233

Chapter VIII

A CASE STUDY OF BHOGDOI RIVER BASIN

8.1: Recent History of the River

The river Bhogdoi is a small but perennial river coming down from the

foothills of Assam-Nagaland border into the plains of Assam and finally pours into the

Brahmaputra. The course of the river has undergone tremendous changes due to

physical and anthropogenic factors during the last two centuries. In view of this, it is

realised that a detailed chronological study of these changes will enable us to

understand the behaviour of the river and relevant issues. Many of these changes are,

however, associated with the historical events.

The historical documents of Assam describe the river as Disoi. The river also

finds its reference and recognition in many of the Bihu songs prevailing in the state.

Each and every Assamese person knows that the Bhogdoi flows through Malow

Pathar, an extensive paddy field in the southern plain of Brahmaputra, noted for its

bumper rice production since time immemorial.

8.1.1: Diversion of the River Course

Historically, river Bhogdoi had been a tributary of Brahmaputra. Under

circumstances, Bhogdoi lost its direct link with the Brahmaputra and in today’s

context it is no longer a tributary of the mighty river.

During reign of Ahom king Swargadew Kamaleswar Singha (1769-1810) a

new channel was dug up from Disoi river to Kaliani river by Purnananda

Burhagohain. The digging of this new channel was an attempt to bring communal

harmony among the various groups of people, which got slackened following a long

234

rebellion by the Mowamoriya, Kachari, Moran and Singphow communities residing in

the kingdom (Gohainbaruah, 1937, p. 185). After the end of the rebellion and in 1795,

the capital of the Ahom Kingdom was shifted from Rongpur to Jorhat through which

river Bhogdoi traversed. It can be assumed that the digging of the new channel took

place during the last decade of the 18th

century or the first decade of the 19th

century.

Fig. 8.1: Location of the Bhogdoi basin in the study area

In view of the socio-political unrest following the rebellion, the capital of the

kingdom was temporarily shifted from Rongpur to Jorhat. The King and Purnananda

Burhagohain intended to shift the capital back to Rongpur soon after the settlement of

the rebellion. But owing to certain reasons the idea could not be materialised. As such,

Purnananda Burhagohain formulated a plan to develop Jorhat as the new capital of the

kingdom. The Na-ali, Cheuni-ali, Kamarbandha-ali, Raja Bahar ali are some of the

235

roads built by him as a part of his plan. In order to solve the problem of drinking water

several tanks were dug in and around Jorhat and the river Disoi was diverted to flow

by the side of the Disoi Rajabahar (royal rest house) at Jorhat (Barbaruah, 1981, p.

287). This is how Bhogdoi underwent a diversion of its course in the plains of Assam.

It can be added here that the name ‘Jorhat’ was coined only after the shifting

of the capital from Rongpur to it. In fact, the word ‘Jorhat’ means a couple of hats

(weekly markets). There existed two weekly markets in the area presently occupied by

the city of Jorhat. These were known as Chaki-hat and Machar-hat and both of them

were on the bank of Disoi river. The area around these two weekly markets was given

a new name as ‘Jorhat’ and it became the last capital of the Ahom kingdom.

8.1.2: Renaming of the River

The historical sources stated above, however, say that the people engaged in

the digging operation were offered Bhog (religious meal) by the spiritual icons locally

known as Gosain-Mahanta-Adhikars. The Ahom king, the Burhagohain (the chief of

the three ministers of the king) and the Barbaruah (one of the civil officers of the

king) also offered meals to the workers engaged in digging. It was for the offering of

Bhog, the new channel was named as Bhogdoi (Gogoi, 1985). This is how river Disoi

became Bhogdoi.

8.1.3: Impact of the Great Earthquakes

Being located very close to two converging tectonic plate boundaries, Assam

and entire North East India come under a highly active seismic zone. Several great

earthquakes rocked the area in the past. Usually, an earthquake having a magnitude of

8 or above in the Richter scale is considered to be a great earthquake. Such an

earthquake with magnitude 8.7 in the Richter scale rocked Assam and the adjoining

236

areas on 12 June, 1897. Wide spread devastation took place covering an extensive

area. Historical records say that the Kilakuti embankment, located to the east of

Kokilamukh was washed away by the rising water of Brahmaputra following this

great earthquake. The rising water of Brahmaputra also washed away an extensive

area on its southern bank and developed several bils (wetlands) in the lower Bhogdoi

basin. Few of these wetlands were Ouana-bil, Shalkura-doba, Ghata-kobowa, Na-

lakhai and so on (Sharma, 2008, p. 55). The river was, however, continuing its flow

through these newly developed wetlands to arrive at the Brahmaputra. In course of

time, the newly developed bils (wetlands) became interconnected and turned into a

network or system of wetlands. The course of Bhogdoi had to pass through this

network of wetlands to reach the master stream.

According to a survey conducted in 1917, the channel of the Bhogdoi was said

to be lost in a system of wetlands (Sharma, 2008, p. 55), but its mouth to Brahmaputra

was active. Bhogdoi had been pouring its discharge into the network of the wetland

and the excess water of the network had been flowing to Brahmaputra through its

mouth.

8.1.4: Flood and Embankments

Flood in Brahmaputra during summer has been a common phenomenon since

long back, but written record of flood of the Brahmaputra and its tributaries dates back

to 13th

century (Bora, 2001). Flood in the state, in fact, took the form of a disaster only

after 1950. Another great earthquake measuring 8.7 in the Richter scale rocked Assam

and the entire North East India in the eve of 15th

August, 1950. Extensive landslips on

the Himalayan slopes and loosening of the soil, subsidence and fissuring of ground in

the valley including river beds, increased sediment load of the rivers, change in the

course and configuration of the mainstream as well as tributaries were some of the

237

outcomes of the earthquake (Dutta, 2001, p.84). The rise of the bed of Brahmaputra

after the great earthquake increased the intensity and frequency of flood in the plains

of Assam. The recurrent floods started to occur in Brahmaputra and its tributaries after

the earthquake. This put the people living near the rivers into misery and sufferings.

Prior to 1950 earthquake, the necessity of construction of embankments along the

banks of the rivers was seldom felt in the state. But, the earthquake made it inevitable.

Such an embankment aligned along the southern bank of Brahmaputra blocked the

course of Bhogdoi in 1956. It also closed the link between Brahmaputra and the

network of wetlands. Thus Bhogdoi underwent a dissection of its course by the

embankment little ahead of its mouth. The lower segment of the channel of Bhogdoi

still exists at places on the other side of the embankment. Such a segment is

stretching from Upar Garumara to Banhphala village (12 km north-west of Jorhat

city). People use to call it as Mora Disoi (dead Disoi). Segments of abandoned channel

of Bhogdoi can also be seen in an area north-west of Badulipar in Golaghat district.

8.1.5: Aggradation of the Wetlands

The materials carried by Bhogdoi started to be deposited in the wetlands of the

area. Prolonged siltation made most of them shallow and led to extinction. Only few

are surviving as remnants. Topographical sheets showing the area between the

Brahmaputra and the Jorhat city (83 J/1) gives a distinct idea about the presence of a

network of wetlands in the area. In order to investigate the matter number of elderly

people (70 years or above) living in the villages near the Brahmaputra were

interviewed. The people said that they witnessed several bils (wetlands) in the area

during their teenage. Long continued siltation led nearly all of these bils to extinction.

238

The erection of the embankment along the bank of Brahmaputra in 1956,

which closed the channel of Bhogdoi, accelerated the process of siltation in the area. It

was because the materials carried by Bhogdoi and other small streams, which come

down from the hills of Nagaland could not arrive at the master stream as the

embankment has been acting as a hurdle on the way. All these areas, which were

previously the wetlands and noted for plentiful fishes, are now highly productive

paddy fields suited mostly for wet paddy cultivation.

The dissection of the course of Bhogdoi by the embankment as well as gradual

aggradation of the wetlands made Bhogdoi to pour its discharge into Khalihamari bil,

an extensive wetland which had its existence until the fifties. The elderly people of the

area still remember their association with it. Some of them were roaming in and

around the Khalihamari bil as buffalo-keeper. Others used to visit the bil for fishing

and hunting of various birds. The wetland, which was known to have depths of 8 to

10.5 m (25 to 35 feet) at places some 60 years back from today is now totally

aggraded and levelled up by the deposits of Bhogdoi. Now, a middle sized bamboo

plantation can be seen in the area which was occupied earlier by a water body. The

Khalihamari bil, which was earlier an inaccessible area and known for fishes, wild

animals and various birds is now a highly productive area with intermittent human

settlement. The channel of Bhogdoi meanders through it. The Survey of India

topographical sheet No. 83 J/1 distinctly shows the Khalihamari bil in its north-

western margin.

The Khalihamari bil, which became the terminus of Bhogdoi had an outlet to

river Kakadonga. It flows through the western margin of the Khalihamari bil and it

had its mouth to Gela bil, an abandoned channel of Brahmaputra, which took off from

that river and rejoined it at a point lower down in its course (Alen, 1906). Nothing

239

could be known as to when and how the upstream end (eastern end) of Gela bil was

closed. It could only be assumed that Gela bil had its upstream end (eastern end)

somewhere near Kokilamukh-Nimati Ghat area. When and how Gela bil came into

existence and how its eastern end came to a closure need a thorough geographic

investigation.

The Khalihamari bil had an outlet to Kakadonga. The excess water of the bil

flew down to Kakadonga through this outlet. This outlet met the Kakadonga near its

confluence with Gela bil. As time passed, the Khalihamari bil started to be aggraded

by the materials carried by Bhogdoi. Little downstream of this confluence, there was

another confluence where the joint river (Bhogdoi and Kakadonga together) met Gela

bil. It flows from east to west through Negheriting, Dregaon, Badulipar and finally

meets river Dhansiri at a place called Kuruabahi Rangagora. The distance from this

confluence to the mouth of Dhansiri with Brahmaputra is only two and half km.

8.2: Basin Characteristics

A drainage basin is defined as the area which contributes water to a particular

set of channel of different orders. It forms a convenient unit for the consideration of

the processes determining the formation of specific landscape in various regions of the

earth (Leopold et al, 1964, p. 131). At the same time, a river basin might best be

considered to have a heritage rather than an origin. It is like an organic form, the

product of a continuous evolutionary line through time (Leopold et al, 1964, p. 421).

The geomorphologists treat basin as a geomorphic unit. It is so because its

characteristics can be measured quantitatively and it provides a basis for analysis,

comparison and classification of the geomorphic elements. In an ideal basin all the

three stages of landform development can be visualised in a distinct manner. The

stream channels of the drainage basin and the landforms they drain and mould are

240

bound together in a close causal relationship (David et al, 1982). W.M. Davis linked

streams to the veins of a leaf, and the drainage basin to a whole leaf. A drainage basin

can be treated as a working system with energy inputs of sunlight and precipitation,

and outputs of stream discharge and load (McCullagh, 1983, p. 16).

The basin of the river Bhogdoi includes the geographical territories of both

Assam and Nagaland. The part of the basin in Nagaland is formed by low hills not

exceeding 1400 m above msl (mean sea level) while the part in Assam is

topographically plain. As a geographic entity the Bhogdoi river basin links two

important physiographic divisions of North East India – the Tertiary hills of the Naga-

Patkai range on the south and the floodplains of Brahmaputra on the north. It is really

a highland-lowland interacting system representing the most dynamic and sensitive

linkages among the elements of ecological units within the hills and the plains. Any

change in the highland environment brings proportionate change to the lowland

environment (Bora, 1998). In this context the study of the river and its basin is very

relevant.

From the source to mouth, the total length of the channel of Bhogdoi is 162.5

km. The first 100 km of the channel from the source is in the hills and it constitutes

61.50 per cent of the total length of the channel. The length of the river channel in the

plains is only 62.50 km and it constitutes 38.50 per cent of the total length of the river

channel. Traversing for a length of 162.50 km and draining water from an area of

945.88 sq km, river Bhogdoi pours its discharge to Kakadonga.

241

Fig. 8.2: The Bhogdoi basin

Bhogdoi basin lies between river Janji on the east and river Kakadonga on the

west. Geographically, the basin lies between 26º17′17″ and 26º49′22″ north lines of

latitudes and 94º1′30″ and 94º29′2″ east lines of longitudes and covers an area of

945.8750 sq km including both plains and hills within it. Of this, 537.60 sq km spans

over the foothills of Assam-Nagaland border, which constitutes 56.78 per cent of the

total area of the basin. Only 408.28 sq km area of the basin falls on the plains of

Assam and it constitutes 43.22 per cent of the total area of the basin.

242

The shape of the basin is very peculiar. Its central portion is highly squeezed

and twisted giving it the shape of the letter ‘V’ with its pointed tip eastward (Fig. 8.2).

Here, the basin has a width of only four km. This width gradually increases towards

north. A general westward slope, which exists in the southwestern part of Jorhat

district compels several streams of the area to flow westward to meet Kakadonga. This

pushes the water divide between Bhogdoi and Kakadonga towards east. The Janji

basin on the east also protrudes into the Bhogdoi basin from the east. Thus the central

portion of the basin becomes very narrow. Away from this point, the width gradually

increases towards north and south.

8.2.1: The Hilly Course

The Bhogdoi river has its source in the Mokokchung district of Nagaland. Its

source has an elevation not exceeding 1400 m above msl. So, it is totally a rain fed

river. Its source is the point where 94°27΄23˝ east meridian and 26°29΄6˝ north parallel

meet each other. Near the source the river is known as Tsujenyong Nala. The first

three and half kilometre the river flows from south to north following the steep slope

of a hill ridge. Then it creates a valley between two parallel ridges. The valley has an

elevation between 200 m to 300 m above msl. The channel flows nearly 25.5 km

along the valley from north-east to south-west. At this point, the channel of the river

crosses the 200 m contour and takes a sharp hair-pin turn. After this it starts to flow

from south-west to north-east. As depicted in the Survey of India topographical Sheets

bearing No. 83 J/6 and 83 J/7, this part of the channel acts as the inter-state boundary

between Assam and Nagaland.

In the hills, a large number of streams meet the channel of Bhogdoi on both

right bank and left bank. These all develop into a complex drainage network in the

243

hilly portion of the basin. From the source of Bhogdoi to its mouth, the tributaries

meeting the river on its left bank are Chitit nala, Tipu Ayong, Longnak nala, Ingtsu

nala, Tzusemsa nala, Aotzu nala, Jongchen nala, Tesangba nala, Borokchu nala,

Thichu nala, Kikenchiven nala, Kikanchu nala, Atemtomchu nala, Inchita nala, Piphi

nala, Japu ghoki, Pukalo ghoki, Ajekheki ghoki, Alulipha ghoki, Longba nala,

Pongkhong nala, Kungkhung nala, Litsumchung nala, Wanglak nala, Kurali nala,

Watichang nala, Amonghen nala and many others. Those meeting it on its right bank

are Chiyong Nala, Tsumeren nala, Tsurung rung, Champi nala, Lampi nala, Abaki

ghoki, Metsutsung nala, Azuphapa ghoki, Zunachi ghoki, Alupakiyi ghoki, Akula

ghoki, Mangat tsu, Ailang nala, Nanaka tsu, Shsrsshapa tsu, Changsang tsu, Tikhang

tsu and many others. Some of them are seasonal while some others flow all round the

year. Inside the territory of Nagaland, the channel of Bhogdoi is known by different

names at different places. The entire hilly course of the river traverses through the

terrain dotted with scattered human settlements and covered by thick vegetation cover

which is now degraded due to human interference.

8.2.2: The Plain Course

The elevation of the point, where river Bhogdoi enters the plains of Assam is

140 m. This place is called Nagajanka located at a distance of six kilometre from

Mariani. In fact, Nagajanka is a hilly stream that meets Bhogdoi at that place and thus

the place is named after the stream. Coming down to the plains of Assam, the river

starts to flow northward until it reaches Jorhat city. From Nagajanka, the river has

developed several cut-off lakes along its plain course. The plain course of the river

passes through an area dotted with a large number of villages and two urban centres

viz. Mariani and Jorhat.

244

In its plain course, the river has only two major tributary streams which meet it

on the right bank. These are Kaliapani jan and Cheni jan or Ranga jan. In the plain

course, the water divide passes very close to the river channel and the western bank is

wider than the eastern bank. In the plains, the eastern bank is nowhere wider than 4

kilometre from the river channel and the water divide moves nearly parallel to the

river channel.

On the western bank of the channel in its plain course, the Dhala jan is the

major stream that meets Bhogdoi. The Saru Soikata jan and Bar Soikata jan are the

two streams that join together to form Dhala jan. It flows northward and its

downstream reach is known as Bar Dholi. Before meeting the channel of Bhogdoi,

Bar Dholi embodies Saru Dholi. These two have developed a system of streams that

pours the joint discharge to Bhogdoi near its mouth. Few other streams like Tarajan

(its downstream reach is called Saru Charai jan), Rawriya jan, Tocklai jan also meet

the plain course of Bhogdoi on its left bank. The part of the Bhogdoi basin in the

plains shows that all the streams flow from south-east to north-west direction. This

clearly indicates that there exists a general north-westward slope in the area.

8.2.3: The Longitudinal Profile

Rivers are linear systems which show a gradient of characters along their

length. Ideally the longitudinal profile of a river is concave with a steep upper portion

near the source, giving way to reaches of progressively less gradient as the mouth is

approached. Other features of the river are associated with this progression. An

attempt is made here to analyse the longitudinal profile of Bhogdoi river. The length

of the channel is measured against elevation categories at 100 m interval. The river

has its source above 1200 m from the msl. The local base level of the river at its

245

mouth is 80 m. It is for this reason the first elevation category is considered from 80 m

to 100 m.

Table 8.1: Lengths of channel against the elevation categories

Elevation

category

(m)

Length of

the channel

(km)

% to total

length of

the

channel

Length below

the elevation

(km)

Cumulative %

of the length

below the

elevation

Gradient

(cm per km)

80-100 42.50 26.15 42.50 26.16 47.05

100-200 87.50 53.85 130.00 80.00 114.28

200-300 27.50 16.92 157.50 96.93 363.63

300-400 2.00 1.23 159.50 98.16 5000.00

400-500 1.00 0.62 160.50 98.78 10000.00

500-600 0.75 0.46 161.25 99.23 13333.33

600-700 0.25 0.15 161.50 99.39 40000.00

700-800 0.25 0.15 161.75 99.54 40000.00

800-900 0.25 0.15 162.00 99.68 40000.00

900-1000 0.20 0.12 162.20 99.81 50000.00

1000-1100 0.20 0.12 162.40 99.93 50000.00

1100-1200 0.10 0.06 162.50 99.99 100000.00

Source: Measured from Topographical sheets

Table 8.1 shows the length of the channel against the elevation categories. It is

observed that the river has 87.50 km of its channel length between 100 and 200 m

elevation, which forms 53.85 per cent of the total length of the channel. This segment

of the channel encompasses both the plains and the hills. The channel section above

the elevation of 140 m is in the foothills and the remaining section falls in the plains.

Starting from the source, the first 100 km of the channel (61.50 per cent of the total

length) is in the hills and only 62.5 km (38.50 per cent of the total length) is in the

plains.

246

Fig. 8.3: The longitudinal profile of river Bhogdoi

As revealed in table 8.1 nearly 96.93 per cent of the total length of the channel

is below 300 m elevation and the remaining 3.07 per cent of the channel is above the

elevation of 300 m from msl. This 3.07 per cent representing only 5 km of the total

length of the river channel confined between elevations of 300 m and 1200 m. It thus

indicates a very steep gradient of the channel (Fig. 8.3). Again, more than 99 per cent

of the total length of the channel is below the elevation of 600 m and nearly one per

cent of the total length of the channel lies above that elevation. The gradient values

presented in the table 8.1 reveal that the river has very high erosion potentiality. The

channel gradient has its direct relationship with its erosion potentiality. As such the

erosional activities of Bhogdoi show a conspicuous difference above and below the

elevation of 300 m contour. Figure 8.3 represents the longitudinal profile of river

Bhogdoi.

0

200

400

600

800

1000

1200

0 40 80 120 160

ALT

ITU

DE

(m)

CHANNEL LENGTH (km)

LONG PROFILE OF RIVER BHOGDOI

247

The lower reach of the channel with a length of 62.5 0 km flows through the

plains of Brahmaputra and the elevation of this section is below 140 m. This channel

section shows a concave profile and its segment above 100 m has a gradient of 114.28

cm per km and the section below has a gradient of 47.05 cm per km. The fall of

gradient from 114.28 cm per km to 47.05 cm per km in the plain course is due to large

scale deposition of sediments on the river bed causing bed aggradation.

8.3: Hydrological Parameters of the Basin

The Bhogdoi basin encloses a geographical area of 945.8750 sq km

with a basin perimemer of 232 km. As we know that

Hence, the basin circularity ratio measures to be 0.2450. This indicates that the

basin is an elongated one in its shape.

The hilly portion of the basin shows a denser network of streams than its

counterpart in the plains. Considering the whole basin, the total length of all the

streams (including the main channel) of the basin is 2043.50 km and thus the drainage

density is estimated at 2.16 km per sq km. When the drainage densities are worked out

separately for the hilly and plain portion of the basin, it appears that the drainage

density of the hilly portion is nearly four times (actually 3.60 times) greater than that

calculated for the plain portion of the basin. This indicates a considerable difference of

drainage densities between the upper and lower Bhogdoi basin. The drainage density

of the hilly part is worked out at 3.14 km per sq km. On the other hand, the plain

portion of the basin has the drainage density of 0.87 km per sq km

248

Bhogdoi has an average annual water discharge of 6072 m3s

-1 (Bora, 2001).

The hydrologic regime of the river responds to the rhythm of the south-west monsoon.

Meteorological data received from Tocklai Experimental Station of the Tea Research

Association from 1969 to 2005 reveal that the part of the basin on the plains of Assam

receives an average of 1983.80 mm annual rainfall. Bhogdoi basin receives 87 per

cent of the total annual rainfall from April to September (NBSS&LUP, 2004). Hence,

only 13 per cent rainfall is left for the winter months. On the other hand, rainfall data

received from the Tocklai Experimental Station indicate that 35.55 per cent of the

total annual rainfall is received in the months of July and August every year (Fig. 8.5).

It implies heavy concentration of rainfall in the summer months with more than one

third of the total annual rainfall occurring in two months only. Bhogdoi is entirely a

rain fed perennial river and it flows all throughout the year. However, there exists

great variation between the water discharge and water levels of summer and winter.

The Universal Soil Loss Equation (USLE) is an erosion model for prediction

of long-term average annual soil loss from a field area with specified cover and

management conditions. The equation predicts the soil loss due to sheet and rill

erosion. It computes soil loss for a site as a product of six factors. The soil loss

equation is-

A = R.K. L S.C.P

Where,

A = Soil loss per unit area (computed) which is expressed in tons/hectare/year.

R = Rainfall erosivity factor which is the number of rainfall erosion index units for a

particular location.

249

K = Soil erodibility factor which is the soil loss rate per erosion index unit measured

on a unit plot of 22.13 m (72.6 ft) length supposed to have uniform 9 per cent

slope continuously in clear-tilled fallow.

L = Slope length factor which is the ratio of soil loss from the field slope length to that

from a 22.13 m (72.6 ft) length under identical conditions.

S = Slope-steepness factor which is the ratio of soil loss from the field slope gradient ti

that of a 9 per cent slope under otherwise identical conditions.

C = Cover and management factor which is the ratio of soil loss from an area with

specified cover and management to that from an identical area in tilled

continuous fallow.

P = Conservation practice factor which is the ratio of soil loss with a support practice

like contouring, strip-cropping, or terracing to that with straight-raw farming up

and down the slope.

Derived information on the factors of erosivity (R), soil erodibility (K),

topographic factor (L & S), cover and management (C) and conservation practice (P)

are available with the National Bureau of Soil Survey and Land Use Planning

(NBSS&LUP). This organization has generated quantitative data of soil loss covering

the state of Assam using USLE. Based on soil loss estimates the Bureau divides

Assam into six classes of erosion hazards as shown in table 8.2.

The area of the Bhogdoi basin comes under the very severe soil loss category.

As much as 82 per cent of the area of the Bhogdoi basin comes under very severe soil

loss class, where the rate of soil loss is more than 40 tons per hectare per year. The

remaining portion of the basin comes under moderately slight class with the soil loss

ranging from 5 to 10 tons per hectare per year. These results are evident from the

measurements made on the map prepared by NBSS&LUP.

250

Measurement of area covering the entire study area under the above soil loss

categories reveals a great deal of valuable information. It is noted that 56.47 percent of

the study area falls under very severe soil loss class (Fig. 8.4). This class dominates

the southern portion of the study area. The entire foothills zone comes under the very

severe soil loss class. The inclusion of the foothills zone under the very severe soil

loss category affirms the preconceived idea that the land cover change in the foothills

has a direct link to the soil loss in the area. Severe soil loss in the area is the direct

outcome of wide spread deforestation and human interference in the foothills.

Table 8.2: Soil loss classes for Assam

Soil loss class Qualitative description Quantitative description

I slight Below 5 tons/ha/year

II Moderately slight 5-10 tons/ha/year

III Moderate 10-15 tons/ha/year

IV Moderately severe 15-20 tons/ha/year

V Severe 20-40 tons/ha/year

VI Very severe Above 40 tons/ha/year

Source: NBSS&LUP

The soil loss in the foothills is also reflected in the aggradation of the channels

of the rivers coming down from the foothills. As the loose soil materials finally come

down to the rivers, these are carried down by the running water to some distance. Near

the foothills, the river channels show a steeper gradient (table 8.1) indicating higher

load-carrying potentiality. With increasing distance from the foothills, the load

carrying capacity of the rivers decreases as the gradient of the channel drops. This

causes siltation of the suspended soil materials on the river bed leading to rise of the

river bed. With regard to aggradation of the Bhogdoi channel, a discussion is

presented in the last part of this chapter.

Table 8.3: The study area under different soil loss classes

251

Soil loss class Area (sq km) % to total geographical area

Very severe 4930.85 56.47

Severe 770.14 8.82

Moderately severe 841.74 9.64

Moderate 505.57 5.79

Moderately slight 888.90 10.18

Slight 455.80 5.22

Water bodies 338.79 3.88

Source: NBSS&LUP

The areas under different soil loss classes of the study area are shown in table

8.3 and the map prepared therefrom (Fig. 8.4) gives a general idea that loss of soil is

much higher in the southern areas than that of the northern areas of the study area.

Bhogdoi is one of the highly sediment charged rivers of the southern bank of

Brahmaputra. It carries down sediments mainly from the foothills of the Assam-

Nagaland border which spans over an area representing 56.78 per cent of the total

geographical area of the basin. According to measurements made by the Water

Resource Department, Govt. of Assam since the early seventies of the 20th

century,

the highest, the lowest and the average water discharge of Bhogdoi were 410 m3s

-1,

0.1 m3s

-1 and 75 m

3s

-1 respectively. The average annual suspended sediment carried

by the river is 83 hectare meter (Sharma, 2008, p. 56). The Upper Assam Investigation

Division under the Water Resource Department, Government of Assam has a gauge

site for this river at the A.T. Road crossing site in Jorhat city. Hydrological data for

river Bhogdoi have been collected from the office of the Upper Assam Investigation

Division at Jorhat.

252

Fig. 8.4: Mean monthly rainfall of the basin

Annual maximum water discharge (or the annual highest peak discharge)

values for any river are very important in fluvial studies because they can be used to

understand the behaviour of the river in the temporal scale. Annual maximum and

minimum water discharge for the period from 1971 to 2008 of river Bhogdoi have

been represented in table 8.4 and 8.5 respectively.

The annual minimum water discharge for the period ranges between 4.61 m3s

-1

and 0.11 m3s

-1 with the average and standard deviation values of 1.49 and 1.01

respectively. Within one standard deviation about the mean 84.21 per cent (or 32 out

of 38) of the variables are observed. As the value is more than 68.27 per cent, it is a

symmetric distribution. The annual minimum discharge series shows an increasing

trend over the years.

0

100

200

300

400

500

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

RA

INF

ALLl (

mm

)

MEAN MOMTHLY RAINFALL (1969-2005)

253

Fig. 8.5: Soil Loss classes

254

Data on suspended sediment load of the river were collected from the Water

Resource Department, Govt. of Assam, which has its gauge site at A.T. Road

crossing, Jorhat. This gauge site is in the plains of Assam and 23 km away from the

foothills. Sediment load data for all the years are not available. However, data were

collected for twelve different years between 1971 and 1999 which are presented in

table 8.9.

8.4: Hypsometric and Hypsographic Study of the Basin

Both hypsometric and hypsographic analyses of the Bhogdoi river basin have

been carried out in order to understand the denudational characteristics as well as

geomorphic status of the basin.

Table: 8.4: Annual maximum water discharge of river Bhogdoi (1971-2008)

Year Maximum Water

Dischargr (m3s

-1)

Year Maximum Water

Dischargr (m3s

-1)

1971 257.17 1990 146.72

1972 381.64 1991 116.84

1973 236.37 1992 134.99

1974 153.97 1993 292.40

1975 76.30 1994 126.64

1976 323.45 1995 278.88

1977 401.98 1996 410.98

1978 369.93 1997 163.04

1979 56.32 1998 248.10

1980 297.06 1999 95.05

1981 183.17 2000 98.66

1982 239.95 2001 80.29

1983 346.81 2002 201.75

1984 101.71 2003 136.41

1985 239.17 2004 203.24

1986 99.72 2005 113.08

1987 221.34 2006 186.50

1988 115.17 2007 210.32

1989 298.56 2008 79.61 Source: Water Resource Department, Govt. of Assam

255

Fig. 8.6: Annual maximum water discharge of Bhogdoi (1971-2008)

Table: 8.5: Annual minimum water discharge of river Bhogdoi (1971-2008)

Year Minimum Water

Dischargr (m3s

-1)

Year Minimum Water

Dischargr (m3s

-1)

1971 1.76 1990 1.06

1972 1.66 1991 1.54

1973 1.54 1992 2.83

1974 1.09 1993 1.35

1975 0.54 1994 1.22

1976 1.44 1995 2.08

1977 0.80 1996 3.10

1978 0.60 1997 1.97

1979 0.68 1998 2.56

1980 0.74 1999 0.64

1981 0.31 2000 0.91

1982 0.11 2001 0.60

1983 .071 2002 0.59

1984 .080 2003 2.61

1985 0.69 2004 1.49

1986 0.71 2005 4.30

1987 0.97 2006 1.83

1988 0.96 2007 1.77

1989 2.34 2008 4.61 Source: Water Resource Department, Govt. of Assam

y = -3.1704x + 265.07

0

100

200

300

400

500

19

71

19

75

19

79

19

83

19

87

19

91

19

95

19

99

20

03

20

07

DIS

CH

AR

GE

(C

UM

AC

) .

ANNUAL MAXIMUM WATER DISCHARGE OF BHOGDOI

256

Fig. 8.7: Annual minimum water discharge of Bhogdoi (1971-2008)

Hypsometric analysis of watershed expresses the complexity of denudational

processes and the rate of morphological changes which are taking place within a

basin. Therefore, it is useful to comprehend the erosional status of watersheds and

prioritize them for soil and water conservation measures (Singh et al. 2008).

Hypsometric analysis was first introduced by Langbein (1947) to express the overall

slope and the form of drainage basin. Hypsometric analysis of drainage basins is an

organizational tool that may be used for quantifying the relative distribution of

drainage-surface areas at different elevations (Oertel, 2001). The hypsometric curve is

related to the volume of soil mass in the basin and the amount of erosion that occurred

in the basin against the remaining mass (Hurtrez et al. 1999). The shape of the

hypsometric curve is important and it describes the stages of the landscape evolution,

which also provides an indication of erosion status of the watershed (Singh et al.

2008).

y = 0.044x + 0.6283

0

1

2

3

4

5

1971 1975 1979 1983 1987 1991 1995 1999 2003 2007

DIS

CH

AR

GE

(C

UM

AC

)

ANNUAL MINIMUM WATER DISCHARGE OF BHOGDOI

Source: Watre Resource Department, Govt. of Assam

257

Fig. 8.8: Hypsometric curve of the Bhogdoi basin

The hypsometric curve of river Bhogdoi shown in figure 8.8 is the plot of

height ratio plotted along the ordinate and the area ratio plotted along the abscissa.

The curve so obtained provides a measure of the distribution of landmass volume

remaining beneath or above a basal reference plane. The computed data for the curve

are presented in table 8.6. In the table ‘A’ is the total area of the basin, which is

945.875 sq km and ‘H’ represents the highest elevation which is 1290 m. The ratio of

the total area of any elevation category to the total area of the basin is called relative

area. The highest relative area is recorded in the elevation category of 0 to 100 m. It is

observed that the relative area goes on decreasing as the elevation category moves

from lower to higher category. Again, the ratio of the elevation above the local base

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

HE

IGH

T R

AT

IO (

h/H

)

AREA RATIO (a/A)

HYPSOMETRIC CURVE OF BHOGDOI

258

level (of any elevation category) to the highest elevation of the basin is called relative

elevation. The relative elevation goes on increasing as the elevation category moves

from lower to higher category.

Table-8.6: Relative height and relative area of Bhogdoi basin

Elevation

Categories

(m)

Area

within the

category

(sq km)

Area above that

elevation category

(sq km)

(a)

Relative

area

(a/A)

Elevation

above the

local base

level (m)

(h)

Relative

elevation

(h/H)

0-100 327.115 618.760 0.6542 20 0.0165

100-200 243.750 375.010 0.3965 120 0.0991

200-300 217.215 157.795 0.1668 220 0.1818

300-400 66.415 91.380 0.0966 320 0.2644

400-500 40.460 50.920 0.0538 420 0.3471

500-600 25.000 25.920 0.0274 520 0.4297

600-700 10.265 15.655 0.0165 620 0.5123

700-800 7.080 8.575 0.0090 720 0.5950

800-900 4.250 4.325 0.0045 820 0.6776

900-1000 2.305 2.020 0.0021 920 0.7603

1000-1100 1.240 0.78 0.0008 1020 0.8329

1100-1200 0.600 0.180 0.0002 1120 0.9256

1200-1300 0.180 0.000 0.0000 1220 1.0000

Note: Here, total area of the basin (A) is 945.875 sq km and the difference between the highest and

lowest elevation within the basin (H) is 1210 m.

259

Fig. 8.9

260

Fig. 8.9

261

Fig. 8.9

262

Fig. 8.9

263

Fig. 8.9

264

The hypsometric integral (HI) is a numerical index of drainage basin

morphology. It is a geomorphological parameter used to classify the geologic stages

of watershed development. It assumes importance in estimation of erosion status of

watershed and subsequent prioritization for taking up soil and water conservation

activities (Singh et. al., 2008). The hypsometric integral explains the erosion that has

taken place in the watershed throughout the geological time scale due to hydrologic

processes and factors of denudation. It is usually derived from the hypsometric curve.

The hypsometric integral of the Bhogdoi basin is computed to be 0.11. The integral

value as expressed in percentage for the basin is 11 per cent. The value reveals that

only 11 per cent of the total volume of land mass of the Bhogdoi basin exists above

the base level of the river. Conversely, the river has already eroded 89 per cent of the

total volume of land mass of the basin.

The hypsometric integral is also an indication of the ‘cycle of erosion’. The

entire period of cycle of erosion is divided into three stages viz. monadnock or old (HI

below 30 %), in which the watershed is fully stabilized, equilibrium or mature stage

(HI between 30 % and 60 %) and inequilibrium or young stage (HI above 60 %), in

which the watershed is highly susceptible to erosion (Strahler, 1952). Hence,

according to Strahler’s classification the Bhogdoi basin is a fully stabilized one

representing monadnock or old stage of basin development. The shape of the

hypsometric curve explains the temporal changes in the slope of the original basin.

Strahler (1952) interpreted the shapes of the hypsometric curves by analyzing

numerous drainage basins and classified the basins as young (convex upward curves),

mature (S-shaped hypsometric curves which is concave upward at high elevations and

convex downward at low elevations) and peneplain stage (concave upward curves).

River Bhogdoi shows a concave upward curve which indicates that the basin attains

265

the peneplain stage. The hypsometric integral of Bhogdoi also indicates the same that

it a is fully stabilized basin showing monadnock or old stage of basin development.

Table 8.7: Proportion of basin area against height above sea level

Note: A is the total area of the basin, which is 945.875 sq km

A hypsographic curve is a generic model which provides both a generalized

qualitative description of what the morphology of an area is like, and an indication of

the stage of evolution it has reached. It is a deductive model in the sense that it is

based on the assumed rapid uplift of a relatively level surface (McCullagh, 1983, p.

21). Hypsographic analysis deals with the earth’s topologic configuration above sea

level, especially the measurement and mapping of land elevations with respect to a

given datum or local base level. It is the representation or description of the earth’s

Elevation (m) Area

(sq km)

Cumulative

area (sq km)

(a)

Proportion of area

to basin area

{(A-a) / A}

Height above sea

level (m)

(h)

0-100 327.115 327.115 0.6542 20

100-200 243.750 570.865 0.3965 120

200-300 217.215 788.0800 0.1668 220

300-400 66.415 854.4950 0.0966 320

400-500 40.460 894.9550 0.0538 420

500-600 25.000 919.9550 0.0274 520

600-700 10.265 930.2200 0.0166 620

700-800 7.080 937.3000 0.0091 720

800-900 4.250 941.5500 0.0046 820

900-1000 2.305 943.8850 0.0021 920

1000-1100 1.240 945.0950 0.0008 1020

1100-1200 0.600 945.6950 0.0002 1120

1200-1300 0.180 945.8750 0.0000 1220

266

topologic features above any datum as on a map. A hypsographic curve indicates the

proportion of the area of the surface at different elevations above or depths below a

given datum in a region. It can be effectively used to understand the elevation

characteristics of a river basin against the basin area.

The table 8.7 gives the necessary calculation for the preparation of the

hypsographic curve of the Bhogdoi river basin. The curve is represented in figure

8.10. The tabulated data on proportion of area to basin area and height above the sea

level are taken for the preparation of the hypsographic curve of the basin. The curve

reveals that 65.42 per cent (or proportion to the basin area is 0.6542) of the basin area

lies above the elevation of 20 meters from msl. Similarly, 9.66 per cent of the basin

area lies above the elevation of 320 metre from msl. This is observed that only less

than one per cent (0.91 %) of the basin area lies above the elevation of 720 metre from

msl. The actual area of the basin below the elevation of 200 metre is 570.865 sq km.

This forms 60.35 per cent of the basin. The local base level of the basin is 20 metre

above msl. Therefore, it can be said that 60.35 per cent of the basin area is existing

between 20 metre and 200 metre.

River Bhogdoi shows a concave hypsometric curve. The concave curve

indicates that the denudational process in the river basin has been continuing for long

time and much of the earth materials of the basin have already been eroded down. The

curve shows that the lower part of the hypsometric curve runs nearly parallel to the

abscissa. It implies that the area near its mouth attains the peneplain stage and the

lower few kilometers of the channel of river Bhogdoi approaches the local base level.

267

Fig. 8.10: Hypsographic curve of the Bhogdoi basin

The recent history of the river states that a lot of changes have taken place in

the lower Bhogdoi basin during the last few centuries. During the fifties of the last

century, an embankment erected along the Brahmaputra has stopped the direct link of

Bhogdoi to Brahmaputra. In view of all these changes the ideal peneplain condition in

the lower part of the basin is not distinctly visible. On the other hand, Bhogdoi river is

characterized by rising flows and the fluvial activities of Brahmaputra are much

stronger to wash away some of the distinctive features of the lower Bhogdoi basin.

The natural as well as the anthropogenic factors, which brought about severe geo-

environmental changes to the lower part of the basin during the last few centuries, are

also responsible for morphological changes of the basin.

0

100

200

300

400

500

600

700

800

900

1000

1100

1200

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

HE

IGH

T A

BO

VE

SE

A L

EV

EL (

m)

.

PROPORTION OF AREA TO BASIN AREA

HYPSOGRAPHIC CURVE OF BHOGDOI

268

It is observed in table 8.7 that among all the elevation categories, the 0-100 m

category includes the largest area. This category alone occupies more than one third of

the basin area. In the same way, as much as 83.31 per cent of the basin area lies below

the 300 m contour. Again, little more than 90 per cent of the basin area is below the

400 m contour. When these values are compared to the area of the basin under hills

(56.78 per cent) and plains (43.22 per cent), it is understood that much of the area of

the basin has been eroded down. The depositional activities of Bhogdoi in its lower

part and the inundation caused by it, however, give indications of the peneplain stage

of lower part of the basin.

Table- 8.8: Area of the basin shown against elevation categories

Elevation

categories (m)

Area

(sq km)

% to the basin

area

% of area below that

elevation

0-100 327.115 34.58 34.58

100-200 243.750 25.77 60.35

200-300 217.215 22.96 83.31

300-400 66.415 7.02 90.33

400-500 40.460 4.28 94.61

500-600 25.000 2.64 97.25

600-700 10.265 1.09 98.34

700-800 7.080 0.75 99.09

800-900 4.250 0.45 99.54

900-1000 2.305 0.24 99.78

1000-1100 1.240 0.13 99.91

1100-1200 0.600 0.06 99.97

Above 1200 0.180 0.02 99.99

It is noted in table 8.8 that more than one third (34.58 %) of the basin area lies

in the 0–100 m elevation category. The area between 100 and 200 m elevation forms

25.77 per cent of the basin area. When the area of these two elevation categories (i.e.

0-100 m and 100-200 m) is considered together, it appears that little more than 60 per

269

cent of the basin area lies below 200 m from msl. In the same way, more than 90 per

cent of the basin area lies below the elevation of 400 m from msl. The area of the

basin above the elevation of 800 m from mean sea level forms less than one per cent

only. Another aspect noted in table 8.8 is that as the elevation category goes high, the

area of the basin diminishes.

8.5: Flood Frequency Analysis

Hydrologic designing often faces the problem of estimating maximum floods.

Its necessity is severely felt in assessing hydrological and hydraulic considerations in

the construction of bridges and sewers, dams, embankments, diversion canals,

detention ponds etc. Accurate estimation of flood frequency and probability assures

safety of these structures. Flood data are analyzed to gain understanding of the river’s

past hydrologic behaviour in order to provide guidance for unexpected future floods

(Reddi, 1998). Flood frequency analysis uses historical records of peak floods to

predict the expected behaviour of future floods. Two primary applications of flood

frequency analysis are (i) to predict the possible flood magnitude over a certain period

and (ii) to estimate the frequency and probability with which flood of certain

magnitude may occur.

Flood frequency analysis provides information about the trend of flood

occurrence. Flood frequency is calculated against a return period. The return period

(also known as the recurrence interval) is the statistical average time duration

between floods of a certain magnitude. A flood with 100 year return period occurs

once on average every 100 years. It has a = 0.01 or 1 % exceedance

probability in any given year. Similarly, a 25 year flood has = 0.04 or 25 %

exceedance probability in any given year. The longer the period of record the better is

the result of likelihood estimation. The return periods commonly used for flood

270

frequency analysis are 2, 10, 25, 50, 100 and even 500 year flood. In fact, the

statistical estimation of flood events is affected by flow characteristics, length of the

period of record and consistency of the hydrologic conditions within the drainage

basin.

A flood is a relatively high flow that overtops the natural or artificial banks in

any reach of a stream. When banks are overtopped, water spreads over the floodplains

and causes havoc through loss of property and life. Since floodplain is a desirable

location for human settlement and activities, it is very important to moderate the

impact of floods, thereby bringing its damage potential to an acceptable minimum

(David et al, 1982). In many parts of the world including India HFL (highest flood

level) is of prime consideration at the time of planning, designing and executing any

construction activity. The volume of mean low flood in a river is also of great interest

to those designing storage works for irrigation and water supply. Its importance is

increasingly realized in the field of draught management and water harvesting.

The gauge site of Bhogdoi is at the A.T. Road crossing in Jorhat city. Data with regard

to mean daily water discharge for every month, mean daily water level for every

month and mean daily suspended sediment load for every month have been collected

for 13 different years (in three time spans) between 1971 and 1999 as continuous data

covering the entire period are not available (table 8.9). The first period covers a span

of five years from 1971 (from June onwards) to 1975. The second period covers data

for a span of another five years from 1981 to 1985 and finally, the third period covers

a span of three years data from 1997 to 1999. For each of these years, the mean

monthly water discharge, mean monthly water level and mean monthly suspended

sediment load values have been collected.

271

Table 8.9: Mean daily water level, discharge and suspended sediment load of river Bhogdoi Source: Water Resource Department, Govt. of Assam

Months Mean Mean Mean daily volume of

daily water level daily water discharge suspended sediment

(m) (m3s

-1) (% to volume of water)

Jun,71 89.0400 62.4400 0.1016

Jul,71 89.0500 57.6100 0.1030

Aug,71 89.0500 72.3600 0.0753

Sep,71 88.9400 65.2900 0.0944

Oct,71 88.7900 44.6700 0.0482

Nov,71 88.5000 16.4000 0.0235

Dec,71 88.4400 5.6400 0.0032

Jan,72 88.3500 2.6400 0.0009

Feb,72 88.3500 3.6800 0.0025

Mar,72 88.3100 1.9800 0.0018

Apr,72 88.5100 7.9700 0.0064

May,72 88.7700 32.7300 0.0199

Jun,72 88.4000 33.1000 0.0230

Jul,72 89.0400 58.2000 0.0329

Aug,72 89.1100 63.4300 0.0704

Sep,72 88.6500 26.0400 0.0380

Oct,72 88.4500 16.9600 0.0192

Nov,72 88.1700 34.0600 0.0060

Dec,72 88.1600 5.7800 0.0025

Jan,73 88.2700 3.4600 0.0010

Feb,73 88.2700 2.5600 0.0013

Mar,73 88.2800 2.2400 0.0008

Apr,73 88.4000 3.2700 0.0010

May,73 88.5200 3.0341 0.0075

Jun,73 88.8400 27.6300 0.0113

Jul,73 88.9700 56.0400 0.0164

Aug,73 89.2500 88.1800 0.0308

Sep,73 88.9600 42.2000 0.0254

Oct,73 88.6600 23.8000 0.0229

Nov,73 88.5400 17.2700 0.0101

Dec,73 88.4700 9.7900 0.0050

Jan,74 88.4300 3.5900 0.0022

Feb,74 88.3900 1.8700 0.0011

Mar,74 88.3200 2.4800 0.0020

Apr,74 88.4300 7.2200 0.0032

272

May,74 88.5800 18.9500 0.0140

Jun,74 88.9200 46.0700 0.0358

Jul,74 89.0600 64.6300 0.0282

Aug,74 88.9300 50.5500 0.0393

Sep,74 88.9700 50.5300 0.0414

Oct,74 88.8800 38.0100 0.0522

Nov,74 88.5300 10.2900 0.0108

Dec,74 88.4400 4.7000 0.0078

Jan,75 88.3400 2.3200 0.0085

Feb,75 88.3400 2.1200 0.0136

Mar,75 88.2700 1.3400 0.0072

Apr,75 88.3000 3.1700 0.0067

May,75 88.4000 3.4100 0.0106

Jun,75 88.6900 13.8100 0.0042

Jul,75 88.8900 13.6000 0.0035

Aug,75 88.8300 14.8800 0.0037

Sep,75 89.0900 44.0300 0.0078

Oct,75 88.8800 23.5500 0.0033

Nov,75 88.4500 5.6000 0.0030

Dec,75 88.3600 4.1400 0.0035

Jan,81 88.3100 1.4300 0.0019

Feb,81 88.3000 0.9400 0.0015

Mar,81 88.4200 1.8600 0.0018

Apr,81 88.3800 2.4100 0.0016

May,81 88.9100 29.3500 0.0034

Jun,81 88.7400 17.1500 0.0047

Jul,81 89.4000 93.7600 0.0083

Aug,81 88.9900 31.9200 0.0143

Sep,81 89.0800 44.0100 0.0086

Oct,81 88.5200 11.8800 0.0064

Nov,81 88.3700 3.1300 0.0034

Dec,81 88.3200 0.5600 0.0018

Jan,82 88.2900 0.2500 0.0018

Feb,82 88.3000 0.4000 0.0015

Mar,82 88.2500 0.1800 0.0026

Apr,82 88.5400 10.6800 0.0031

May,82 88.5500 12.6500 0.0127

Jun,82 89.0500 67.8900 0.0047

Jul,82 88.8900 64.8600 0.0078

273

Aug,82 89.1600 90.8100 0.0099

Sep,82 88.9500 39.9900 0.0124

Oct,82 88.7400 28.2000 0.0042

Nov,82 88.4300 5.3200 0.0029

Dec,82 88.3300 2.2200 0.0016

Jan,83 88.3100 1.4300 0.0019

Feb,83 88.3000 0.9400 0.0015

Mar,83 88.4200 1.8600 0.0018

Apr,83 88.3800 2.4100 0.0016

May,83 88.9100 29.3500 0.0034

June,83 88.7400 17.1500 0.0047

July,83 89.4000 93.7600 0.0083

Aug,83 89.0300 42.4200 0.0097

Sep,83 89.1500 51.4900 0.0051

Oct,83 88.8200 25.2400 0.0038

Nov,83 88.5400 7.0400 0.0026

Dec,83 88.4400 1.9000 0.0017

Jan,84 88.4400 3.9300 0.0020

Feb,84 88.3900 2.5900 0.0027

Mar,84 88.3400 1.0700 0.0014

Apr,84 88.4500 4.9200 0.0044

May,84 88.6300 19.0400 0.0054

June,84 89.1300 41.6000 0.0076

July,84 88.8600 29.7100 0.0078

Aug,84 88.9100 28.6700 0.0091

Sep,84 89.0400 29.2800 0.0091

Oct,84 88.5900 17.1000 0.0076

Nov,84 88.4200 1.8900 0.0073

Dec,84 88.3800 1.3200 0.0049

Jan,85 88.3100 1.1500 0.0053

Feb,85 88.3400 1.2300 0.0055

Mar,85 88.4300 5.2200 0.0057

Apr,85 88.8500 41.8800 0.0067

May,85 88.5400 21.9300 0.0044

June,85 89.9000 67.2600 0.0058

July,85 89.0200 52.4500 0.0056

Aug,85 88.8100 29.1500 0.0051

Sep,85 89.1200 54.7500 0.0079

Oct,85 88.7500 31.0900 0.0053

274

Nov,85 88.4700 9.4500 0.0035

Dec,85 88.4000 2.5700 0.0031

Jan,97 NA 2.7600 0.0000

Feb,97 NA 2.6000 0.0000

Mar,97 NA 2.8000 0.0000

Apr,97 NA 2.8900 0.0000

May,97 NA 7.6700 0.0020

Jun,97 NA 51.1700 0.0180

Jul,97 NA 69.3300 0.0250

Aug,97 NA 62.0100 0.0230

Sep,97 NA 59.5300 0.0220

Oct,97 NA 14.1800 0.0060

Nov,97 NA 7.6100 0.0030

Dec,97 NA 6.9400 0.0030

Jan,98 NA 4.8100 0.0010

Feb,98 NA 3.2100 0.0000

Mar,98 NA 7.4900 0.0020

Apr,98 NA 27.2000 0.0070

May,98 NA 23.0000 0.0320

Jun,98 NA 54.6300 0.0180

Jul,98 NA 76.4800 0.0320

Aug,98 NA 75.1900 0.0320

Sep,98 NA 53.0600 0.0190

Oct,98 NA 18.8300 0.0070

Nov,98 NA 18.0800 0.0110

Dec,98 NA 13.3700 0.0050

Jan,99 NA 4.5000 0.0010

Feb,99 NA 1.9200 0.0000

Mar,99 NA 1.1400 0.0000

Apr,99 NA 2.5200 0.0000

May,99 NA 12.3000 0.0050

Jun,99 NA 40.2300 0.0002

Jul,99 NA 51.8200 0.0000

Aug,99 NA 38.5300 0.0002

Sep,99 NA 42.6400 0.0003

Oct,99 NA 33.5600 0.0005

Nov,99 NA 15.7300 0.0006

Dec,99 NA 6.9200 0.0008

NA: Data not available

Source: Water Resource Department, Govt. of Assam

275

These are presented in three separate graphs in fig. 8.9. Plotting of the mean

daily discharge values for every month indicates that the water discharge remains high

from May to October. The daily average discharge value for any month was observed

to be 93.76 m3s

-1. This value appears twice in July, 1981 and in July, 1983. Based on

the daily average discharge for the months during 12 different years (the year 1971 is

not been considered as the values are available from June to December, the average

annual water flow of the river is calculated in table 8.10. The average annual water

discharge of the river is calculated to be 8236.83 m3.

Table 8.10: Measurement of average annual volume of water flow of Bhogdoi river

Years Volume of daily water flow (derived

from 12 monthly values of mean daily

discharge for each month)

(m3 per day)

Volume of annual water flow

(m3)

1972 23.88 8716.20

1973 23.29 8500.85

1974 24.91 9092.15

1975 11.00 4015.00

1981 19.87 7252.55

1982 26.95 9836.75

1983 22.92 8365.80

1984 15.09 5507.85

1985 26.51 9676.15

1997 24.12 8803.80

1998 31.28 11417.20

1999 20.98 7657.70

Volume of average annual flow of water 8236.83 m3

During the period from 1971 to 2008, the annual highest peak

discharge values range between 410.61 and 56.32 m3s

-1 which give a mean of 202.255

m3s

-1 with standard deviation value (σn-1) of 101.01 and the CV (co-efficient of

276

variation) value of 49.94. The CV value ranges from 1 to 100. A higher value of CV

shows higher inconsistency in the data, whereas a lower value of CV shows higher

consistency in the data. In the present analysis, the CV shows a much higher value

which indicates that the set of data under study (annual highest peak discharge values

for a period of 38 years) has little consistency representing highly erratic nature of the

annual highest peak discharge. Similar study considering the annual minimum

discharge gives standard deviation (σn-1) and CV values to be 1.02 and 68.59

respectively. The CV for this series also shows much higher inconsistency of

discharges.

Analysis of meteorological data indicates that 35.60 per cent of the total

annual rainfall is received in the months of July and August every year. In as many as

23 out of 38 years or 60.50 per cent of the time between 1971 and 2008, the highest

peak flood occurred during these two months. During this period, the month-wise

occurrence of number of annual highest peak discharge is shown within brackets

against the months such as May (1), June (4), July (12), August (11), September (7)

and October (3). It supplements that the distribution of month-wise rainfall

corresponds to the occurrence of the month-wise number of annual peak discharge.

The annual maximum water level of Bhogdoi fluctuates between 90.74 m in

1981 and 89.26 m in 2008. During this period, it is observed that July and August are

the two months with highest occurrence of annual maximum water level. The annual

maximum water level occurs between May and October. The month-wise occurrence

of number of annual highest water level is shown within brackets against the months

as May (1), June (6), July (12), August (10), September (5) and October (4). This

analysis also shows a positive relationship between the month-wise distribution of

rainfall and month-wise occurrence of the highest water level.

277

Fig. 8.11: Month-wise occurrence of the highest and lowest water discharge

Annual minimum water discharge values range between 4.61 m3s

-1 and 0.11

m3s

-1. The month of March has the distinction of being the month with maximum

occurrence of lowest annual discharge for as many as 16 years out of 38 years. This

month is followed by February with 7 years, December with 6 years, January with 4

years, April with 3 years and September, October and November each with one year

of occurrence. These are represented in figure 8.11. Similarly, March again has the

distinction of being the month with highest occurrence of minimum water level with

10 years out of 38 years. Such values for the other months of the year are shown

within brackets as November (1), December (9), January (3), February (9), March (0)

and April (6). These are represented in figure 8.12.

Fig. 8.12: Month wise occurrence of the highest and lowest water level

1

4

12 11

7

3

0

2

4

6

8

10

12

14

MAY JUN JUL AUG SEP OCT

NO

. OF

OC

CU

RR

ENC

E

PEAK DISCHARGE

6

4

7

16

3

0

2

4

6

8

10

12

14

16

18

DEC JAN FEB MAR APR

NO

. OF

OC

CU

RR

ENC

E

LOWEST DISCHARGE

1

6

12

10

5 4

0

2

4

6

8

10

12

14

MAY JUN JUL AUG SEP OCT

NO

. OF

OC

CU

RR

ENC

E

OCCURRENCE OF HIGHEST WATER LEVEL

1

9

3

9 10

6

0

2

4

6

8

10

12

14

NOV DEC JAN FEB MAR APR

NO

. OF

OC

CU

RR

ENC

E

OCCURRENCE OF LOWEST WATER LEVEL

278

The variations of annual maximum and minimum discharge from their

respective means have been shown in table 8.11. Here, the variations of annual

maximum and minimum discharge have been expressed as percent of the respective

mean values. For as many as 21 years out of the 38 years or 55 per cent of years under

observation, the annual maximum discharge was below the average and it remained

above the average for only 17 years or 45 per cent of years under observation. The

highest deviation about the average took place in 1996 and the annual maximum

discharge was + 103.20 per cent higher than the average. At the same time, the lowest

deviation of the annual maximum discharge about the average is as low as 72.15 per

cent below the average. These two extremities show a difference of 175.35 per cent

between them.

The variation of annual minimum discharge from the mean is more acute than

the annual maximum discharge. In this case, the two extremities are + 210.02 and

92.60 per cent about the mean. Hence, they show a variation of 302.62 per cent

between them. Like the annual maximum discharge, the annual minimum discharge

also shows 55 per cent of the years under observation below the average while only 45

per cent of the time it is above the mean. When the variation in these two sets of

data representing annual maximum and minimum discharge is studied, higher

variation is observed in the series of minimum discharge. The CV values shown in the

table 8.11 for both the series also indicate the same.

The probability estimation of high flow of river Bhogdoi has been made based

on data collected for a period of 38 years from 1971 to 2008. These are represented in

table 8.12.

279

Table 8.11: Variation of annual maximum and minimum water discharge from their means

Yea

r

An

nu

al m

axi.

Dis

char

ge

(m3s-1

)

Mea

n o

f an

nu

al

max

. d

isch

arg

e

(m3s-1

)

Var

iati

on

of

ann

ual

max

.

dis

char

ge

as %

of

the

mea

n

C

o-e

ffic

ien

t o

f

var

iati

on

of

max

.

dis

char

ge

(%)

An

nu

al m

ini.

dis

char

ge

(m3s-1

)

Mea

n o

f an

nu

al

min

. d

isch

arg

e

(m3s-1

)

Var

iati

on

of

ann

ual

min

i.

dis

char

ge

as %

of

the

mea

n

C

o-e

ffic

ien

t o

f

var

iati

on

of

min

.

dis

char

ge

(%)

1971 258.17 202.255 +27.646 49.94 1.76 1.487 +18.359 68.59

1972 381.64 +88.692 1.66 +11.634

1973 236.37 +16.867 1.54 +3.564

1974 153.97 -23.873 1.09 -26.698

1975 76.13 -62.359 0.54 -63.685

1976 323.45 +59.922 1.44 -3.161

1977 401.98 +98.749 0.80 -46.200

1978 369.93 +82.903 0.60 -59.650

1979 56.32 -72.154 0.68 -54.270

1980 297.06 +46.874 0.74 -50.235

1981 183.17 -9.436 0.31 -79.153

1982 239.95 +18.637 0.11 -92.603

1983 346.81 +71.472 0.71 -52.253

1984 101.71 -49.712 0.80 -46.200

1985 239.17 +18.252 0.69 -53.598

1986 99.72 -50.696 0.71 -52.253

1987 221.34 +9.436 0.97 -34.768

1988 115.17 -43.057 1.96 +31.809

1989 298.56 +47.616 2.34 +57.364

1990 146.72 -27.458 1.06 -28.716

1991 116.84 -42.231 1.54 +3.564

1992 134.99 -33.258 2.83 +90.316

1993 292.40 +44.570 1.35 -9.213

1994 126.64 -37.386 1.22 -17.956

1995 278.88 +37.885 2.08 +39.879

1996 410.98 +103.199 3.10 +108.473

1997 163.04 -19.389 1.97 +32.482

1998 248.10 +22.667 2.56 +72.159

1999 95.05 -53.005 0.64 -56.960

2000 98.66 -51.220 0.91 -38.803

2001 80.29 -60.303 0.60 -59.650

2002 201.75 -0.250 0.59 -60.323

2003 136.41 -32.555 2.61 +75.521

2004 164.82 -18.509 1.49 +0.202

2005 113.08 -44.090 4.30 +189.173

2006 186.50 -7.790 1.83 +23.067

2007 210.32 +3.988 1.77 +19.032

2008 79.61 -60.639 4.61 +210.020

280

Table 8.12: Estimation of Probabilities of high flow of river Bhogdoi (1971-2008)

Yea

rs

(wit

ho

ut

Ran

kin

g)

An

nu

al

Max

imu

m

Dis

char

ge

(m3s-1

)

Yea

rs (

afte

r

ran

kin

g)

An

nu

al

Max

imu

m

Dis

char

ge

(m3s-1

)

Ran

k

M

Pro

bab

ilit

y

P=

M/(

N+

1)

Ret

urn

per

iod

T=

(N+

1)/

M

P

(in

%)

1971 258.170 1996 410.98 1 0.0256 39.0000 2.56

1972 381.640 1977 401.98 2 0.0513 19.5000 5.13

1973 236.370 1972 381.64 3 0.0769 13.0000 7.69

1974 153.970 1978 369.93 4 0.1026 9.7500 10.26

1975 76.130 1983 346.81 5 0.1282 7.8000 12.82

1976 323.450 1976 323.45 6 0.1538 6.5000 15.38

1977 401.980 1989 298.56 7 0.1795 5.5714 17.95

1978 369.930 1980 297.06 8 0.2051 4.8750 20.51

1979 56.320 1993 292.40 9 0.2308 4.3333 23.08

1980 297.060 1995 278.88 10 0.2564 3.9000 25.64

1981 183.170 1971 258.17 11 0.2821 3.5455 28.21

1982 239.950 1998 248.10 12 0.3077 3.2500 30.77

1983 346.810 1982 239.95 13 0.3333 3.0000 33.33

1984 101.710 1985 239.17 14 0.3590 2.7857 35.90

1985 239.170 1973 236.37 15 0.3846 2.6000 38.46

1986 99.720 1987 221.34 16 0.4103 2.4375 41.03

1987 221.340 2007 210.32 17 0.4359 2.2941 43.59

1988 115.170 2002 201.75 18 0.4615 2.1667 46.15

1989 298.560 2006 186.50 19 0.4872 2.0526 48.72

1990 146.720 1981 183.17 20 0.5128 1.9500 51.28

1991 116.840 2004 164.82 21 0.5385 1.8571 53.85

1992 134.990 1997 163.04 22 0.5641 1.7727 56.41

1993 292.400 1974 153.97 23 0.5897 1.6957 58.97

1994 126.640 1990 146.72 24 0.6154 1.6250 61.54

1995 278.880 2003 136.41 25 0.6410 1.5600 64.10

1996 410.980 1992 134.99 26 0.6667 1.5000 66.67

1997 163.040 1994 126.64 27 0.6923 1.4444 69.23

1998 248.100 1991 116.84 28 0.7179 1.3929 71.79

1999 95.050 1988 115.17 29 0.7436 1.3448 74.36

2000 98.660 2005 113.08 30 0.7692 1.3000 76.92

2001 80.290 1984 101.71 31 0.7949 1.2581 79.49

2002 201.750 1986 99.72 32 0.8205 1.2188 82.05

2003 136.410 2000 98.66 33 0.8462 1.1818 84.62

2004 164.820 1999 95.05 34 0.8718 1.1471 87.18

2005 113.080 2001 80.29 35 0.8974 1.1143 89.74

2006 186.500 2008 79.61 36 0.9231 1.0833 92.31

2007 210.320 1975 76.13 37 0.9487 1.0541 94.87

2008 79.61 1979 56.320 38 0.9744 1.0263 97.44

281

The probability estimation of low flow of river Bhogdoi has also been made. It

uses data for a period of 38 years from 1971 to 2008 which are presented in table 8.13.

Table 8.13: Estimation of Probabilities of low flow of river Bhogdoi (1971-2008)

Yea

rs

(wit

ho

ut

Ran

kin

g)

An

nu

al

Min

imu

m.

Dis

char

ge

m3s-1

Yea

rs (

afte

r

ran

kin

g)

An

nu

al

Min

imu

m.

Dis

char

ge

m3s-1

Ran

k

M

Pro

bab

ilit

y

P=

M/(

N+

1)

Ret

urn

per

iod

T=

(N+

1)/

M

P

(in

%)

1971 1.760 2008 4.61 1 0.0256 39.00 2.56

1972 1.660 2005 4.300 2 0.0513 19.50 5.13

1973 1.540 1996 3.100 3 0.0769 13.00 7.69

1974 1.090 1992 2.830 4 0.1026 9.75 10.26

1975 0.540 2003 2.610 5 0.1282 7.80 12.82

1976 1.440 1998 2.460 6 0.1538 6.50 15.38

1977 0.800 1989 2.360 7 0.1795 5.57 17.95

1978 0.600 1995 2.080 8 0.2051 4.88 20.51

1979 0.680 1997 1.970 9 0.2308 4.33 23.08

1980 0.740 1988 1.960 10 0.2564 3.90 25.64

1981 0.310 2006 1.830 11 0.2821 3.55 28.21

1982 0.110 2007 1.770 12 0.3077 3.25 30.77

1983 0.710 1971 1.760 13 0.3333 3.00 33.33

1984 0.800 1972 1.660 14 0.3590 2.79 35.90

1985 0.690 1991 1.570 15 0.3846 2.60 38.46

1986 0.710 1973 1.540 16 0.4103 2.44 41.03

1987 0.970 1976 1.440 17 0.4359 2.29 43.59

1988 1.960 1993 1.350 18 0.4615 2.17 46.15

1989 2.360 2004 1.270 19 0.4872 2.05 48.72

1990 1.060 1994 1.220 20 0.5128 1.95 51.28

1991 1.570 1974 1.090 21 0.5385 1.86 53.85

1992 2.830 1990 1.060 22 0.5641 1.77 56.41

1993 1.350 1987 0.970 23 0.5897 1.70 58.97

1994 1.220 2000 0.910 24 0.6154 1.63 61.54

1995 2.080 1977 0.800 25 0.6410 1.56 64.10

1996 3.100 1984 0.800 26 0.6667 1.50 `66.67

1997 1.970 1980 0.740 27 0.6923 1.44 69.23

1998 2.460 1983 0.710 28 0.7179 1.39 71.79

1999 0.640 1986 0.710 29 0.7436 1.34 74.36

2000 0.910 1985 0.690 30 0.7692 1.30 76.92

2001 0.600 1979 0.680 31 0.7949 1.26 79.49

2002 0.590 1999 0.640 32 0.8205 1.22 82.05

2003 2.610 1978 0.600 33 0.8462 1.18 84.62

2004 1.270 2001 0.600 34 0.8718 1.15 87.18

2005 4.300 2002 0.590 35 0.8974 1.11 89.74

2006 1.830 1975 0.540 36 0.9231 1.08 92.31

2007 1.770 1981 0.310 37 0.9487 1.05 94.87

2008 4.61 1982 0.110 38 0.9744 1.03 97.44

282

Besides, analysis and estimation of flood magnitude for high flows have

been made for river Bhogdoi following Gumbel’s Extreme Value Distribution

Method. In this regard, probable flood magnitudes have been measured against

several return periods as shown in table 8.14.

Table 8.14: Estimation of flood magnitudes (QP) using Gumbel’s Extreme Value Distribution

for Bhogdoi river at A.T. Road crossing, Jorhat

Return

period in

years

(T)

Mean High

Peak Flow

(m3s

-1)

(QP)

Standard

Deviation

(σn-1)

Frequency

Factor

(K)

Kσn-1 QP =QP+ Kσn-1

10 1.8482 186.68 388.94

20 2.3020 232.52 437.78

25 202.255 101.01 2.4440 246.86 449.12

50 2.8891 291.82 494.08

75 3.1559 318.77 521.03

100 3.3486 338.24 540.05

Similar analysis and estimation of low flow probability have been carried out

for river Bhogdoi following Gumbel’s Extreme Value Distribution method. Probable

occurrence of low flow has been worked out against several return periods as shown

in table 8.15.

Table 8.15: Estimation of low flow magnitudes (QP) using Gumbel’s Extreme Value

Distribution for Bhogdoi river at A.T. Road crossing, Jorhat

Return

period in

years

(T)

Mean Low Peak

Flow (m3s

-1)

(QP)

Standard

Deviation

(σn-1)

Frequency

Factor (K)

Kσn-1 QP =QP+ Kσn-1

10 1.8483 1.8852 3.3652

20 2.3020 2.3480 3.8280

25 1.487 1.02 2.4440 2.4930 3.9730

50 2.8891 2.9468 4.4268

75 3.1559 3.2190 4.6990

100 3.3486 3.4155 4.8955

In addition to the above method, analysis and estimation of flood magnitude

for high flows of river Bhogdoi has been carried out following Log Pearson Type-III

283

distribution method. Here also, the probable flood magnitudes have been shown

against different return periods as presented in table 8.16. The annual number of flood

waves of Bhogdoi has been collected from the Upper Assam Investigation Division of

Water Resource Department, Govt. of Assam. As revealed from the available data, the

water level of the river above the Danger Level at A T Road crossing gauge site is

89.0 m. Table 8.17 presents the annual number of flood waves against the years.

Table 8.16: Estimation of flood magnitudes (Q) using Log Pearson Type-III Distribution

method for Bhogdoi (1971-2008)

Return

Period in

years

(T)

Coefficient

of

skewness

(CS)

Frequency

factor

(K)

Standard

Deviation

(S)

KS XT=X+KS

(m3s

-1)

Antilog of

XT

(m3s

-1)

(Q)

2 0.033 0.0076 2.2568 180.6342

5 0.830 0.1917 2.4409 275.9942

10 0.2195 1.301 0.231 0.3005 2.5497 354.5683

25 1.818 0.4199 2.6691 466.7668

50 2.159 0.4987 2.7479 559.6287

500 2.472 0.5710 2.8202 660.9977

200 2.763 0.6382 2.8874 771.6138

Graphical plotting of the flood waves against the years gives a temporal

pattern of their occurrence. In this respect river Bhogdoi shows a distinctive pattern

with a rising trend from 1971 to 1991. The trend after 1991, however, shows a steady

fall in the number of annual flood waves. These are shown in figure 8.13 and table

8.17.

284

Fig. 8.13: Annual number of flood waves of river Bhogdoi (1971-2008)

Table 8.17: Annual number of flood waves of river Bhogdoi (1971-2008)

Year No. of flood waves Year No. of flood waves

1971 7 1990 19

1972 10 1991 21

1973 9 1992 20

1974 11 1993 19

1975 8 1994 12

1976 10 1995 18

1977 16 1996 18

1978 16 1997 12

1979 8 1998 11

1980 14 1999 16

1981 12 2000 10

1982 13 2001 14

1983 12 2002 9

1984 16 2003 12

1985 17 2004 8

1986 14 2005 8

1987 17 2006 5

1988 19 2007 7

1989 18 2008 4

Source: Water resource Department, Govt. of Assam

y = -0.049x + 13.851

0

5

10

15

20

25

1971 1975 1979 1983 1987 1991 1995 1999 2003 2007

NO

. OF

WA

VES

ANNUAL NUMBER OF FLOOD WAVES

285

8.6: Analysis of Suspended Sediment Load

Data relating to suspended sediment load of river Bhogdoi had been collected

by the State Water Resource Department from its gauge site at the AT road crossing in

Jorhat city. It is a volumetric measurement of coarse, medium and fine suspended

particles taken for each of the months of the year. The suspended sediment is

measured as cubic centimeters per liter of water and then expresses it as per cent to the

volume of river water. The sum of the coarse, medium and fine suspended particles

gives the total suspended sediment load of the river water.

The mean daily suspended sediment discharge for every month have been

collected for three different spans of time (i) from June, 1971 to December, 1975, (ii)

from January, 1981 to December, 1985 and (iii) January, 1997 to December, 1999.

These are presented in table 8.9. Graphical plotting of the monthly values of mean

daily water level, mean daily water discharge and mean daily suspended sediment

discharge for every month of each of the years is shown in fig. 8.9. The graphs

representing five years from 1971 to 1975 display very high volume of suspended

sediment load. The month of July, 1971 has been noted for maximum suspended

sediment load for all the time under study with 0.103 per cent of the volume of water.

Similar condition continued until 1974, but in 1975 the amount of suspended sediment

load dropped considerably. The graphs representing five years from 1981 to 1985

show a similar condition to what had happened in 1975. The highest monthly

suspended sediment load recorded during this span of time was 0.0143 per cent of

total volume of water. It was recorded in the month of August, 1981.

The 12 monthly average suspended sediment loads for the period from 1971 to

1975 and 1981 to1985 have been found to be 0.02036 and 0.005037 per cent of the

286

volume of water respectively. This is a significant variation indicating an abrupt fall of

suspended sediment load between the two spans of time, i.e. 1971-75 and 1981-85.

The graphs representing the 12 monthly average of suspended sediment load

for 1997-99 witnessed the highest daily value of suspended sediment load for any

month being 0.032 per cent of the volume of water and it was recorded in May, July

and August, 1998. The average suspended sediment load for this period was found to

be 0.010028 per cent of the volume of water. The period from 1997 to 1999 records a

steady rising trend of both monthly maximum and monthly average suspended

sediment load of the river. These values have been shown in figure 8.14 and it depicts

that suspended sediment load of the river was highest during the early seventies. It

came down to an abrupt low during early eighties and finally started to show an

increasing trend during late nineties.

Fig. 8.14: Maximum and average monthly suspended sediment load of river Bhogdoi

Primary investigation identifies two aspects responsible for such changes in

the amount of suspended sediment load in the river. Human settlement began in the

foothills of the Bhogdoi basin in the sixties of the last century. Clearing of forest for

settlement and shifting cultivation caused heavy erosion in the foothills. The eroded

materials were transported to the plains of Assam by the river. These transported

0.00

0.02

0.04

0.06

0.08

0.10

0.12

1971-75 1981-85 1997-99

% O

F V

OLU

ME

OF

WA

TER

SUSPENDED SEDIMENT LOAD

MAXIMUM MONTHLY LOAD AVERAGE MONTHLY LOAD

287

materials got deposited at the foot of the hills where the river enters the plain. This

caused the river bed to rise and consequently decrease in the gradient of the river.

Decrease of gradient, on the other hand, reduced the load carrying capacity of the

river.

Till the seventies of the last century the river was capable of carrying the

suspended materials to a long distance. The decrease of gradient in the subsequent

period reduced the load carrying capacity of the river. The gauge site of the river is

located 19 km downstream from the foothills. For this reason, the suspended sediment

load decreased significantly between the seventies and the eighties of the last century.

It is noted that suspended sediment load shows some increase between the eighties

and the nineties of the last century. It is because various construction activities have

been going very fast in Jorhat district since the eighties of the last century. River

Bhogdoi is the only source of sand for construction purpose in the district. Heavy

collection of sand, particularly from the upper reach of its plain course has been taking

place at an ever-increasing rate since the eighties. This has caused lowering of the

river bed to a significant extent and resultant steepening of the gradient of the river

bed. This has been identified to be one of the reasons for increase in the load carrying

capacity of the river between the eighties and the nineties of the last century.

Table 8.18: Textural composition of suspended sediment load of river Bhogdoi at Jorhat

Time span Coarse particles

(% of the total volume of

suspended sediments)

Medium particles

(% of the total volume of

suspended sediments))

Fine particles

(% of the total volume of

suspended sediments)

1971-75 19.92 33.64 46.44

1981-85 35.97 39.21 24.82

1997-99 36.57 41.83 21.60

Source: Water Resource Department, Govt. of Assam

288

The composition of the coarse, medium and fine suspended sediment particles

at Jorhat gauge sight is studied. Considering the entire period from1971 to1999 the

average percentages of coarse, medium and fine particles are found to be 26.01, 36.24

and 37.75 per cent respectively. It implies that volume of fine suspended particles is

more than that of the coarse and medium-size particles.

The table 8.18 shows the textural composition of the suspended sediments.

Analysis of data for the period 1971-75 reveals that the coarse, medium and fine

particles constituted 19.92, 33.64 and 46.44 per cent of the total volume of the

suspended sediments where fine particles dominated the other two types. Between

1981 and 1985, the shares of coarse, medium and fine sediments were 35.97, 39.21

and 24.82 per cent respectively. Again, during 1997-99, coarse, medium and fine

particles constituted .it is also noted that the coarse suspended particles in the river

water have been increasing over the years. The medium particles, however, show a

steady increase.

Impact of urbanization of Mariani town and Jorhat city as well as collection

of sand from Bhogdoi bed at various places in the upstream portion of the gauge sight

has caused such size distribution of sediments. A discussion about the collection of

sand from Bhogdoi bed has been made in the subsequent part in this chapter.

8.7: Depositional Characteristics of the River

Aggradational activities and consequent extinction of wetlands in the lower

Bhogdoi basin has been discussed in the introductory part of this chapter. The

depositional behavior of the river with regard to the changes of gradient in its plain

course has already been narrated. The impact of the 1950 great earthquake,

construction of embankment along the bank of Brahmaputra which closed the mouth

289

of river Bhogdoi in 1956 and consequent meeting of river Bhogdoi with river

Kakadonga are all interlinked and associated with the rise of the bed of river Bhogdoi.

In view of these, the river started to show changes in its depositional behaviour,

particularly after 1956.

The rise of the river bed as a result of deposition of sediments started at an

enhanced rate since the sixties of the last century. It initiated an intermittent problem

causing inundation on both of its banks. As the siltation and rise of the river bed are

ongoing and continuous processes, the frequency and intensity of inundation increased

with time. Presently, deposition and aggradation of the river bed can be viewed

distinctly by naked eyes along the 23 km reach of Bhogdoi from Gormur to Solmara.

In this segment, the river bed appears to stand above the agricultural fields and

villages on either side of its channel. The city of Jorhat is also located in this segment

of the river channel.

Primary investigation with regard to rise of the river bed discloses that the bed

of the river near Jorhat city area rose by 1.35 m during the last 38 years. In the year

1972 a two storied building was constructed in the city near a railway bridge across

Bhogdoi. The plinth level of the building was kept at an elevation of 5.5 m (18 feet)

from the basement of the pillars of the railway bridge the upper surface of which was

visible at that time. The plinth of the same building is now only 4.15 m (13.5 feet)

above the river bed. This clearly indicates that the upper surface of the basement of

the pillar is buried under 1.35 m thick layer of sand.

In an unpublished official report prepared by the Embankment & Drainage

Division, Water Resource Department, Government of Assam relating to construction

of embankment along the banks of Bhogdoi, the reduced level (RL) of the river bed

was 86.34 m in 1965 at A.T. Road crossing near Jorhat, which became 88.69 m in

290

2009 at the same point. This also indicates that there is an upheaval of the river bed by

1.75 m in the last 44 years. This started to cause floods at different locations of the

plains. Bhogdoi is now flowing above the average level of the Jorhat municipal area.

This demanded the erection of embankments on both of its banks in different phases.

During the first phase the section of embankment from J B Road to Pujadubi was

completed in 1972. It was further extended from J B Road to Chengeli Ati in the

second phase and from Pujadubi to Garmur in the third phase. Since then raising and

strengthening works of these embankments have been undertaken for two times by the

Water Resource Department, Govt. of Assam. Now, Bhogdoi has a span of 23 km

long embankment from Solmara to Garmur along its banks.

Table 8.19: Collection of sand from Bhogdoi river bed by trucks

Source: Personal survey covering all the sand collection sites

The erection of embankments on both the banks of the river caused deposition

more acute than ever. All the materials coming down from the foothills started to be

deposited in the narrow strip between the embankments. This raised the river bed in a

much rapid way. Mention may be made that scouring activity in the river bed has been

going on at several places. The state Forest Department has 6 sand mahals (quarries)

from Mariani to Jorhat city. These 6 sand mahals have 8 sand collection sites along

Sand

collection

sites

Average daily no. of

trucks in winter months

Average daily no. of trucks

in summer months

Daily average

no. of trucks

Mariani – 1 35 20 27.5

Mariani– 2 25 10 17.5

Kathanibari 42 18 30

Pukhuria 30 14 22

MES Gate 15 8 11.5

Jorhat 45 30 37.5

Malow Ali 30 18 24

Solmara 1 1 1

Total 223 119 171

291

the river channel. The materials deposited in the river bed are being carried away by

trucks. The sand of the upper reach of the river is used for construction works and that

of the lower reach is used mainly for filling purpose. It is important to note that

collection of sand from the bed of Bhogdoi continues all the year round except few

days in summer when the water level goes up. There is, however, marked seasonal

variation in summer and winter collection of sand.

An attempt has been made to estimate the annual total volume of sand

collected from the bed of Bhogdoi (table 8.19). The table shows that on average

daily171 trucks of sand are being taken out from the eight collection sites. Of these

124 trucks are mini trucks and 47 trucks are large trucks. Considering the load

carrying capacity of one mini truck as 2.80 m3 (100 cubic feet) and that of a large

truck as 7.50 m3 (265 cubic feet), the average daily volume of sand collected from the

river bed is 700 m3. At this rate the annual volume of sand collected from the river bed

rests at 2,55,500 m3.