Open access e-Journal Earth Science India, eISSN: 0974 – 8350 Vol. 5(II), April, 2012, pp. 51-59 http://www.earthscienceindia.info/ Study of Barak River Meander and Associated Hazard around Silchar Town, Assam, using Remote Sensing and GIS Pulak Das Department of Ecology and Environmental Science, Assam University, Silchar-788011, Assam, India Email: [email protected] Abstract Present paper deals with the study of a meander in River Barak around Silchar town in Assam using remote sensing and GIS, encompassing a period of thirty-four years between 1976 and 2010. Changes in planform characteristics of River Barak are analysed with the help of six remote sensing images of the years 1976, 1979, 1988, 1999, 2003, and 2010. The sinuosity of the meander is observed to be quite high (>2.3) exhibiting a decreasing trend in thirty-four years. The meander ratio ranging from 1.05 to 1.12 also exhibited a decreasing trend. The decreasing trends of sinuosity and meander ratio undermine the probability of cut- off across the meander in near future. To analyse the meander shift, three Ground Control Points (GCPs) were selected. It is observed that the western arm of the meander bend around Silchar town has shifted about 132.45 m towards northwest while the eastern arm of the bend has shifted about 61.59 m towards east and 9.88 m towards northwest at two locations respectively from 1976 to 2010. Over all the meander is observed to be shifting towards north-west direction towards downstream and can be categorised as down-valley meander migration. Cases of river bank erosion are observed at two locations without any bank protection leading to destruction of property and displacement of families. A third location is comparatively stable with river bank protection by boulders. The study of past meandering characteristics may indicate towards the conditions which were prevalent and will be helpful in predicting the future channel pattern and associated hazard of river bank erosion around Silchar town. Key Words: Barak River; Meander shift; River bank erosion; Silchar Introduction Alluvial rivers, depending upon their planform characteristics, can be classified into straight, meandering, braided, and anastomosing. Leopold and Wolman (1957) classified rivers into straight, meandering (SI> 1.5), and braided based on its sinuosity, where as Schumm (1963) recommended five classifications. These planform characteristics are determined primarily by slope, confinement, sediment supply, channel and valley materials, and riparian vegetation (Knighton, 1998). It is very rare to find straight alluvial channels, because most streams tend to meander unless they are confined in a narrow valley or gully. A curve along the channel introduces an additional form of energy dissipation not present in a comparable straight reach, an energy loss due to change of flow direction (Leopold et al ., 1960). Meandering nature are common in alluvial river mainly of low gradient. On flat ground a river tends to form a relatively broad channel that slowly wanders back and forth. Meandering streams are more dynamic, and their tendency to shift location across the valley bottom increases with sinuosity (ratio of stream length to valley length), bed load, and slope. The sinuosity is the degree to which a river departs from a straight line (Schumm and Khan, 51 Study of Barak River Meander and Associated Hazard around Silchar Town, Assam, using Remote Sensing and GIS: Das 1972). Meanders tend to occur at regular intervals along the river channel, depending on the flow rate. The higher the flow rate, the greater the distance between meanders tends to be. The evolution of a meandering planform is characterized by three basic processes: (1) the continuous elongation of the river axis, with single or compound lobe formation, (2) the downstream (sometimes upstream) migration of the meander loops, and (3) the occurrence of cutoff events (Camporeale et al., 2008). Through time, the configuration and position of rivers changes as part of meander evolution, development, and migration processes. The process of migration does not occur simultaneously along the entire length of a channel but, at discrete location at any one time, leading to the alteration of individual meanders (Burke, 1984; Hagerty, 1984; Chang, 1992). Channel migration or meander migration (Crosato, 2008) is a combination of translation and extension of river channels (Brice, 1984). Bank erosion and accretion leads to bank retreat and advance respectively (Crosato, 2008). Changes in the configuration of a single bend may include rotation, elongation, and a shift in meander axis (Hooke, 1984). When meanders change their shape and shift in their position, the associated erosion and deposition that enables these changes to occur can cause loss or damage to private properties and structures. River Barak is of highly meandering nature. The name Barak itself originates from Bodo-Baak (Bodo– Big; Baak– Bends/meanders). It took its rise a little to the west of Maothana, on the northern boundary of Manipur. Taking a south-west turn from its source near Tipaimukh in Manipur, it then come to north and for a considerable distance formed the boundary line between the region and the state of Manipur. After its junction with the river Jiri, this too for a considerable length, formed the border between Barak valley region and the Manipur. The Barak River valley is represented by a narrow E-W trending elongated alluvial filled basin and is located at the northern edge of N-S trending fold ridges of the Indo-Burma frontal fold belt. The River has undergone substantial changes in its channel position at several places in Barak valley with strong northward shift towards west of Silchar (Das et al., 2007). The entire area of Cachar district is represented by i) unconsolidated, ii) semi- consolidated and iii) consolidated formations. The hydraulic gradient of ground water is from North to South in northern parts and ground water flows from South to North-West in southern parts (CGWB, 2008). The river, flowing through Barak valley in southern Assam (Fig.1) is of alluvial channel characteristics. The river exhibits several changes through time and space with indications of shifting of channel at various places in the region. The intensely meandering Barak River exhibits abandonment of several loops and overall northward shift (Das et al., 2007). Consequent upon the frequent river bank cutting and loop abandonment, the habitation and agricultural practices are affected resulting in loss of soils. Silchar town, a major part of which is encircled by the river, is among one of the busiest town of northeast India and a commercial hub for the states of Tripura, Southern Assam, Manipur, and Mizoram. The river planform characteristics around the town are associated directly with natural hazards such as bank erosion and floods, affecting the social and economic condition of people living in and around Silchar town. Shifting river course on one hand is displacing people from its bank and on the other raises doubts on the stability of river course in extreme flood conditions. Erosion of banks due to meandering nature of river Barak is a major safety issue in the region threatening households, agricultural lands, and roads. As about half of the town boundary is circumscribed by the meander loop, it is very important to have an idea for the possible ways in which the meander loop will migrate. In the wake of this, the meandering Open access e-Journal Earth Science India, eISSN: 0974 – 8350 Vol. 5(II), April, 2012, pp. 51-59 http://www.earthscienceindia.info/ Fig. 1: Location of Barak River and Silchar town. history of the river around Silchar town may throw some light regarding the future probability of migration. Considering these points, a study has been conducted using remote sensing and GIS to analyse the changes in planform geometry of the Barak river meander around Silchar town between 1976 and 2010. The study deals with changes in some important features of a meander such as sinuosity index, meander ratio, and channel shift during a period of 34 years and discusses about its impact on human habitations and household displacement from the river bank. Materials and Methods In order to assess the Barak river meander around Silchar town, a segment with a length of about 12.5 km is selected; coordinates of start and end point of this segment is 24.8505 N latitude-92.7775 E longitude, and 24.8342 N latitude- 92.8356 E longitude. Satellite images of Landsat- multispectral scanner system (MSS), thematic mapper (TM), and enhanced thematic mapper plus (ETM+) for January 1976, January 1979, November 1988, December 1999, January 2003, and February 2010 was available (Table-1). For using satellite images it is assured that the images are georeferenced which leads to less error and better image interpretation. For calculation of the planform characteristics, above mentioned meander around the Silchar town was digitized for all the six images and calculation was done for Sinuosity index (SI) (following Schumm, 1963) (SI=Observed length (O L)/Expected straight length (E L)) and Meander ratio (MR) (following Garg, 1987) (MR=Meander belt (M B)/Meander length (M L)). For digitization of river and geometrical calculation QGIS and AutoCAD2006 softwares were used. Channel boundaries were digitized as a polygon into a geographic information system. For image processing IDRISI software is used. To quantify the channel shift, three Ground Control Points (GCPs) (Fig. 2) were selected and plotted 53 Study of Barak River Meander and Associated Hazard around Silchar Town, Assam, using Remote Sensing and GIS: Das (following Das et al ., 2007) and distance between GCPs and river banks were measured using QGIS software for different years. For ground verification of change in river course and consequent river bank erosion three erosion sites (E1, E2, E3) (Fig.