Mirror of Assam - Natural Hazards
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NATURAL HAZARDS

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 :

FLOOD

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 Arunachaal Pradesh.

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 district.

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 :

Year Land affected Value of damage in Lakh Rs
1954 31,590 km2 1,750
1962  16,290 km2 2,064
1966  18,046 km2 2,447
1972  11,024 km2  2,462
1986 15,000 km 40,000
1987 22,230 km2 70,000
1988 20,250 km2 70,884

  

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 :

1. Heavy 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 erosion.

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 and worms.

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 the field.

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 erosion.

4) Stopping 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.

5) Undertaking 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 rivers.

EARTHQUAKE

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 the region.

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.

Some Famous 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.

 

 
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