INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 4, No 1, 2013

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1 INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 4, No 1, 2013 Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN Flood risk zone mapping of the Subansiri sub-basin in Assam, India Chinmoyee Gogoi 1, Dulal C. Goswami 2, Sarat Phukan 3 1,2- Department of Environmental Science 3- Department of Geological Sciences Gauhati University, Guwahati , Assam, India chinmoyee.gogoi@gmail.com ABSTRACT Flood plain zoning of a river basin is an essential management strategy to regulate land use in order to restrict the damages in frequently flooded areas. The river Subansiri is one of the largest north bank tributaries of the Brahmaputra which creates flood havoc almost every year bringing untold miseries for the people living in the lower part of the basin, i.e. the part which falls in the Dhemaji and Lakhimpur districts of Assam. Flood plain zonation of this area is done in this study on the basis of the flood inundation map of 2004 collected from the Dartmouth Flood Observatory website and the flood inundated areas during the flood of 2008 where water remained stagnant even in the post flood period.these areas are delineated from post flood season imagery of October The final flood zonation map includes three zones viz. the chronically inundated, occasionally inundated and rarely inundated zones. A large part of the agricultural lands and villages of Dhemaji and Dhakuakhana lying on the left bank of the river Subansiri were submerged by flood compared to the right bank. This may be because of the low-lying nature of the basin in Assam where these parts are always vulnerable not only to the floods of the Subansiri and its tributaries, but also to breaching of the embankments of the river Brahmaputra. Keywords: Flood, Lower Subansiri basin, inundation, flood plain, flood zonation 1. Introduction A river basin is a geographical area that drains through a common point downstream and forms a distinct hydrological unit. Basically, a river flows down from the high hill region towards the plain before meeting another river or ocean. Flooding denotes the process of inundation of an area by unexpected or sudden rise of water due to snowmelt, extreme rainfall intensity in the upper reaches as well in the lower part of a basin or due to dam failure in which life and property in the affected area are under risk (Nyarko, 2000). Flooding is a natural and recurring event for a river. It is a result of heavy or continuous rainfall exceeding the absorptive capacity of soil and the conveyance capacity of river channel. This causes a river to overflow its banks on to the adjacent lands. Floodplains are, in general, those lands situated adjacent to rivers and streams that are subject to recurring floods. Leopold et al, (1964) defined a flood plain as a strip of relatively smooth land bordering a stream and overflowed at a time of high water. Flood is a major problem in the lower part of the Subansiri Sub-basin in Assam. Floods in this region occur due to excessive rainfall during the monsoon season from June to October almost every year. The channels of the Subansiri and its tributaries in the Brahmaputra valley are inadequate to carry the abnormal quantity of water arising from monsoon rains causing the river to overflow its banks and inundate the surrounding low lying country. Almost the entire Lakhimpur and Dhemaji districts are covered by the floodplains of the river Subansiri Submitted on June 2013 published on August

2 and its tributaries. The floodplains of the Subansiri are mostly flat and fertile due to annual deposition of silt. About % of the study area is covered by agricultural lands which are very much essential for the sustenance of its large populations. Every year devastating flood occurs in this part of the basin causing extensive damage to the crops. So, for proper land use planning and effective development of the area, flood zoning of the lower part of the Subansiri basin is very much important. However, a detailed zonation of the Lower Subansiri basin has not been done as yet. The present study is therefore made to delineate the different zones of flooding of the Subansiri river an understanding of which will help in planning and designing appropriate flood mitigation measures as well as for adopting sustainable agricultural practices. 2. Study area The Subansiri is an important north bank tributary of the Brahmaputra flowing from the higher Himalayas of Tibet through middle and sub Himalayas of Arunachal Pradesh to the plains of Assam before meeting the river Brahmaputra.On entering the plains of the Brahmaputra valley after flowing down the steep gradient of the Himalayas, the river spreads its enormous discharge and load of sediments forming an anastomosing pattern of its channel which assumes a meandering pattern further downstream. The Subansiri basin covers an area of 35,771sq.km of which 4350 sq.km falls in Assam. The maximum discharge recorded at Chowldhoaghat is 12, cumec in the year The catchment of the river in Assam almost entirely represents the floodplain and is ravaged by floods almost every year. Annual floods cause inundation to vast areas on both banks and as a result, cultivable lands and human dwellings of these areas are adversely affected causing distress to the people residing in the area. Since flood waters from river Subansiri and its tributaries generally inundate the floodpains in Lakhimpur and Dhemaji districts of Assam, this part of the basin in Assam has been selected for the present study (Figure 1). The floodplains of the Lower Subansiri basin are extensively cultivated. Almost sq.km of the study area are used for agricultural purpose. The present population in the study area is estimated to be 23,03,383 (2001 census). A large section of the population belongs to the ST (plain), SC and OBC communities which spread over a total of 1790 villages of Lakhimpur and Dhemaji districts. Figure 1: Location map of the study area 76

3 3. Causes and consequences of flood in the lower Subansiri basin Flood and drainage congestion is a major problem in the plains of the Lower Subansiri basin. Heavy rainfall in the hill segment as well as in the plains, lack of adequate gradient to drain out the high discharge of the rivers, increasing silt load due to high rates of deforestation and landslide in the upper catchment, breaching of embankments are the main causes of flood in this part of the basin. Geotectonically fragile and seismically sensitive sub-himalayan segment of the basin further accentuates the flood and erosion scenario. After the Great Assam Earthquake of 1950 (magnitude, 8.7) and the associated flood, the morphology of the river has changed a lot. Because of that earthquake, landslide occured and blocked the river forming a natural dam in the higher gorge section at Sipoumukh, 9 km away from Gerukamukh. It blocked the river for almost three days after which the dam had suddenly breached and the sudden release of the stored water washed away a large portion of the Pathalipam tea garden which was situated near the river in the foothill zone during that time together with hundreds of people and innumerable cattle living in the numerous villages downstream. During that flood, the river was charged with enormous quantities of silt which consequently altered the conditions of flow and morphology of the river channel, causing untold miseries to the people living in its low-lying areas (District Gazetteers, 1976, Goswami, 1985). The large amount of sediment generated by landslides was brought down into the rivers raising their beds considerably (Poddar, 1952, CSE 1991). Figure 2: Breach points along with the extents of area of breaching Since 1954, the river has been confined between embankments. This also leads to riverbed aggradations, because the silt which earlier used to be deposited in the adjoining floodplains, now gets deposited inside the river channel (CSE Report, 1991). According to Schumm and Litchy (1963) floods of very high magnitude may be a contributing factor to channel widening and river bank erosion along with associated changes in the channel pattern. The map evidence, together with record of flood history, indicate that regular large floods have breached the embankments, created areas of bar development and caused bank erosion and channel migration. Floods caused by sudden breaches of embankments are far more 77

4 damaging than normal floods where overtopping of banks takes place during high flow conditions in the river. Every time a breach occurs, a sizable area and population is affected. Further, inundation caused by breaches leads to deposition of coarser sediments in large areas, leaving the land unsuitable for cultivation. The recovery period of such land is considerably high (Das et.al, 2012). Figure 2 shows the points of breaching and their area of impact in different years which further aggravates the flood problem in this basin. Every year a large part of the basin is flooded causing damage to agricultural lands and affecting large number of people residing in the flood plain areas. Table 1 shows the damage statistics due to flood and erosion in the basin from 1987 to Table 1: Flood damage statistics of the lower Subansiri basin from 1987 to 2007 Area Area No of No of No. of Crop area affected affected villages villages people affected by flood by erosion affected Year of affected affected by flood (in sq. (in sq. by erosion Flood by flood by flood (sq. Km.) Km.) km) NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA means data are not available (Source: Water Resources Department, Govt. of Assam, Guwahati) 4. Climate The whole of the Brahmaputra valley in Assam including the Lower Subansiri basin forms an integral part of the Southeast Asiatic Monsoon regime having a unique climatic personality in comparision to any other parts of India. Because of the intense spell of heavy rainfall in the upper reaches as well as in lower part of the basin during the months June through September due to the prevailing south westerly monsoon, flood occurs in the lower part of the basin in Assam. This is due to the fact that during the pre-monsoon months of April and May, North East India receives a fairly good amount of rainfall on account of large scale thunder storm activity over this region causing a favourable antecedent condition of saturated ground and nearly bank full stage in rivers which help to a large extent in producing high flood in the month of June. Generally, high floods occur in the months of June, July and August. Figure 3 78

5 shows the monthly rainfall pattern based on the data recorded at North Lakhimpur from 1993 to The highest monthly rainfall recorded at this station was mm in the month of August 1993 and the lowest was.7 mm in the month of November The Subansiri basin in Assam is characterised by highest temperatures during the southwest monsoon season i.e. from June to about the beginning of October which begins to drop considerably by the end of November. The average monthly maximum temperature is 4. and minim m is.4 as re orded at orth a himp r. t also e perien es a highly h mid atmosphere throughout the year. Figure 3: Monthly Rainfall recorded at North Lakhimpur from 1993 to Methodology The main objective of the study is to carry out a flood risk zoning exercise in the Subansiri basin in Assam using GIS and Remote Sensing techniques. The Remote Sensing and GIS software ERDAS IMAGINE 9.1 and ARC GIS 9.3 are used as tools for identifying different flood risk zones in the study area with reference to the existing flood inundation map of 2004 and the post flood imagery of In most of the cases, overtopping of the river or breaches of embankments by flood water generally affect the low lying areas along the banks of the rivers outside the natural levees or embankments. The situation is further aggravated by water logging in many places, caused by the embankments due to the presence of which rain waters accumulated in the floodplain fail to drain into the river. These areas along the river channels usually remain inundated for several weeks or even months after recession of the flood wave. These tracts of land can be easily identified in the post monsoon satellite images due to the presence of water and/or high soil moisture. In the present case, these tracts of land are delineated from the post monsoon satellite image and identified as the chronically inundated zones of flood. The July 2004 flood in the Brahmaputra valley is considered to be the biggest recorded flood event in recent history of the state. It broke all previous records of flood damage affecting 28.5 million ha. of land, 12.3 million people, million ha. of cropland, and 10,560 villages. All the 27 districts were affected by the flood and the total damage was estimated at Rs.6500 crore (Goswami, 2008). This flood inundated an extensive area in the Lower Subansiri river basin also. The inundated areas due to this flood were digitized from the flood inundation maps available in the Dartmouth Flood Observatory website (DFO, 2006). DFO releases World Atlas of flood hazard that illustrates river floods observed by MODIS, 79

6 LANDSAT 7, RADARSAT, NOAA-AVHRR (Advanced Very High Resolution Radiometer) and other satellites since 1985 ( Prasad et.al., 2006).These maps were available through the Dartmouth University website. The area between the chronically inundated zone and the furthest limit of the inundated areas of the July 2004 flood is considered here as the occasionally inundated zone, since these areas get inundated only in case of a very large flood. The areas outside the reach of the chronically and occasionally inundated zones are considered as the rarely inundated zone of flooding. These areas are not presently affected by any flood, but there is a possibility of inundation in future because of large floods. 6. Database used 1. Post flood satellite imagery of IRS LISS-3 October, MODIS Flood inundation map of 2004 collected from Dartmouth University Website. ( 3. Land use map of the study area prepared from visual interpretation of LISS-3 satellite image of the year Revenue village map of Lakhimpur and Dhemaji districts collected from the Revenue Department, Govt. of Assam. 7. Land use mapping Land use is a comprehensive term that includes various human interventions on the surface of the earth. Land use maps present an inventory by aggregation of individual land use classes, depicting their location, extent and geographical distribution on real-time basis. These maps are scientific tools for analyzing the environment and the relationship between vegetation and land use and the site in which it occurs. Figure 4 (a): Land use map of the Subansiri Basin in Assam The land use map of the Lower Subansiri basin is prepared on the basis on interpretation of IRS LISS-3 satellite images of November, Since automatic image classification algorithms (supervised, unsupervised or neural network) did not yield satisfactory results in 80

7 the study areas primarily because of high chlorophyll density, the present work has been done entirely based on visual interpretation and on screen digitization using GIS and Remote Sensing softwares. A total of 15 land use classes have been identified from the interpretation of IRS LISS-3 satellite images of November, Figure 4(a) represents the different land use categories of the Subansiri basin in Assam, while figure 4(b) provides the area-wise percentage of each of these categories. Figure 4 (b): Pie diagram showing the areawise (%) land use category of the Subansiri basin in Assam. 8. Results and discussion The analysis of satellite image and flood inundation maps using the methodology mentioned above yielded the flood risk zone map shown in the Figure 5. The three zones of probable flood that were identified during study are Zone 1 - chronically inundated zone Zone 2 - occasionally inundated zone Zone 3 - rarely inundated zone 81

8 Figure 5: Flood risk zonation map of the Lower Subansiri basin The Zone 1, Zone 2, and Zone 3 covers an area of sq.km sq.km and sq.km. respectively in the study area. It is seen that most of the chronically inundated and occasionally inundated zones fall on the left bank of the Subansiri river. These areas are normally affected by the floods of the Subansiri and its tributaries and also due to breaching of embankments of the Brahmaputra river. The villages falling under different flood risk zones are identified by overlapping the village map of the study area on the flood risk map as presented in Figure 6. The Zone 1 covers 963 villages, while zone 2 covers 1505 villages and zone 3 covers 780 villages. Out of 963 villages of zone 1, around 784 villages are on the left bank of the Subansiri river, while remaining 187 villages are on the right bank. Heavy rainfall or even a flood of relatively low magnitude is sufficient enough to inundate these areas. Out of 1505 villages of the zone 2, around 1095 villages are on the left bank of the Subansiri river, while remaining 410 villages are on the right bank and out of 780 villages of the zone 3, around 388 villages are on the left bank of the Subansiri river while remaining 396 villages are on the right bank.. 82

9 Figure 6: Villages and Reserved forests covered by different flood risk zones Most of the villages of Lakhimpur and Dhemaji districts situated on the left bank of the river are submerged by flood waters every year bringing untold miseries to the people. This may be because of the low-lying nature of this part of the basin. In spite of the active sedimentation of the Subansiri and its tributaries and the Brahmaputra, the low-lying nature of the area as currently observed appears distinctly anomalous. This anomaly can only be explained by assuming sinking of the whole or part of the basin (GSI, 1977). Parts of the Pabha Reserved Forest, Kadam Reserved Forest, Ranga Reserved Forest, Dulung Reserved Forest, Kakoi Reserved Forest and Subansiri Reserved Forest are also covered by the chronically inundated and occasionally inundated zones. Every year a large amount of flora and fauna are affected by the flood. Figure 6 represents the different villages and Reserved Forests that are covered by different flood risk zones. The different land use categories that fall under different flood risk zones are identified by overlapping the land use map of the study area on the flood risk map and the results are presented in the form of maps. Figures 7, 8 and 9 represent the different land use categories that are covered by the zone 1, Zone 2 and Zone 3. Also, tables 2, 3 and 4 represent the area wise landuse categories that are covered by different flood zones. It is seen that most of the areas of Zone 1 are covered by agricultural land and Swampy/marshy land. Some Swampy/marshy areas are also used as occasional agricultural land. Zone 2 covers a large part of agricultural land and homestead plantation and rural settlement and Zone 3 covers a large part of agricultural land and dense forest. A large part of the Subansiri RF, Dulung RF, Ranga RF and Kakoi RF are under this zone. 83

10 Figure 7: Land use categories covered by the flood risk zone 1 Table 2: Areas covered by different land use categories of Zone 1 Land use category Area in sq.km. Agricultural fallow Agricultural land (plantation) Built up land (Urban) Dry river channel Homestead plantation and rural settlement River Sand bar Scrub land Seasonal Agricultural land Swampy/ marshy land Vegetated bar Waste land Water body

11 Figure 8: Land use categories covered by the flood risk Zone 2 Table 3: Areas covered by different land use categories of Zone 2 Land use category Area in sq.km. Agricultural fallow Agricultural land (plantation) Built up land (Urban).504 Dense forest Dry river channel Homestead plantation and rural settlement River Sand bar Scrub land Seasonal Agricultural land Swampy/ marshy land Vegetated bar Waste land Water body

12 Figure 9: Land use categories covered by the flood risk Zone 3 Table 4: Areas covered by different land use categories of Zone 3 Landuse category Area in sq.km. Agricultural fallow Agricultural land (plantation) Built up land (Urban) Degraded forest Dense forest Dry river channel Homestead plantation and rural settlement River Sand bar.922 Scrub land Seasonal Agricultural land Swampy/ marshy land Vegetated bar.082 Waste land Water body Summary and conclusion Every year due to heavy rainfall in the hilly terrain as well as in the plains, three to four waves of flood pass over the lower part of the Subansiri basin in Assam. Various flood protection measures like earthen embankments, spurs, porcupines etc. are used to protect the area from flood. Embankments provide reasonable protection to the flood affected areas locally and for a limited period. But, their effectiveness decreases due to rising of the river bed gradually, which is due to the fact that the river carries large silt load annually a considerable part of which is deposited in the channel leading to aggradation of the river bed (Goswami, 1998). Breaching of embankments occurs mainly due to overtopping, seepage and erosion. The materials used for construction of embankments are mostly sand and silt which 86

13 are susceptible to erosion. Due to lack of proper maintenance of the existing embankments and meandering nature of the river, the erosion problem leading to breach of embankments is increasing year by year causing heavy flood in this portion. For flood vulnerability mapping and proper management of the basin in order to mitigate the flood problem, a detailed study of the area on the basis of probable flood zones, villages and land use affected by flood, and socio-economic condition of the people living in the basin, is highly essential. The present study is one of the initial efforts to identify the flood risk zones which will be useful for future planning of the area. The primary purpose of producing these kinds of maps is for public dissemination of flood related information and knowledge which will serve to improve general awareness and aid the process of planning and management of flood risks and hazards. This exercise is also expected to help empower individuals and officials to take appropriate preparatory and response measures and to take rational decisions in regard to use, layout and design of an area of land exposed to the hazard of flood. Among the various measures urgently needed in the study area for reducing flood damages and increasing food, health as well as ecological security, mention may be made of an effective flood forecasting and warning system, evacuation of probable flood affected people before they are overwhelmed by flood, use of eco-friendly materials and techniques such as Geotextile bags (or Geobags) for construction of embankments to protect river banks from severe scouring and erosion, and most importantly, selection of variety of crops that are best suited to soil and water conditions for cultivation in the non-flood period, raising of plinths of houses especially the granaries, and deployment of life saving emergency actions. 10. References 1. CSE Report, (1991), Annual Mayhem in Assam in A. Agarwal and S. Narain (series ed.), Floods, Flood plains and Environmental Myths, pp Das A.K., Sah R.K. and Hazarika N., (2012), Bankline change and the facets of riverine hazards in the floodplain of Subansiri Ranganadi Doab, Brahmaputra valley, India. Natural hazards, 64(2), pp District Gazetteers, Lakhimpur district, Govt. of Assam, (1976). 4. DFO (2006), Dartmouth Flood Observatory Goswami, D.C., (1985): Brahmaputra River, Assam, India: Physiography, Basin denudation and Channel Aggredation, Water Resources Research, American Geophysics Union, Vol. 21, pp Goswami, D. C., (1998), Fluvial regime and flood hydrology of the Brahmaputra River, Assam in V. S. Kale (ed.), Flood studies in India, Memoir 41, Geological society of India, Bangalore. 7. Goswami, D.C., (2008), Managing the wealth and woes of the river Brahmaputra GSI, (1977), Contributions to geomorphology and geohydrology of the Brahmaputra Valley. Geological Survey of India, Miscellaneous publication, 32, p

14 9. Leopold, LB., Wolman, M.G., and Miller, J.P., (1964), Fluvial processes in Geomorphology, San Francisco, California: W.H. Freeman. 10. Nyarko, Benjamin Kofi., (2006), Flood risk zoning of Ghana: ACCRA experience, International archives of photogrammetry and remote sensing, XXXIII (B7), Amsterdam. 11. Poddar, M.C., (1952), Preliminary report of the Assam earthquake, 15 th August, 1950, Bulletin of geological survey of India, 2, pp 11-13, 12. Prasad, K.C., Kakoti, R., Burman, S.K., (1981), Geology and geomorphology of a part of the Subansiri river valley. Geological survey of India, Miscellaneous publication 46, pp Prasad A. K., Kumar K. V., Singh S., Singh R., (2006), Potentiality of multi- sensor satellite data in mapping flood hazard, Journal of the Indian society of remote sensing, 34(3), pp Schumm SA. And Lichty RW., (1963), Channel widening and floodplain construction along Cimarron river in southwestern Kansas. US geological survey professional Paper 352-D: