basin landform. The river Disang has
been flowing with significantly developed meanderings (Fig 4.12) almost in its
entire course.
It is observed that the river has a great tendency to shift its banks. It is
observed in the field and map as well that out of the total 97 major bends in the
plain reach of the Disang river from Dibrugarh district extending to
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Disangmukh in Sivasagar district, as many as 40 concave bends have been
attacked by major to minor bank erosion. Out ot these 40 bends, 10 bends are
having stomgly active erosion mainly in the areas of Mahmora, Lakwa, Betbari,
Rajabari (Near the N.H. 37), etc. of Sivasagar district of Assam.
The process of constant shifting of banks of the Disang river has been
continuing since long past and more so after the great earthquake of 1950.
While the river banks are shifting laterally, the river has been attacked its banks
here and there resulting in active erosion on those places and points. Due to
bank erosion breaches of the embankments has been oceuring regularly. Such
breaches o f embankments cause extensive damage of lives, properties and crops
and land as well.
The basin is located over the structural unstable and geologically
loose rock formations which covers up a zone of very high seismicity with the
occurrences of several earthquakes. Futhermore, the basin contains very high
rainfall in its hilly catchment. Moreover heavy deforestation in the hilly
catchment of the river leads to considerable soil erosion on the slopes of the
hills. The eroded top soil and land debris which are being transported by the
river water in the hilly courses pour into the river beds. This happens only in the
areas where river bed slope is very steep. The transported materials get
deposited in the river bed due to abrupt change of river slope, flow of water, etc.
Slope from a very steep grade to a mild one in the plains leads to meander
development in the plain areas. This ultimately results in bank erosin. Bank
erosion increases the suspended and bed load of the river which are sorted at the
lower reaches of the Disang river and the basin as well. Bank erosion and
deposits have thus changed the landform characteristics of the basin and the
channel particularly in the lower part of the Disang river basin.
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7.1.4 Changes due to Sedimentation
Sedimentation acts as one of the major factors for change of fluvially
developed landforms. Fluvial deposits are of two types - lateral and vertical
accretions. Lateral accretion of deposits result from redistribution of coarse
bedload in laterally shifting channels. Fine-grained material deposits over the
banks during high flood constitute accreation of deposits. In the middle part of
the basin, the channel has some transitory channel deposits mainly of bed load
type. Lateral accreations of channel fill deposits are very common in the lower
part of the basin which is marked by abandoned channel segments. In the
Disang river, marginal and point bar deposits have been distinct few kilometres
downstream from the point of the Tawkak river meeting the Disang river.
7.1.5 Changes due to Human Interferences
Human beings are always recognised as an important agent in the change
or modification of landform. Man can use water of the river,change channel
directions, and construct reservoir, irrigation channels,etc. In these ways,
geomorphology of the channels and basins gets changed.
The drainage basin of the Disang river is a complex one being composed
of different geological materials. It is observed that the upper catchment of the
basin has been ravaged by deforestation which creates geo-ecological problems
in this part of the basin. Further more, for the development of urban settlement
and industrial centres, construction of roads, rock blastings and hill cutting have
abruptly influenced on the eco-system and changed the landform of the basin at
least to a certain extent. The effect of landuse in the hills has a far reaching
impact on the hills. The indiscriminate mass movement caused on the hills and
hill slopes largely affects the flood plain areas of the basin resulting in rapid
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aggradation on drainage channels and silting on the swampy areas,etc. This has
also indirectly aggravated the flood situation in the plain. This phenomenon is
common in the Disang river basin.
7.2 Impact of Landform Changes on Channel and Basin
Morphology
The changes of landforms of the Disang river basin can well be defined
in terms of changes in relief, slope, dissection of topography, drainage density,
frequency, texture and drainage profiles,etc. All these changes have a great
impact on the channel and basin morphology.
The landform of the Disang river basin has developed because of
changes of fluvial dynamics of the river even as the geomorphic foundation was
due to geological activities. It is observed in the plains that downcutting by the
river has been lesser than the lateral erosion. Still then, the river has been able to
transport load. Such a phenomenon has led to widespread deposition of silt,
sand, clay in the valley side slopes and the channel bends too. The valley side
slopes have developed concave profiles (Fig 5.1).
The term channel morphology can be defined as the configuration of
channel and channel landform developed because of dynamics of channel water
flows and sediment passing over the base matrial of the riverine area. The river
system uses to transport water and sediment from the source to mouth. The
channel morphology as a whole is mainly adjusted or is in the process of
adjustment with the elements of downstream areas (Gogoi,2003). It is observed
that erosion and deposition while bringing about morphological and process
adjustment cannot occur if the basin materials are too resistant. Under this
condition deposits may conform the channel to a non-equilibrium position. The
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amount of silt and clay within channel boundaries have also been effective in
determining the depth and width allocation in a particular cross-section
(Schumm,1977). Vegetation cover is also influencing the flows of water and
sediments along the channel. It is observed in the field that vegetation has a dual
action on shaping and changing the river morphology. It can prevent scouring,
bank erosion and protect bed and banks by the way of binding action of
vegetation roots.
The Disang river has been flowing with meanders almost in its entire
plain course and it is observed in the field that the river has a tendency to shift
its banks. It has already been mentioned that there are 97 major bends in the
plain reach of the Disang river and as a result different abandoned channels and
other geomorphic features have been formed mainly in the Sivasagar district.
Moreover, the overbank flow is a regular phenomenon and as a result there has
been a spcetacular deposit of large quantity of sediments like sand, clay,
pebbles, etc. along the beds and its banks of the channels. Such type of
sediments with alluvial deposits makes the agricultural field became fertile.
Moreover, due to the regular sedimentation in the plain reach of the river,
a number of grazing lands have developed giving rise the cattle and goat rearing
in addition to development of human settlements. Such a geomorphicoeconomico-environment existence is best observed in the Disangmukh area (the
confluence of Disang with Brahmaputra).
It has already been mentioned in chapter 5 that the Disang river and its
main tributaries passing over the old alluvial and built up plains in the basin are
more or less unstable and hence the river has created a large number of wetlands
and abandoned channels such as Mara-Disang beel, Dighali beel, Dherua beel,
Phekalai beel, Majumelia beel to mention only a few. The existence of these
wetlands has largely changed not only the geomorphic scenario but also the
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resource potential and economic environment in addition to changing the
overall natural as well as cultural environments. Even though in the negative
side the shifting of channels and creation of wetlands have thrown many of the
human settlements out of past location and characteristic landform to a new one,
the potential resources have come to attract people, specially the schedule castes
people to earn their livelihood from the wetland resources.
In the hilly areas of the Disang river basin due to deforestation, there has
occurred removal of top soils giving rise to huge quantity of sediment that rest
on the river beds and depressed areas of the basin. Thus it affects soil climate
and climate of the region (as a result of deforestation) and some other positive
activities inherent in the hill slopes and tops.
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