A Hazard is a situation which poses a level of threat to life, health, property or environment. Most hazards are dormant or potential, with only a theoretical risk of harm, however, once a hazard becomes ‘active’, it can create an emergency situation. The unique geo-climatic conditions of the regions make Assam and the entire northeastern region of India very prone to natural disasters like flood, earthquake and landslide. The state of Assam experiences perennial floods, river bank erosion, landslide and other environmental catastrophes.
Disasters cause sudden disruption to the normal life of a society along with enormous damage to property associated with high casualty of human life. A review of the past disasters indicates that the state had to bear the devastations of two natural disaster floods and earthquake. Natural Hazards:
Earthquakes in India are caused by the collision of the Indian Plate with the Eurasian Plate in the north. Every year the Indian Plate moves roughly 5 centimeters northward, pushing under the Eurasian Plate and forming the Himalayas and other great mountain chains in the northern part of the Indian peninsula.
For great earthquakes the movement can be several meters and can covers several hundreds of square kilometers of the contact surface between the two plates. Most earthquakes in the northern part of the Indian Sub-continent are a result of this complex process. Earthquakes which occur at the margins of plates (or plate boundaries) are known as inter-plate earthquakes and account for 95% of the global seismic activity annually.
The North-eastern region of India is an earthquake prone area. The region has experienced a large number of earthquakes of tectonic origin. The risk probabilities of earthquake are less over the entire Brahmaputra valley. The region of Northeast India is seismically very active. Two major earthquakes of magnitude 8.7 occurred in 1897 and magnitude 8.6 in 1950 causing large scale damage of lives and properties in this region. Sir Edward Gait (1933) has mentioned about the occurrence of destructive earthquakes in this region in 1548, 1596, 1607, 1642, 1663, 1696-1714, 1869, 1882, and in 1897. In the present century, destructive earthquakes occurred in 1918, 1923, 1930, 1932, 1938, 1943, 1947, 1950 and in 1988.
Much of Assam lies in the Brahmaputra River Valley, except for a few southern districts. The northern and eastern parts of this valley are bounded by the Himalayan Frontal Thrust (HFF). In the eastern parts along with the HFF, there is the Lohit and Naga Thrusts. Among the large earthquakes in this region were the events in 1869 and 1897. The 1897 earthquake is well known for the dramatic accounts of violent upthrow during the shock.
Source: For preparation of Environmental Atlas of Assam, ASTEC
SIGNIFICANT EARTHQUAKES IN ASSAM
12 June 1897 – Near Rangjoli, Assam, Mw. 8.0
Latitude: 25.500 N, Longitude: 91.000 E
This was one of the most powerful earthquake in the Indian sub-continent. The earthquake damaged upto Kolkata where dozens of buildings were badly damaged or partially collapsed.
The earthquake caused great destruction to many towns in Assam and Meghalaya. 1500 people were killed and hundreds more hurt. In Shillong, where most of the structures like the Telegraph House, were demolished. Landslides were reported all across the Garo Hills. The towns of Dhubri, Goalpara, Guwahati and Kuch Bihar in Assam and West Bengal was heavily damaged. Earthquake fountains, some 4 feet high, were reported from Dhubri. At Goalpara, a 10-foot wave from the Brahmaputra, swept into the area, destroying the bazaar and many pukka buildings. Ground waves were reported from Nalbari, where the rice fields rise and fall as the waves passed under them. At Guwahati, the earth subsided along the Brahmaputra and several sand vents were formed. The Brahmaputra is also reported to have risen by 7.6 metres and even reversed its flow during the shock. The earthquake formed frozen earth waves of “jinamis” in a rice field in lower Assam, where the crest to trough difference was between 2 to 3 metres. Fissures and sand blows occurred over a wide area of Assam, Meghalaya, West Bengal and northern Bangladesh.
The entire state of Assam lies in Zone V. Here earthquakes of upto MM intensity IX can be expected. According to a hazard map by the Global Seismic Hazard Assessment Programme (GSHAP), the state can expect to have a peak gravitational acceleration (PGA) of 0.24g to 0.48g. The region where the highest PGA can be expected is along the state’s border with Meghalaya, the site of the Great Indian earthquake of 1897.
10 January 1869 – Cachar (Assam), India, M 7.5
Latitude: 25.000 N, Longitude: 93.000 E
It caused serious damage in the region. The impact of the shock was felt over 6,50,000 square kilometres. There was heavy damage in the towns of Cherrapunji, Silchar, Shillong and Sylhet and also in Manipur. Fissures opened on the banks of the Surma river and sand vents threw up great amounts of sand and water. The epicentral tract was 30 – 45 kilometres long and 5 – 6 kilometres wide lying on the northern border of the Jaintia Hills. The hypocentre had a depth of 50 kilometres.
21 January 1941 – Near Tezpur, Assam, Ms 6.5
Latitude : 26.500 N, Longitude : 92.500 E
23 October 1943 – Near Hojai, Assam, Mw 6.9
Latitude : 26.000 N, Longitude : 93.000 E
The quake was centred 13.7 kilometres E of Hojai (Assam, India) and 32.7 kilometres NW of Lumding (Assam), India, and 74.5 kilometres W of Dimapur
(Assam), India. The MM intensity near Dimapur was VIII to IX.
15 August 1950 – Indo-China Border Region, Mw 8.6
Latitude: 28.500 N, Longitude: 96.500 E
There was widespread devastation in Upper Assam, the Abor Hills and the Mishmi Hills. Districts of Jorhat, Lahkimpur, Sibsagar and Sadiya, in Assam. Dibrugarh and Saikoaghat were among the worst affected areas. Railway communications were disrupted due to damage to tracks and bridges. However, the area that suffered damage and encompassed by the isoseist VIII was nearly 75,000 square miles. There were fissures in the earth, from which water and sand was emitted. These are called sand vents and represent liquefaction due to intense ground shaking. Vast areas of land either were elevated or subsided, altering the drainage of the region. There were huge landslides in the mountains and these dammed tributaries of the Bramaputra River, like the Dihang, Dihing and Subansiri. Dykes blocked the tributaries of the Brahmaputra; that in the Dibang valley broke without causing damage, but that at Subansiri opened after an interval of 8 days and the wave, 7 metres high, submerged several villages and killed 532 persons. The earthquake was followed by a large number of aftershocks, most of which were of magnitude 6.0 or greater. The aftershock zone extended from 900 east longitude to 970 east longitude.
31 December 1984 – Silchar, Assam, Mw 6.0
Latitude :24.640 N, Longitude : 92.890 E
In the Cachar district, 20 persons were killed and 100 were injured. There was extensive damage in southern Assam. Underground pipes were broken and the ground was cracked. Sand, mud and water spewed forth and subsidence occurred along wide stretches of the Sonai River. Two bridges on the Sonai – Kachidharan road collapsed and others were damaged. An intensity of VI was recorded at Banskandi and Palanghat and intensity of V in Kumbhirgram, Lakhipur, Rajabazar, Silchar and Udarband.
Flood the most frequent natural calamities faced by India. A relatively high flow or stage in river, marked by higher than the usual, causing inundation of low land or a body of water rising, swelling and over flowing land not usually covered by water is termed as flood. It is essentially a natural hydrologic phenomenon with a large volume of surplus water that inundates the flood plains, interfere greatly socio – economic condition. Studies shows that flood levels to the extent of 2 meters could be brought down and severity of floods could be substantially reduced in the rivers along with generation of hydropower of the order of 30,000 MW in N.E Region (M.U. Ghani).
The mean annual rainfall in the Brahmaputra basin is around 2300 mm. Internationally the Brahmaputra rank 10th in the world in terms of discharge. The water resources of NE Region has been assessed to be about 31 % of the country. However harnessing of water for irrigation and other beneficial purposes in Brahmaputra is 3%. Monsoon rainfall accounts for 75% to 80% of the annual rainfall. The flood problem has needlessly been continuing since time immemorial. This could have been overcome by the best utilization of huge water resources of the region for poverty alleviation and sustainable development which appear as disastrous flood in some part or other part of the region every year resulting in colossal damage.
Major river systems
The Brahmaputra is one of the largest rivers in the world. Another important river system of Assam is the Barak, the head stream of the Meghna, rises in the hills of Manipur in India and flows south-west for 250 Km. At Lakhipur, it emerges from hills and at Bhanga, it splits into the Surma and Kushiyara which cross into Bangladesh near Karimganj. Northern tributaries of the Brahmaputra and Barak are braided and unstable in their reach. The instability of the river is mainly attributed to high sediment charge, steep
slopes and transverse gradient. Apart from these the entire area is in highly seismic zone and receives Earthquake shocks of severe intensity periodically. This is also one of the factors for unstable character of the river. The Earthquake that occurred in those periods considerably disturbed the drainage network of Assam in 1897 and 1950.
The Brahmaputra and Barak river system is subjected to frequent heavy floods, drainage congestion and bank erosion resulting in extensive submergence of land, loss of life and property as well as disruption in communication system. At times the period of floods above danger level is 40 to 70 days. The impact of floods was not felt to the same extent in the past as is felt now. This is due to rapid increase of population and subsequent increase in the all-round activities of man. The flood plain is gradually occupied to meet ever increasing requirements of food and fiber and consequently the flood problem has been accentuated.
Causes of flood
Floods in the region occur due to variety of causes such as:
River channel carrying flows in excess of the transporting capacity within their banks. This is due to excessive precipitation that occurs in NE-India. Backwater in tributaries at their outfalls into the main river because of non-synchronization of peak floods in them. Heavy rainfall in short space of time.
Aggravation of river bed.
Inadequate waterways at rail and road crossing and encroachment in the flood plains. Degradation of catchment area in forms of deforestation, jhuming and loss of soil mantle in Himalayan friable watershed. Lack of proper control of landus
The rainfall induces drainage problem occurs in naturally low land is severe
and acute in this part of the country. The drainage congestion is caused due to heavy precipitation of short duration and higher flood level in the main river, which doesn’t allow the water to drain into main river quickly. Drainage congestion is also caused due to construction of road, rail and embankments, which obstructs natural flow and encroachment in the riverine areas due to population pressure. The lack of sufficient capacity of drainage channel and natural bowel shape topography of land resulting from defunct river courses also contribute to drainage congestion problem.
Bank erosion of the Brahmaputra and its tributaries has become a matter of serious concern to both people and the Govt. consuming large annual exchequer of the Govt. in erosion control works. Along the valley specially in Dhemaji, Lakhimpur, Morigaon and Dhubri long reaches of river bank are eroded and consequently number of villages on record are seen left inside the Brahmaputra. The causes of heavy erosion are attributed to excessive sediment load, steep bed gradient, transverse bed slope, non cohesive erodible nature of bank material, formation of char island and consequent development of side channel. Majuli island and the Kaziranga National Park are also the worst victim of this erosion process. It is also observed below the confluence of major tributaries.
The extent and magnitude of flood problem is assessed in terms of different types of damages caused by flood. The floods of 1988 and 1998 of the Brahmaputra basin were unprecedented which completely shattered the economy of the state. Comprehensive studies have been undertaken to go into the various aspects of floods and flood control. Various aspects both structural and non-structural measures that are considered for formulation and implementation (M.U.Ghani).
Problems due to flood
The main problems due to floods are inundation, drainage congestion and bank erosion and consequence is damage to property. The problems depend on river system, topography of the place and flow phenomenon. The space borne satellite Remote Sensing technology is found to be an effective tool to
disseminate the proper information in near real time basis.
The issues related to floods in the Brahmaputra are:
Narrow size valley (80-90 Km)
Highest rainfall (average nearly 2600 mm)
Tributaries are within the close range
Peak rainfall profile ranging from mid-May to mid-September
Settlement in vulnerable areas
Source: For preparation of Environmental Atlas of Assam, ASTEC
Satellite Remote Sensing is found to be an effective tool to supplement this in near real time basis. Near Real time information is the key for the user departments in organizing the relief operations. For this flood inundation information has to be furnished to them as early as possible and space borne technology is found to be authentic and cost effective tool for dissemination of the same. For this State Remote Sensing Centre has to play an active role. Under the DMS programme as per guideline the digital flood map is generated at NRSA, Dept. of Space, Hyderabad and transmit the same to the State Remote Sensing Centre through internet for generation and dissemination of hard copy flood maps to the users. To use satellite remote sensing more effectively water level data of the rivers is necessary (source: CWC, Govt. of India). Since rising trend above danger level is indication of probable occurrence of flood, the water level information will facilitate to select proper satellite data during flood peak time. One limitation with operational Indian satellite is non-availability of microwave sensor which has cloud penetration capability, hence one has to depend on satellite having microwave sensor viz. RADARSAT SAR data. If the day happened to be a clear day just after occurrence of Peak flood, IRS series of data is good enough for analysis.
Another interesting area where remote sensing and Geographical Information System (GIS) can be used is in post flood damage assessment due to floods. The people in Assam strongly feel that floods are increasing and thereby the damages. The available data clearly shows the average area affected by floods during 1990’s is higher than in 1980’s. Floods in the Brahmaputra valley have been aggravated due to change of river course, change in bed topography, heavy landslides up stream, siltation, etc. In the recent past in 1988 the flood damage by the Brahmaputra and its tributaries have been considerable gravity as shown in the following estimates:
Total area affected : 4.65 million ha
Crop area affected : 1.33 million ha
No. of houses damaged : 6,15,500
Population affected : 1.2 million
No. of heads of cattle- lost : 46,210
Estimated value of public property damaged : Rs. 225 cror
More than one wave of floods has become a common feature of late, displaying aggravation of overall tendencies. Added with flood associated problem of erosion which has assumed serious proportion. It is estimated that more than 10 lakh ha of land have been eroded in Assam. Other NE States also subjected to erosion resultant is sedimentation in valley area.
Flood damage assessment is required to identify and take up flood damage restoration works and to distribute the calamity relief fund to various agencies/ districts on more scientific basis. Detailed damage assessment is required by 15th September for the entire monsoon season i.e. from May to September. Out of 23 districts of Assam except the two hills districts all other are prone to floods every year. Nine districts are identified as worst affected areas in every year due to floods.
In 1998 as a 1st phase under DMS programme three districts viz. Dhubri, Dhemaji and Morigaon were taken to generate collateral information. User needs the information on inundation within 72 Hrs. If cloud free data is not available alternative is the RADARSAT for which request for acquisition of
data require two days advance information and hence inundation data can be made available within 7 days for this specific case. Current IRS series satellites have enhance the temporal resolution in terms of revisit to same area almost alternate day. During floods in 1998 the available Indian satellite data are as follows.
Damage aspect for these three districts were taken up as usual as the requirement is towards mid of September. For achieving this objective a well knit institutional linkage had to be established among various agencies. For damage assessment pre flood satellite data was procured and rectified using topomaps. Rectified data was used as master image for rectifying other data sets. Pre flood landuse classification is made. Satellite data procured during flood season is geometrically rectified and enhanced. Subsequently the image is classified for extent of inundation and flood inundation maps are prepared. Now the extracted landuse, flood layer and other existing layer such as village were integrated in GIS environment for analysis of damage to crops and number of village affected.
Few technological constraint as seen during the task are cloud cover, temporal frequency of the data, inherent problems with microwave data, turn around time, close contour information, updated database, gaps in data (lack of monitoring network at appropriate locations), reliable historical database for modeling and lack of updated database of villages as per census record.
This acute problem is a major hindrance to the development of the region. The prosperity lies in regional co-operation, since crucial natural link among the states is provided by trans boundary rivers. Proper development and sharing of the water of these rivers could be an effective point for integrated development. There is in urgent need to create a conducive environment for co-operation and solution of the outstanding regional water and related issues and facilitate joint efforts for the benefit of all concerned. The region has chronic devastating flood, which could be mitigated, and development of rich hydropower could be undertaken through several multipurpose dams over rivers flowing through the territory.
H.F.L. EVER RECORDED
106.48 in 1998
87.37 in 1991
66.59 in 1988
51.37 in 1988
37.43 in 1954
30.36 in 1988
122.69 in 1973
103.92 in 1988
86.84 in 1972
95.62 in 1974
96.49 in 1998
91.30 in 1986
79.87 in 1985
78.25 in 1998
61.86 in 1973
57.68 in 1988
54.92 in 1993
55.38 in 1984
50.08 in 1984
46.20 in 2000
30.91 in 1993
21.84 in 1989
22.50 in 1997
16.36 in 1993
DISTRICT WISE FLOOD INUNDATED AREA STATISTICS FOR ASSAM STATE
Based on the analysis of RADARSAT SAR data on 8th & 10th July 2002
Inundated area (in HA) as on 10th July 2002
Inundated area (in HA) as on 8th July 2002
1, 24, 631
Source: Assam Remote Sensing Application Centre.
* There may be over- estimation of inundation area in these districts due to possible ponding in paddy fields.
Landslide, the sudden and rapid down hill movement of soil along the hill slope is another dimension of slope instability. It now becomes a common environmental hazard all over the world. The rate of landslide is increasing day by day. Landslide being a natural hazard occurs either due to increase of load on its head or decrease of support in its toe. The nature of slope and the geomorphic processes induce landslides, but it became more hazardous as soon as the settlement process started on the hills. The landslide in the recent years was primarily man induced, the slopes of the hills of Guwahati in Assam are naturally prone to landslides for its structural peculiarities and prevailing climate of the region. The hills of the Guwahati city is coated with a thick layer of immature soil with low permeability which naturally became more landslide prone during rainy season. Growth of population and construction of houses on the steep slope zones and innumerable roads and footpaths caused removal of support at the toe of steeper part further deteriorate the situation.
The frequency of landslides increases with the increase of settlement. Unauthorized rapid growth of settlement on the hills is said to be the root cause of most of the landslides.
Causes of Deforestation
Forests in Assam are dwindling and the main reasons attributed to the gradual depletion of forests in Assam are:
The hills and plateau of the two districts Karbi Along and N.C. Hills are populated by hill tribes having their own cultural life style intertwined with forests, wildlife and jhum (Shifting) cultivation. This particular process involves ‘slash’ and ‘burn’ of forest area and natural vegetation. Original jhum cultivation had a long jhum cycle of about 20-25 years, which was allowed to elapse before the same plot of land was cultivated. In this process, the forest cover remained intact. But the increase in population demanded more cultivable land, thus shortening the period to about 4-5 years. This greatly effected the vegetation of the area, as well as the total environment.A study using satellite mapping of forest degradation due to shifting cultivation in the hill districts of N. C. hills and Karbi Anglong district, carried out at the Assam Remote Sensing Application Centre, shows that out of 423885 hectares of land under Jhum cultivation in Karbi Anglong district, 6844 hectares, became degraded forest land. Similarly out of 292309 hectares in N.C. hills, 7938 hectares are degraded. The study further shows that indiscriminate felling of tries primarily for shifting cultivation in these two districts is causing certain serious environmental problems like loss of soil fertility, soil erosion, floods and siltation in the plains. Open rearing of domestic animals also contributes towards damage of the environment. Particularly in Karbi-Along district, from where the bamboo, required as the raw materials for the Nagaon Paper Mill at Jagiroad, is procured. As a result it loses its capacity for growth and a large areas in Karbi Anglong have undergone the evil process of deforestation. In other parts of Assam also, setting up of saw mills, veneer
mills and plywood factories has caused rapid depletion of large forest areas, particularly during the last three decades. The data from National Remote Sensing Agency shows that within a period, of seven years (1975-82) about 64 million hectares of forest areas were depleted in the entire north east region. The growth of tea industry has caused depletion of a large forest area. The present spate of growth of small tea gardens, many forest covers have to give way to tea plantation, thus causing further shrinkage in the total forest area. Forests and grasslands have had to be shrunk by the expansion on settled agricultural practice. The link between human poverty and environmental degradation is also one of the important factor behind deforestation. Lack of alternative livelihood for some and greed on the part of unscrupulous traders has led to gradual denudation of the forest resources. Another poverty environment link is seen in the destruction of forest for collection of firewood. The demand for firewood being high, its supply is going on through illegal route. Even large trees are cut deep inside the forest, split into small pieces and supplied as firewood. Population pressure has led to encroachment of the forestland. Since the coming of the immigrants and their settlement in the forests, the process of capturing vote banks has been going on and the result is the gradual destruction of the forest land. Forest land becomes the first choice for rehabilitation of people affected by various natural calamities – thereby adds to further reduction. Use of the forest as a major revenue earner has also contributed much to the depletion of forests. Much destruction of forest occurs also due to building of roads, opening up new industrial towns, construction of communication towers and electrical lines. In illegally procuring coal from Karbi Anglong district, much forest is destroyed. For obtaining limestone from Karbi Anglong district for use in the cement factory at Bokajan, often large tracts of forests are destroyed in blasting. Another privately owned cement factory, due to start operation soon, is going to add to the depletion of forest. Further, the reserved forests along Assam’s border with Nagaland, Meghalaya and Arunachal Pradesh were the worst victims of encroachment and about 12% of reserved forests are under border encroachment. This takes place because, with the developmental activities in the plains people who used to live in hilly areas came down to set up habitation .in the reserved forests in these areas to enjoy the
benefits of development.
Effects of deforestation on Environment
The shrinkage of forest cover has effected the climate of Assam adversely. The rainfall has become erratic, the temperatures have risen and in many places, the sign of desertification has set in. Because of the loss of water retention capacity of the soil, rain in the upper reaches of the rivers have led to heavy soil erosion, leading to siltation of the river beds thereby causing flash floods. Such floods destroy more forest, creating a vicious cycle, and destroying a large number of varieties of precious flora and fauna, including medicinal plants. The consequent floods carry any fertile top soil to fertilise the flooded land, by depositing only sand and silt destroy the quality of the soil. The River Kakodonga passing through the Doyang Reserve Forest began to play havoc on the rice cultivation in the Ghilahdari Mouza in Golaghat district.
The flood was caused due to heavy deforestation of the Doyang forests. In 1984-85, in a bid to stop flooding, water hyacinth was spread along the river. As a result within a few years the water-flow was greatly reduced. The net result is severe drought, affecting as many as nearby 7000 bighas of paddy field. There have been widespread reports of herds of wild elephants coming inside human habitation in search of food, and damaging paddy fields and properties, trampling huts and even killing people. Lack of food in their natural habitat must have driven the pachyderms to human locality. Unless the depletion of forest area is checked, this problem will grow in future, and ultimately it will cause extinction of this great wild specie
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