Flood Disaster and its Preparedness in Assam, a Northeast India

Being a part of eastern Himalayan ranges falls in the world’s wettest monsoon belt, this northeast part of India has always enjoyed the blessing of huge potentiality of water resources. As concerning the average annual potentiality of water in the Brahmaputra’s river basin and estimation of monsoon run off, it has always endowed with tremendous potentiality of water resources in the country. The eight percent of spatial dimension of this Northeast part has endowed the ability of hydropower potentiality of about 59000 MW which implies about 40% of the country’s total hydro power potential. Only 2% that is 1100 MW has been harnessed so far in this part and it has increased for tapping up to 4029 MW which is only 7% of the total hydro-potential in the region. Harnessing of river waters in the different fields like multipurpose hydropoject, irrigation, cultivation, geo-tourism, geo-park scientifically have come more important to reduce the flood havoc in the entire state of Assam, a northeast India.
The Brahmaputra is the fourth biggest river in the world in terms of annual discharge rate of about 19,830 cubic meter per second as its mouth and its catchments area is about 5, 80,000 sq.km located in the eastern Himalayan ranges and its neighbors that has occupied the area belonging to the four countries in percentage i.e. Tibetan Plateau of China (50.52%), India (33.62%), Bhutan (7.76%) and Bangladesh (8.10%). High order drainages of about 52 numbers of tributaries feed the Brahmaputra in the downstream side making it constant viable water resources in the region. Of these, the major 19 tributaries lie in the north bank while 12 others significant tributaries are in the south bank of the mighty river. In general, all total 15 numbers of principal tributaries lies in the Indian Territory. It may be worth mentioned that the Dihang (Siang), i.e. Tsangpo in the upper reaches and Subansiri contributes 37.40% and 10.66% surface run off to the Brahmaputra respectively. Moreover, the other two important tributories like Dibang (sikang) and Lohit contibutes around 7.65% and 9.50% surface runoff to the mighty river. Other tributaries such as Manas contributes water annually 6.75% (38,000 million cubic meters), Sonkosh contributes 3.18% (18,000 million cubic meters) and Burhidihing 2.65% (15,000 million cubic meters) annually to the Brahmaputra basin. Moreover, other important rivers are like Kopili, Jiabharali, Dudhnoi, Krishnai, Pagladia, Beki etc.
It has been seen that different major tributaries of Brahmaputra river such as Dihang, Subansiri, Dibang, Luit, Manas etc. and the significant tributaries and sub tributaries like Jiadhol, Ranganadi, Gai nadi, Simen Nadi, Burhidihing, Kopili, Dudhnoi, Krishnai, Bolbola, Beki, Pagladia are associated with Brahmaputra Valley that have always been causing a periodic flood havoc in the monsoon seasons. In this consequence, watershed prioritization and mamangement of each and every river’s have come most important for mitigation of flood hazards in their downstream areas. The scientific hydrological approach is to bring the tributaries under control so that their water level comes down the danger level during the monsoon period. This huge task seems to be very challenging keeping view in volume of runoff and topographical characteristics. The four tributaries viz Dihang, Dibang,Luit and Subansiri have contributed around 65 percent of water volume to the Basin of mighty Brahmaputra river. The hydrological data of three rivers like Dibang, Subansiri and Luit may be available or may be acquired due to their major catchment area falls in India but there would be required more hydrological data from the Tibetan China as concern to the management of Dihang or Tsangpo River in that part. This is true that Dihang cannot be managed easily due to its large size of hydrological characteristics and there is need to set up a common scientific platform consisting of China and India to manage the Tsangpo along the course. From the satellite images it can be visualized that China’s development activities in the banks of Tsangpo is also going on rapidly and recently they have set up a Air Base with a length of 5 kms and breadh of 1 kms located in the south banks of Tsangpo River near the south of Lhasa. This Air Base has well connected through a bridge with length of about 11 kms which connected to its northern banks and its terrain and another Lhasa river including a 2.5 kms trunnel inside the terrain at 3600 m from the mean sea level.
It is also true that if theses four tributaries come under control than it will be a great help to mitigate the flood hazards in the downstream areas particularly to the upper Assam region. The management of these major rivers including Jiabharali and Burhidihing by collective means of methodologies like aforestation, watershed prioritization, irrigation project, fishing cultivation, ground water recharges, linking of rivers, setting geo-tourism spots at vulnerable location where embankment breached, water way communication, open inundation in the low lying area, multipurpose hydro power projects etc would be very fruitful measures in the long term for minimizing and prevention of flood havoc in the Districts of Dhemaji, Lakhimpur, Dibrugarh, Tinsukia, Sibsagar, and Sunitpur.
The periodic flood havoc is one of the main reasons for which it has always interrupted in increasing trend of per capita income in the state. The state has to lose huge quantity of cultivation on lakhs of hectares of land apart from river bank erosion has been taken place in hundreds of hectares of land. Brahmaputra is the second largest river in terms of load deposition after the yellow river of China where the weathering and Brahmaputra’s bank erosions rate are tremendously increasing due to various factors of geological and technological reasons which will lead to large deposition of siltataion in the river bed. As a result discharge capacities of rivers have come down due to sedimentation on the river bed. On the other hand, the Brahmaputra river system is associated with the higher altitude of eastern Himalayan region resulting maximum weathering leads to huge erosion that has been taken place in the upper reaches. Of course, many non stable islands created during the high flood time in the Brahmaputra River, but erosion causes major concern to the heritage land of Majuli besides posing threat to the Dibrugarh, Tezpur town apart from severe erosion at Koliabor, Morigaon, Palashbari, Dhuburi etc.etc.
Failure of embankments at different places have caused major destruction to thousands of thousands villages along with affecting lakhs of lakhs of peoples. Districts of Dhemaji, Lakhimpur, Dibrugarh, sonitpur, Nagoan, Morigaon, Nalbari, Barpeta are the worst affected region due to failure of embankments. In general, embankments are constructed for temporary protection of villages near the banks of the rivers. In fact, flash flood created due to failure of embankments because of various reasons such as age of embankments placed on the on the recent alluvium, quality of embankments, position and alignment of embankments respective to the truncation of rivers, lack of protection of embankments by the dykes, earth dams, spurs, porcupines where it might have been placed in perpendicular directions of rivers to resist the water pressure etc etc. In Assam embankments failed because of above factors along with shallow depth of the rivers due to silt deposition on the river bed that have decreased the capacity of water discharges of various tributaries. The floods in the Sunitpur, Lakhimpur and Dhemaji district are due to low carrying capacity of water by the river because of deposition of huge siltation on the river bed and tend to flows many directions. These rivers are Jiabharali, Jiadhol, Siemens, Gai, Ranganadi etc. From the preliminary study of high resolution of satellite imagery, it has found that the river systems within the districts of Sonitpur, Lakhimpur, Dhemaji, Sibsagar,Jorhat, have been shifted several times in the recent geological past along with the mighty Brahmaputra. From the road section of Jonai to Lakhimpur, it is found that about 15 nos of significant rivers comes down abruptly from the foothills of Arunachal Pradesh to the Brahmautra and Subansiri in the Dhemaji and Lakhimpur districts. Cultural heritage land the Majuli is also controlled by the Subansiri, Dhansiri and Brahputra River. From the preliminary study of space technology it may be assumed that biggest river Island Majuli was borne after shifting of Brahmaputra River to the southern part in the Jorhat district alongwith the shifting of Subansiririver to ward western part in the Lakhimpur District. In these two districts the shifting of river had occurred in the recent geological past. It has also been found that the catchment area of rivers like Jiadhaol, Gai, Simen are not so large in comparing with the catchment of Subansiri and Ranganadi. The Jiadhol catchment is below the Subansiri catchment and its shape and size of the Catchment is a type of lenticular in nature and water comes down heavily and needs watershed prioritization to minimize the flood havoc in the Dhemaji area. Watershed prioritization prevents erosion and help to reduce damaging run off besides the increasing rate of infiltration to the ground water tables.
This is true that Assam is flood prone state in the world due to its around 40.15% land area are flood prone because of structural set up and around 4.85 % of land eroded along with 4056 kms of length of embankments and all total 176 rivers (48 high order and 128 low order drainages) besides 150 places are sensitive to breach embankments. Due to the complex geological and physiographical set up with the belt of ecologically wettest monsoon the Assam is always a flood prone states. Besides it, there are the major concerns for the issues like global warming phenomena, type of proposed hydro power projects and degradation of vegetation in the upper catchment area which would be the caused for accelerating the flood havoc in the forthcoming years.
The flood disaster management in the long term measures would definitely cover the ongoing multipurpose hydro power projects in the Arunachal Pradesh but should be provision of flood cushion which will definitely help to mitigate the flood and to reduce the risk hazards in the Brahmaputra valley. At the same time, there are also to be thinking that the upstream hydro power plant would be set up in the area of high seismic activity where many of past seismic epicenters are in the Arunachal Pradesh. Height of the Gravity Dam is also a major concern because more height of dam would more generate the power and more capacity of reservoir. But from the various factors such as huge siltation, abnormal behavior of climates & rainfalls due to global warming, lack of availability of hydrological data of Tibetan plateau, eastern Himalayan tertiary rocks, height of the altitude, high seismic prone zone, present river’s discharge capacity, socio economic conditions along the downstream, lack of infrastructure and facilities, lack of disaster preparedness plan, type of proposed Dam , at present ,it is better to keep the Dam height low for the benefit of the people of this region and reducing the risk flood hazard mitigation.
Flood disaster preparedness is purely a scientific phenomenon which basically deals with pre disaster mapping and its scenario for probable flood hazards mitigation with the help of different thematic integration like rainfall estimation, digital elevation models, catchment’s hydrological statistics, periodic flood zoning, population distributions, type of socio economic conditions, social and physical infrastructures, dam break assessment, geo-environmental components apart from capacity and type of resources. In the recent trend of flood disaster preparedness the first and foremost parameters of flood disasters evaluation is a Doppler radar phenomenon .National Weather Service of USA can provide accurate warning system with the help of relevant images acquired from Doppler radars of more than 150 sites.
The centre has come up for setting up 55 doppler radar across the country which is a big achievement in the Indian sub continents. But there is need for installing sufficient numbers of Doppler radar in the entire northeastern regions for the output of images includes reflectivity, velocity and rainfall information. Doppler radar can help to find out the precipitation amount and its dimension which are desperately required in the entire catchment of Brahmautra valley in a network manner for its calibration of accurate result.
Another recent trend of Flood disaster preparedness is the meterological satellites centres of NOAA or INSAT series which can simulate for forecasting of weather and climates in the specific areas. The warning and communication systems will be upgraded if the above technology can be utilized and thereafter peoples of downstream areas can be alerted through Radio, TV and regular bulletins.
Selection of sites and its numbers are important in the installing the Doppler radar to forecast the rainfall in the different catchment and thereafter simulation of result for forecasting of floods in the various downstream areas. This technology should have been used by the NHPC and NEEPCO in their respective catchment areas to manage the runoff and to control flow of rivers respective to the reservoir statistics so that it would not be damaged maximum to the downstream area. There is need to develop communication devices between the state Government and the Hydro Power Company like NEEPCO for the benefit of the downstream people. The hydro project named Ranganadi , a run-of-the-river scheme and involves construction of a 60 m high concrete gravity diversion dam for conveying water through a 10 km long tunnel with 6.8 m diameter to generate 405 MW (3 x 135 MW) of power. It has been proved that the Ranganadi, run of river project having interred transfer basin to Dikrong which releasing water through 6 spillway gate consisting 120 sq m of each gate cannot minimize the flood hazards in the downstream area and has influenced more flood havoc in the lower reaches of Lakhimpur District. Released of water from Umrong and Khondon reservoir of NEEPCO had also created flood havoc in the Nowgaon and Morigaon District in the 2004. The proposed dam with highest produce of 3000 MW of Dibang Project is an also serious concern for the Dibrugarh District due to its physiographic pattern of Dibrugarh and Sibsagar District. Dam break failure and its assessment might have been done before going to complete this project by the NHPC. Inter transfer of river basin option may have been also a good concept for managing the river in the Brahmaputra valley but requires update river database with the help of recent technology.
The State Disaster Management Authority headed by chief Minister of Assam was formed recently under the guidelines of National Disaster management Authority headed by the Prime Minister of India. The Revenue and Disaster Management Department of Assam has taken lot of innovative measures for preventing the flood disasters and has recently stressed scientific inputs for evaluation of disaster preparedness plan for mitigating flood hazards