Global Perspectives on Geography (GPG) Volume 2, 2014

Transcription

1 Global Perspectives on Geography (GPG) Volume 2, The Importance of Geomorphology in Understanding Natural Hazards with Special Reference to Hazards of The Dhansiri River Basin in The Golaghat District of Assam, India Nitashree Mili *1, Shukla Acharjee 2 Centre for studies in Geography, Dibrugarh University, Dibrugarh, Assam, India. 1* neetashree19@gmail.com; 2 suklageo@yahoo.com Abstract A hazard is a naturally occurring process or event which has the potential to cause loss of life and property. Natural hazards are threatening events, capable of producing damage to physical and social space where they take place not only at the moments of their occurrence, but on a long term basis due to their associated consequence. In present day context, the applied geomorphologic knowledge has become very important for better understanding of natural hazards. The knowledge of geomorphology helps in mitigating various hazards or to reduce the impact to a great extent. This paper focuses on the role of Geomorphology in the prevention of various natural hazards with special emphasis to flood and erosion hazards caused by the dhansiri river of Golaghat district of Assam. Keywords Hazard; Geomorphology; Flood; Erosion; Dhansiri; Assam Introduction A hazard implies the probability of a destructive phenomenon occurring at a particular place. If the destructive phenomenon is a product of nature such as flood, landslide, earthquakes or volcanic eruption, it is a natural hazard. Study of natural hazards and their effects happen to be one of the most dynamic areas of Geomorphology. Geomorphology is a branch of physical Geography which studies the origin and development of different types of landforms on the earth s surface and the processes modifying those landforms. Natural hazards are considered within a geological and hydro meteorological conception. These hazards are strongly related to geomorphology since they are important ingredients of the earth s surface dynamic. Hazards are results of sudden changes in long term behaviour caused by minute changes in initial conditions (Scheidegger,1994). Geomorphology has contributed enormously to the understanding and assessment of different natural hazards. Geomorphological work includes not only the understanding but the mapping and modelling of Earth's surface processes. Geomorphologists are becoming increasingly involved in the prevention of hazards, through vulnerability analysis, hazard and risk assessment and management. The work of geomorphologists is therefore of prime importance for disaster prevention. Human activities have had a huge impact on the environment and landscape, through industrialisation and land-use change, leading to climate change, deforestation, desertification, landdegradation, air and water pollution. These impacts are strongly linked to the occurrence of geomorphological hazards, such as floods, landslides, floods, soil erosion, and others (Ayala & Goudie, 2010) which are strongly related to geomorphology since they are important ingredients of the Earth's surface dynamics. The present paper is a attempt to study the role of geomorphology in understanding the natural hazrds. Objectives To highlight the concept of natural hazards. To study the major natural hazards of the study area. To find out the relationship between Geomorphic study and natural hazard management: Methodology The required informations are collected from both primary and secondary data sources. Primary data and information are collected through field survey and participant observation. Secondary data are collected from various relevant books, journals etc. The required survey of India toposheets (Scale 1:50,000) of 1974 were scanned, georeferenced, mosaiced and used as resource maps. IRS-P6 LISS-III, digital image of 2000 and 2011 on 1:50,000 scale obtained from National Remote Sensing Centre (NRSC), Hyderabad, were used to map the 1

2 Global Perspectives on Geography (GPG) Volume 2, 2014 geomorphological attributes during 2008 and The digital image was registered to the base map using a set of Ground Control Points (GCPs) and preprocessed in the ERDAS IMAGINE software environment. Map to image georectification process was adopted for geometrical correction of satellite images. UTM projection was used with spheroid and datum as WGS84. Ground control points were obtained from Survey of India 1:50,000 toposheet. The geomorphological attributes of 2000 and 2011 were prepared by digitizing in Arc GIS and incorporated in a GIS domain for change analysis. ASTER image was used to prepare the slope map of the study area. Study Area The area under study is located in the south bank of the river Brahmaputra in Assam. Among the 27 district of Assam, Golaghat district has been selected as a sample district to understand the problems of natural hazards. Geographically in the Golaghat district, Dhansiri basin considered as one of the important geographical unit with respect to agro economy and human settlement, is the main river of Golaghat district, Assam. The Dhansiri river, one of the major south bank tributaries of Brahmaputra has long been considered as a problematic river in the history of Assam due to recurrent and extensive flooding and bank erosion. As flooding and erosion continues to affect major portion of the growing population in the Dhansiri basin, people suffering from persistent flood and erosion problems are seeking methods to mitigate flood and erosion damages. IN D I A Natural Hazards FIG. I MAP OF THE STUDY AREA The term natural hazard implies the occurrence of a natural condition or phenomenon, which threatens or acts hazardously in a defined space and time. Different conceptualizations of natural hazards have not only evolved in time, they also reflect the approach of the different disciplines involved in their study. In this sense, a natural hazard has been expressed as the elements in the physical environment harmful to man (Burton and Kates, 1964); an interaction of people and nature (White, 1973); the probability of occurrence of a potentially damaging phenomenon (UNDRO, 1982); and as a physical event which makes an impact on human beings and their environment (Alexander, 1993). A natural hazard is a threat of a naturally occurring event that has the potential negative effect on people or the environment. Natural hazards are always potentially disastrous, the extent of damage depends on frequency and magnitude of the event. Those natural hazards that cause losses of human life and great material damage to economics are called Disaster and Catastrophes. Relationship between Geomorphologic Study and Hazard Management One of the major problems in hazard control and management has been the lack of scientific understanding of the complex physical processes involed in the occurance of hazards. Hazards cannot be prevented from occurring but their impacts can be reduced through proper management system. Flood and erosion control and management are necessary not only because they impose a curse on the society but the optimal exploitation of the land and proper management and control of water resources are of vital importance for bringing prosperity in the region. But this cannot be technically feasible without proper understanding the relationship among the factors affecting river flow, sediment load, channel pattern, channel migration etc. and their resultant impacts on man. Geomorphologic knowledge helps for better understanding the effects of the hazards, their frequency and magnitude. Primarily the interest of a geomorphologist concerns regarding flood and bank erosion are the following: Concept of Flood and Flood plain, floodplain delineation, flood deposit recognition. Monitoring of changes in river channel position and pattern. Determination of the probability of flooding from a river. The effects of such flooding The extent of flooding. Flood hazard management measures. Channel Pattern. River flow pattern. Grain size analysis of bank materials. All these information can be used to determine the areas prone to flood and bank erosion. Mapping of flood prone zone is a primary step involed in reducing the risk of the region. The information can also be used 2

3 Global Perspectives on Geography (GPG) Volume 2, to control the landuse in the floodplain areas. Today, the increasing pressure of population on floodplain for agriculture and settlement and intensification of hazards due to flood and erosion, the engineers, hydrologists, fluvial hydrologists and planners need to know as accurately as possible the probable flood magnitudes and their occurrence for implementing various water resources development plans. Flood frequency analysis is thus extremely useful in risk analysis and economic development and planning of flood prone areas by means of providing the probability of occurrence of the future flood events. Similarly the knowledge of channel pattern, river flow and bank materials help in adopting various structural measures. The geomorphic ideas help to a great extent in the implementation of hazard mitigation policies. Now-adays Geographical data and information relating to Remote sensing and Geographical Information System(GIS) have become essential for better understanding of the natural hazards. The remote sensing technology helps in providing accurate information about the spatial and temporal extent of flood by way of delineating the flood inundated areas along the river course. With the availability of repetitive remote sensing data, it is quite possible to monitor the variations in extents of flood inundation. River sediments and related problems like erosion and deposition are also studied through remote sensing. Erosion-prone areas as well as the area of intensive bank erosion can be detected through remote sensing and their extents, dimensions, patterns and process can be analysed leading to formulation of effective erosion control plans. suffers extensively due to floods and river banks erosion. Flood may be considered as the biggest cause of loss of life and property in Assam. The two main river systems i.e. the Brahmaputra and Barak have invariably been inundating the plains of Assam in different time. The flood statistics of Assam shows that the major earthquakes of 1897 and 1950 have caused abrupt changes in the fluvial regime of Assam. The Dhansiri basin in Golaghat is highly vulnerable to flood management strategies. Dhansiri presents challenge in terms of long and recurring flood hazard. Flood in the Dhansiri Basin is characterized by their extremely large magnitude, high frequency and extensive devastation. It occurs in the middle and lower reaches of Dhansiri and the problem is more pronounced in the downstream of NH-37 across Numaligarh. Floods of extreme high magnitude occurred in the sub basin in the years 1954,55,59,60,70,76,86,87,88,89, 91, 98, 2000, 2004, 2010 and Hazards in the Dhansiri River Basin in Golaghat District Of Assam Flood From the geomorphic point of view Dhansiri river is characterised by meandering pattern, Channel shifting, high flow velocity during monsoon period and prone to various fluvio-geomorphic hazards. The two most common hazards of Dhansiri River Basin in Golaghat District are Flood and Bank Erosion. Defining a flood is a difficult task, partly because floods are complex phenomenon and partly because they are viewed differently by different people. A more general definition of Flood is given as A flood is a body of water which rises to overflow land which is not generally submerged. Assam is a land of rivers. Rivers present a potential threat to human populations and property through flood, drought and erosion. They therefore have potential social and economic as well as physical relevance. Flood is probably the most recurring, wide spread, disastrous and frequent natural hazard of Assam. It is a matter of concern that every year Assam FIG. II DISCHARGE HYDROGRAPH TABLE I MAGNITUDE AND DAMAGED CAUSED BY FLOOD IN THE GOLAGHAT DISTRICT DURING No of villages 101 nos 132 no s 206 no s No of affected people Agricultural area affected 42763hec hec 1 lakh 211 hec Crop damaged 4405 hec hec 50 thousand hec Source: The flood in Golaghat District: causes, impacts and remedies - Mandira saikia.{ijcaes journal,issn: } The major portion of the river course is within the original state Nagaland and terminating state Assam. In Nagaland, the river passes through a hilly course in the upper catchment and causes flood only in the foot hill areas of Kohima district, around Dimapur town. But the inundation stays only for a day or two at the most. In its course in Assam also, no major inundation generally takes place upto Bokajan. The problem of flooding 3

4 Global Perspectives on Geography (GPG) Volume 2, 2014 becomes more complicated in the Golaghat district. The main causes of flood in the sub-basin are: i. Heavy rainfall: Precipitation over the basin is not uniform. High intensity of rainfall with average annual monsoon rainfall of mm. Being located in the monsoonal regime the basin receives heavy rainfall during monsoon season. From the analysis of hydrograph, it is found that during monsoon period (May-Oct) the discharge peaks well coincide with corresponding higher water level. The daily discharge and water level (stage) data of Dhansiri river at Numaligarh Station have been plotted for the year 1998, 1999 and 2000 to show the pattern of changes in flow characteristics (Fig. II and Table I). ii. Steep slopes of the river in the hills: Major portion of the river is in the steep hills of Nagaland (Fig. III). Whenever there is rainfall in the hills water rushes down to the plains from extensive catchment areas, flooding the plains. fine grained alluvium. In this particular case the river flows through the unconsolidated alluvial terrain predominantly comprising sands and clays. Besides, the river carry enormous quantities of sediments from the hills and on reaching the plain, sediments are deposited on the channel bed (Fig. IV). The channels are thus filled up and the water dig out different courses. FIG. IV IRS P6 IMAGE OF A PART OF DHANSIRI RIVER.SHOWING HIGHLY MEANDERING COURSE AND THE CUT OFF FORMING OXBOW LAKES FIG. III SLOPE MAP OF THE DHANSIRI RIVER BASIN SHOWING THE UPPER COURSE FALLING UNDER STEEP SLOPE iii. iv. Deforestation and landslide proneness in the upper catchment: Cleaning of the forested hillslopes and also the plains for cultivation, construction of roads, setting up of residence etc. are very common both in the upstream and downstream of the river which is a major cause of flood. Again, the rocks of Nagaland being soft, the earthquakes may easily cause landslide. Landslide also occur due to heavy summer rain and may block the course of the river and flood occurs. Highly meandering and shifting nature of the river in the plain: The geometrical pattern of the course is highly irregular in the plains while maintaining almost a constant and less irregular course in the hilly terrain. According to Leopold and Langbein (1966) the meander usually appears wherever the river flow through Bank Erosion The Dhansiri sub basin falls in highly seismic zone and experiences several earthquakes in a year along with heavy downpour in the hilly catchments. This factor coupled with heavy deforestation in the hill catchment leads to considerable soil erosion on the steep slopes of the hills.the eroded soil is then transported by the river. On reaching the plain the transported materials get deposited at the river bed due to sudden change of the river grade from a very steep slope to gentle slope leading to meander which ultimately results in bank erosion. Erosional processes are more active during monsoon and when the water level recedes the bank level. During the receding stage of the river, different types of shear failure also took place and were observed around Kamargaon, Golaghat areas along the Dhansiri River channel (Dutta, 2007). As water level receded in the channel, saturated levee material lost support from the channel side. These resulted in shearing of blocks from the saturated bank due to its own weight. 4

5 Global Perspectives on Geography (GPG) Volume 2, TABLE III DETAIL AFFECT OF EROSION CAUSED BY RIVER DHANSIRI IN GOLAGHAT REVENUE CIRCLE AREA SINCE THE YEAR 2001 FIG. V A SUBSET OF LOWER PART OF DHANSIRI RIVER SHOWING BANKLINE SHIFT AND BANK EROSION (E=EROSION) IN BETWEEN THE YEAR 2000 AND 2011 TABLE II MAJOR EARTHQUAKES IN THE NORTHEASTERN REGION IN RECENT PAST Place Year Magnitude Remarks Cachar March 21, Numerous earth fissures and sand craters Shillong plateau June 12, About 1542 people died Sibsagar August 31, Property damage Myanmar December 12, Property damage Srimangal July 8, km 2 area suffered damage SW Assam September 9, Property damage Dhubri July 2, Railway lines, culverts and bridges cracked Assam January 27, Destruction of property Nagaland Destruction of property N-E Assam October 23, Destruction of property Arunachal July 7, Destruction of property Upper Assam July 29, Severe damage Upper Assam August 15, About 1520 people died. One of the largest known quake in the history Patkai Range, Arunachal Property damage Manipur- Burma border Property damage Darjeeling Property damage Indo-Myanmar August 6, border No casualty reported Source: STATUS OF SEISMICITY IN THE NORTHEAST INDIA AND EARTHQUAKE DISASTER MITIGATION - R.P.Tiwari, Department of Geology, Pachhunga University College, Mizoram University. Year Total Damage A B Nil Nil 2007 Nil Nil Nil Nil Total 418 bigha 80 nos Area Eroded (A)* {area in bigha} Families become homeless(b)* Source: Revenue circle Office, Golaghat. Bank erosion, channel shifting and damage of human habitats as well as agricultural land etc. have been common phenomenon of the river Dhansiri. The Dhansiri river basin is one of the most erodible basins in India. The earthquake in 1950 may be said to be a threshold point after which the bank erosion problem has become more severe and intensive throughout the Brahmaputra valley of Assam (Table II). The Dhansiri had alone eroded away over 418 bighas of land between 2002 and 2011 (Fig. V). Loss of vast fertile land has made nearly 100 families homeless and landless particularly in the last 10 years (Table III). Flooding during the monsoon period due to heavy rainfall and fast flow velocity are also the causes of bank erosion. Being located in the monsoonal regime, the Dhansiri basin receives heavy rainfall during the monsoon season. Dhansiri gets floods from May to September. Over 80% of the flow and over 95% of the sediment contribution take place during this period. Due to meandering nature of channel pattern and excessive sediment discharge, there is constant shifting of the channel. The process of constant shifting due to bank erosion of the river has been continuing through ages. Mitigation Strategies Taken up So Far Construction of embankment along both side of the Dhansiri for flood mitigation. The total length of embankment constructed in Dhansiri sub basin is only km. A number of anti erosion schemes have been taken up by the Government from time to time to provide protection in the erosion affected reaches. Altogether 9612 hac of land including Golaghat township have been protected by various anti-erosion measures. Conclusion Hazards are a key interaction between humans and 5

6 Global Perspectives on Geography (GPG) Volume 2, 2014 physical environment. It is a complex Geo- Environmental phenomenon; therefore, it needs a integrated approach composed of Geomorphological, Hydrological, Meterological and Anthropogenic aspects to lessen the disastrous effects of Natural hazards. Geomorphologic knowledge can contribute to the choice and design of various strategies for hazard management. The dynamism of the Earth's surface is enclosed within a temporal and spatial scale. The response of the landform to the changes caused by the processes corresponds to the magnitude and frequency of the events, the resistance of the involved materials and the size of the concerned landform (Summerfield, 1991). Natural hazards take place in a certain place and during a specific time, but their occurrence is not instantaneous. Time is always involved in the development of such phenomena. For example, the intensity and duration of rainfall in conjunction with the nature of the fluvial system, developed also on a time basis, would determine the characteristics of the flooding. The contribution of geomorphology to the field of natural disasters is mainly through the elaboration of hazard assessments. In general, such assessments comprise stages like mapping, modelling, prediction and management proposals, using field observations, photogrammetry, geographical information systems and remote sensing, the zonation and mapping of different hazards is done. Modelling approaches consider not only the understanding of present, but past events, leading to accurate predictions of the consequences a geomorphic hazard may have on a determined landscape under a given conditions. Thus, geomorphological work includes not only the understanding but also the mapping and modelling of Earth's surface processes, many of which directly affect human societies. In addition, geomorphologists are becoming increasingly involved with the dimensions of societal problem solving, through vulnerability analysis, hazard and risk. Dhansiri, has been experiencing a continuous process of migration by way of bank erosion during flood periods. The severity of floods has further amplified under the influence of human intervention. If this recurrent phenomenon is not mitigated, it would be difficult to achieve the economic development. ACKNOWLEDGMENT The authors are thankful to Dr. Munindra Konwar for his constructive suggestion during the preparation of the manuscript. REFERENCES [1] Alcántara-Ayala, I. & Goudie, A.S., 2010, Geomorphological Hazards and Disaster Prevention Alcántara-Ayala, I., 2002, Geomorphology, natural hazards, vulnerability and prevention of natural disasters in developing countries, Geomorphology 47, p [2] Alexander, D., Natural Disasters. UCL Press and Chapman & Hall, New York, 632 pp. [3] Burton, I., Kates, R.W., The perception of natural hazards in resource management. Nat. Resour. J. 3, [4] Dutta, M.K., Dhansiri River Channel, Assam A geomorphologic attribute, Unpubld. Ph.D. Thesis, Dibrugarh University, Assam. [5] Scheidegger, A.E., Hazards: singularities in geomorphic systems. Geomorphology 10, [6] Summerfield, M.A., Global Geomorphology: An Introduction to the Study of Landforms. Wiley, New York, 537 pp. [7] UNDRO, Natural Disasters and Vulnerability Analysis. Office of the United Nations Disaster Relief Coordinator. Geneva, Switzerland. [8] White, G.F., Natural hazards research. In: Chorley, R.J. (Ed.), Directions in Geography, Methuen, London, pp Nitashree Mili was born in Dibrugarh on 19 September, 1985 and received Master degree in Fluvial Geomorphology from Gauhati University, Assam(India). Currently she is a Lecturer in the Centre for Studies in Geography, Dibrugarh University, Assam. She joined there in the year April Her research areas are mainly in the field of Fluvial geomorphology and environmental science. She participated, presented research paper in national and international level seminars and conferences and published research papers in the reputed journal. Ms. Mili, Lecturer can be reached at neetashree19@gmail.com. Miss Shukla Acharjee got her Ph.D. in the year 2010 and she has published a few research papers in refered journals. 6