North East India
experiences a number of natural hazards almost every year. Of the common
natural calamities, occurrence of floods, river bank erosion, droughts
hail storms and not infrequent earthquakes are the main. Geographical
factors like situation, location, underlying geological structures,
relief, etc. are primarily responsible for the occurrence of these
calamities. Of late, human activities like destruction of the forests.
unplanned construction of embankments and roads, cutting of hill slopes,
etc. have accentuated by the region have been described below :
Flooding of the plains
and valleys during the rainy season is a common hazard in North East
India. It causes immense destruction of crops, property and even of life
in the region. The floods are a common feature occurring in this region
since time immemorial. These used to leave silt on the agricultural fields
making them fertile every year. But their intensity and destruction have
increased since the great earthquake of August 15, 1950. This earthquake
disturbed the courses and beds of many rivers like the Brahmaputra,
Subansiri, Burhi Dihing Jia Bharali, etc. causing them either to change
their courses or by raising the beds resulting in occurrence of floods
even after a few showers at their catchments area. Besides, the earthquake
raised the beds of many ox-bow lakes and swamps, which had earlier acted
the reservoirs of excess water.
Flood hazards occur
regularly in about 14 districts of the Brahmaputra Valley, two districts
of Barak Valley, four districts of Manipur plain and two districts of
During the rainy season,
the rivers not only become filled up with water, but the excess amount of
water also spill over their banks flooding their neighbouring regions. At
the same time bank erosion occurs in almost all the large rivers
destroying human habitats, crop fields, roads and railway tracks.
It is noticed that about
2,900,000 ha of land in Assam alone is flood prone. Similarly large areas
of Manipur plain and western Tripura are also flood prone, During the last
few decades there were heavy floods in Assam in almost every alternate
year, especially in 1954, 1957, 1966,1968,1987,1988,1990,1991,and 1998. It
is estimated that average annual loss due to destruction of crops alone by
floods in North East India stands at about Rs.15 crores. Besides, there
are destruction of roads, railways, bridges and human settlements and loss
of human and animal life. In certain years floods may occur more than once
in one summer.
The rivers that
generally cause flood havoc in their surrounding areas are as follows: The
Lohit, Balijan and Na Dihing rivers in the plains of Dibang Valley and
Lohit districts of Arunachal Pradesh. The Kundil, The Kundil, Lohit, Na
Dihing and Dhola in Tinsukia district. The Burhi Dihing and Brahmaputra in
Sibsagar district. The Brahmaputra, Bhogdoi and Gelabeel in the Majuli
Island and the riverine tract of Jorhat district . The Brahmaputra,
Gelabeel and Dhansiri in the northern part of Golaghat district including
the Kaziranga National Park. The Brahmaputra and Kapili in Nagaon
The Brahmaputra, Kapili
and Kolong in Marigaon district. The Burhi Suti, Gai and Jia Dhol in
Dhemaji district.The Subansiri and Dikrang in Lakhimpur district. The
Puthimari, Baralia, Pagladia and Brahmaputra in Kamrup district . The Beki,
Manas, Chaulkhown (Nakhanda) and Brahmaputra in Barpeta district. The
Brahmaputra in Goalpara district and the Brahmaputra, Gadadhar, Gangadhar
and Jinjiram in Dhubri district. The Barak in Cachar district and the
Kushiyara and Singla in Karimganj district. In the Manipur Plain flooding
is often caused by the Imphal and Iril rivers in the two Imphal districts
and by the Thoubal river in Thoubal district. In Tripura occasional floods
are caused by the Khowai and Gumti rivers in the West Tripura districts.
Side by side with
floods, bank erosion of some major rivers also causes immense long-term
destruction every year. The river Brahmaputra has been shifting slowly
southward. Many of its tributaries have also been shifting-some eastward,
some westward and some like Burhi Dihing, Dhola, etc. towards south. Such
shifts slowly come about through bank erosion in every rainy season is
caused by Na Dihing and Dhola in Tinsukia district, by the Brahmaputra in
Dibrugarh district, by the Burhi Dihing and Brahmaputra in Dibrugarh
district , by the Brahmaputra in Majuli and Neamati areas of Jorhat
district, by the Gelabeel (a branch of the Brahmaputra ) in Golaghat
district. In fact the Brahmaputra causes bank erosion also in the
Bhuragaon of Marigaon district, Palasbari area of Kamrup district and
South Salmara area of Dhubri district. In the north bank immense
destruction of built up land is caused by the bank erosion of the Gai, Jia
Dhol, Subansiri, Dikrong, Pagaldia, Beki and Gangadhar rivers. Arunachal
Pradesh also experiences river bank erosion by the rivers by the Luhit,
Dibang, Na Dihing ,etc. especially in the districts of Luhit, Dibang
Valley and East Siang.
The extent of damage
caused by floods and river -bank erosion in Assam along can be seen from
the following estimate :
||Value of damage in Lakh Rs
|| 16,290 km2
Apart from floods and
bank-erosion, landslide, especially in the hill states of North East India
is also a common phenomenon occurring in the rainy season. Frequent
landslides block the roads linking the towns and villages with one another
thus causing great inconvenience to transport and communication and trade
and commerce of the region.
Causes of Occurrence of Floods :
As stated above, there
are a number of geographical factors which are responsible for occurrence
of floods. These factors are often accentuated by certain human
activities. However, the factors are discussed below :
Rainfall and snowmelt water in Summer :Throughout North East India,
average annual rainfall is very high, varying between 100 cm to 1300cm.
Besides, about 80% of the annual rainfall comes mainly in the five summer
months from May to September. This is also the period when the snow over
the Himalayas melt and water there from rolls down the rivers to the
Brahmaputra Valley. The supply of water thus becomes excessive in the
rivers and flooding occurs
2. Location of the
Plains : Each of the
Brahmaputra, Barak and Manipur Plains are surrounded on there side by high
hills and mountains. Whenever there is rainfall in the hills and mountains
water rushes down to limited Plains from extensive catchments areas,
flooding the Plains.
3.Low Gradient of the
Plains : The Brahmaputra, Barak and Manipur river valleys grade very
gently toward their down stream . It is found that the gradient the
gradient of the Brahmaputra between Sadiya and Dhubri is 13 cm per km on
the average. In case of Barak and Manipur Valley also the gradient is
similarly low. Therefore, excessive rain and snow-melt water flows down
very slowly causing floods in these plains.
4. Choking of the River
Channels : (a) The hills and
mountains of the Himalayas, eastern Arunachal, Nagaland, North Cachar
Hills, Manipur, Mizoram and Tripura are made of relatively soft Tertiary
rocks. The heavy monsoon rains that fall on them further soften the
materials causing solifluction and sheet erosion down the steep slopes.
The streams and rivers carry these sediments easily down the steep hill
slopes, but as soon as they (the rivers) reach the plains their carrying
capacity is reduced and the sediments are deposited on their beds, thus
choking the channels. They, therefore, overflow the channels causing
flood. (b) Further, the great earthquake of the 15th August
1950, raised the bed of the Brahmaputra, Dibang, Subansiri and many other
rivers. It is estimated that the earthquake raised the bed of the
Brahmaputra by about 22 cm on the average. The rising of the bed naturally
reduced the volume of the channels making the rivers shallow. The water
carried by them, therefore overflows in summer causing floods. (c) Serious
sheet -erosion is caused also by human interference of nature. With rapid
increase of population and technological development , man has been
indiscriminately cutting the forest cover of the catchments areas of the
rivers. Thus the top soil retaining capacity of the catchment areas has
decreased and the lossened materials of the soil is washed down by the
streams and rivers. These are deposited on the channel beds in the plains
making the channels shallow. Moreover, man also constructs dams and
embankments for flood control. All these have adverse effect by way of
occurrence of flood and water -logging in some other areas.
5. Shifting of the
River Courses : The river of North East India, especially the ones
that come down from the Himalayas and the Tertiary hills change their
course frequently. This is mainly because the rivers carry enormous
quantities of sediments from the hills and on reaching the Plain the
sediments are deposited on their own beds. The channels are thus filled up
and the water in the following summers dig out different courses
abandoning the earlier ones. Besides, the seismic instability of the
region also contributes to such shift. The result is that the rivers in
the process of their shift causes devastation through floods and bank
The floods that occur
almost every summer in the plains of the North East India cause both harm
and benefit to the areas concerned. The most important benefit they
provide is the deposition of fertile silt on the crop fields. They also
spread the fish seeds and wash the countryside. But the immense harm that
the floods cause often outweighs the benefits. Floods cause devastation by
destroying crops and houses and by washing away domestic and wild animals
and sometimes even causing death to people. Floods are often followed by
epidemics and endemics due mainly to contamination of water. In the low
lying areas where drainage facilities area absent, pools of stagnant water
remained standing for long, preventing any economic use of such lands and
providing breeding grounds for mosquitoes and such other harmful insects
Uncertainty of places
and times of occurrence of flood is another important aspect which adds to
the misery of the people. Partly because of siltation of the river beds
partly because of unplanned construction of embankments, flooding occurs
in a summer. These uncertainties have adverse effect on the enthusiasm of
the farmers to do hard work, for, they hardly know if their labour shall
be at all rewarded with bountiful harvest.
Measures of Checking
Floods and Necessary Relief Activities :
No effects were made
till about 1940 to check flood havocs in North East India. During the
period between 1942 and 1951, only about 262 km of embankments, was
constructed to check flooding. Apart from construction of embankments ,
digging of canal, and making provision of sluice gates to evacuate floods
water can be a fruitful measure to reduce the impact of floods. But these
have to be done after a thorough study of the causes and consequences in
During the First Five
Year Plan as much as 1000 km. of embankments along the banks of the
Brahmaputra and some of its tributaries and 500 km of embankments that of
the Barak and its tributaries were constructed. Besides, measures to save
the Dibrugarh town area from severe bank erosion were also taken up.
During Second Five Year Plan measures were taken to check Brahmaputra bank
erosion of Dibrugarh again and at Palasbari, Soalkuchi, Tarabari,
Goalpara, Dhubri and six other places. A River Research Station was also
started in this period in Assam (at Byrnihat, later shifted to Vashishtha
area). During the Third and Fourth Five Year Plan periods several schemes
were taken up to control flood, besides making some embankments permanent,
constructing some sluice gates, building sand and boulder spurs in the
spots of bank erosion and making provision for collecting data about the
flow of water, sediments etc. of the important rivers.
The Flood Control
Department of Assam has so far constructed 984 km of embankments on the
banks of the Brahmaputra, 2567 km of embankments on the those of its
tributaries, besides constructing 600 km of drainage canal, 56 sluice
gates and 421 bank protection spurs. Similarly, in the Barak Valley 738 km
of embankment, besides construction 600 km of drainage canal, 56 sluice
gates and 421 bank protection spurs. Similarly, in the Barak Valley 738 km
of embankment, 247 km of drainage canals and 29 sluice gates were
completed. In the North Cachar and Karbi Anglong 28 km of embankment and
54 bank protection spurs have been completed. It is said that these
measures have been able to protect 16.08 lakh hectares out of the total of
31.50 lakh hectares of flood prone area of Assam. The fruits of these
measures have, however, been found to be not permanent. In the year 1988
as much as 38,268 lakh hectares of agricultural land was affected by
floods in Assam alone. However, the efforts to control flood. have still
been going on. In 1981 the Brahmaputra Board was reconstituted with the
aim of checking bank erosion, producing hydroelectricity and irrigating
agricultural land besides controlling flood. The Board also has plan to
construct dams across the Subansiri and Burhi Dihing rivers to control
flood and produce as much as 8,500 mw of electricity. Further, the Board
undertakes to control floods caused by the Barak river by constructing a
dam across the river at Tipaimukh (Tuivaimukh) at the trijunction of
Assam, Manipur and Mizoram.
The important measures
necessary to be taken immediately, for controlling flood and mitigating
the miseries caused by it, are listed below :
1) Construction of
embankment in a planned manner in some selected areas only.
2) Controlling the major
river by constructing dams and reservoirs.
3) Checking bank
deforestation and taking up afforestation in the catchment areas. This
will stop soil erosion at the headstream regions and siltation of the
river beds at the plains.
protection work of settlements, both rural and urban.
6) Construction of
drainage channels, culverts and sluice gates wherever necessary.
7) Construction of
raised platforms near the settlements of the flood prone area for taking
shelter (for both man and animal )in the period of high flood.
It is by many scholars
that in case of flood, protective measures with lest interference causing
to the rivers are more durable than the positive measures interfering the
Next to flood,
earthquakes are common natural calamities experienced in North East India.
The region falls in the immediate neighbourhood of the newly formed
Himalayan Mountain zone. Therefore, it is tectonically weak and
seismically unstable. It is said that Himalayas are still rising. This
condition affects North East India causing frequent earthquakes. Some of
the important region, as to why earthquakes are so frequent in the
north-eastern India, are noted below :
1. The Himalayas are the
youngest folded mountains. These began to be built up the middle Tertiary
Period. They are still rising. Besides they are still to be compensated
isostatically. Hence these move, causing earthquakes in the region.
2. There are several
large faults in this region. Of them, The Main Boundary Fault
running east-west along the foot of the Himalayas and the Haflong-Disang
Fault running north east-southwest from the southern Meghalaya border
to beyond Margherita are the main. These faults may cause instability to
3. North East India has
many river and streams and widespread heavy rainfall. Millions of tonnes
of materials of the crust are removed from the hills and mountains and
deposited in the plains. This process may also cause isostatic
instability, resulting in frequent tremors.
4. It is now said that
the mechanism of Plate Tectonics working in this region is the main cause
of the frequent earthquakes of North-East India. The region is located at
the north-easternmost margin of the Indian Plate. This plate is migrating
slowly to the northeast pushing itself onward below the South-East Asian
and Central Asian Plates. The forces and frictions produced in this
process cause tremor in North-East India which is lying over this margin.
A study of the location
of the epicentres of the earthquakes reveals that most of them lie
along the Himalayan and Patkai-Arakan mountain chains. Besides, the
Meghalaya Plateau and its immediate surrounding areas also contain some
epicentres. The following account of the famous earthquakes of the region
clearly shows that most of the epicentres of these earthquakes lay in the
above mentioned two areas of North East India.
Earthquakes of North East India :
North East India has
been experiencing earthquakes since time immemorial. The Tungkhungia
Buranjee, a book on history of the Ahom rule, written in 1696, recorded
some important earthquakes of North East India in the mediaeval period.
Following are the main earthquakes of the region that occurred since 1869
1. In 1869 there
occurred an earthquake on January 10 that probably had its epicentre at
North Cachar Hills. It affected an area of 647 km 2 3. with
deep cracks and eruption of mud and water through them.
2. The great earthquake
of 1897 occurred on Jun 12, It affected more than four lakh square km of
area covering North East India , the then undivided Bengal and eastern
Bihar. It intensity was 8.7 in the Richter Scale. Its epicentre was in
Meghalaya Plateau. This devastating earthquake badly affected the Plateau
and its surrounding region, especially western Assam and Sylhet of
Bangladesh. Many houses, building, Bridges and roads were destroyed. There
are cases of upliftment of some areas and sinking of some others. Many
river-beds were found to have been raised and many rivers changed their
courses. Even a forested foothill area about 40 km to the south-west of
Guwahati sank down to give rise to the present Chand Dubi Lake, which
still bears the submerged stumps of the large trees.
In 1918 there occurred
an earthquake tremor of severe intensity on July 8 in this region . Its
influence was felt over an area of 225, 234 km2. The probable
epicentre of this earthquake was at Sri Mangal. The sympathetic shocks of
this tremor was felt even in the Arakan and Coromondal Coasts on both
sides of the Bay of Bengal.
4. A very serious
earthquake was also felt on the nineth September of 1923 in North East
India. Its epicentre was at a place to the north of Cherrapunji in the
southern part of Meghalaya ( 2505`N 910 5` E). Its
tremor was felt all over eastern India including Chotangpur Plateau,
undivided Bengal and North East India. Destruction due to this earthquake
was extensive in the western part of Assam and Bangladesh.
5. Another devastating
earthquake occurred in the region on July 3, 1930. It affected an area of
906,115 km2 with its epicentre lying at 25 0 8` N
and 90 0 2` E near Dhubri.
6. On August 14, 1932
there occurred an earthquake with its epicentre at north western Myanmar.
It affected North East India , undivided Bengal and northern Bihar.
7. Another serious
earthquake was felt in this region on January 21, 1941. Its epicentre was
in the upper Brahmaputra Valley.
8. On October 23, 1943
there occurred another devastating earthquake with its epicentre at a
place some kilometres to the north of Dibrugarh (270 5`N 9305`E).
It also affected whole of eastern India.
9. A major earthquake
was also felt in the region on July 29, 1947. Its epicentre was in West
Siang District (2805` N 940 E ). Apart from North
East India, it affected the Himalayan foothill region upto Bihar.
10. The most devastating
earthquake of the region of recent memory is that of August 15, 1950. It
occurred in the evening (7-40 P.M. ) of the Independence Day with its
epicentre at 2802`N and 970 E in the easternmost
part of Arunachal Pradesh. Its tremor read 8.6 in the Richter Scale.
Starting from the early evening the tremor continued throughout the night.
Many houses and buildings of the eastern part of Assam were razed to the
ground. There developed innumerable wide and deep cracks on the ground and
mud, sand and water burst out damaging the roads, settlement sites and
crop fields. There were many incidences of landslide in the Himalayan
region. In fact such a landslide at the head stream portion of the river
Subansiri kept the water of it blocked for a few days. Subsequently this
caused devastating floods on both of its banks in the plains when the dam
burst. Many Mising villages were washed away causing loss to men, material
and animals, both domestic and wild. In many areas large patches of land
sank down while in many areas there were upheavals. This naturally caused
change of many river courses and rise of river beds. It is found that the
bed of the Brahmaputra itself rose up in many areas, causing bank
inundation even for a slight rise in its water level. It is also noticed
that a large number of lakes, marshes and swamps in Assam became shallow
as a result of this earthquake.
After this devastating
earthquake, the region has not experienced any severe earthquake so far.
However, two tremors of medium intensity were felt during this period, one
on July 29, 1970 and other in August. 1988. Further, it is noticed that
the frequency of mild earthquake tremors has increased since last 15
years. Considering this situation one can say that North East India is
seismically an unstable region.