Study of Barak River Meander and Associated Hazard Around Silchar Town, Assam, Using Remote Sensing and GIS

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,

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

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. 2) were visited and discussions were made with local people.

Table-1: Satellite data used in the study.

Year Features 1976 1979 1988 1999 2003 2010 Spacecraft ID Landsat 2 Landsat 3 Landsat 5 Landsat 7 Landsat 7 Landsat 5 Sensor ID MSS MSS TM ETM+ ETM+ TM Acquisition 1976-01- 1979-01- 1988-11- 1999-12- 2003-01- 2010-02- date 26 19 10 19 28 08 WRS Path 146 146 136 136 136 136 WRS Row 043 043 043 043 043 043 Resolution 68 x 83 68 x 83 30 x 30 30 x 30 30 x 30 30 x 30 (m)

Fig. 2: Barak River meander migration around Silchar town from 1976 to 2010 and locations of Ground Control Points (GCPs) and Erosion sites (E).

Results and Discussion

Analysis of study segment shows that the expected length (EL) of the channel is 5.06 km. The observed length (OL) exhibits a decreasing trend, with a change of 0.12 km in 34 years. The sinuosity index (SI) of the river is quite high (>2.3) for all the studied years; the lowest value being 2.37 and the highest being 2.46 (Fig. 3). The SI exhibits an alternating Open access e-Journal Earth Science India, eISSN: 0974 – 8350 Vol. 5(II), April, 2012, pp. 51-59 http://www.earthscienceindia.info/ increasing and decreasing pattern with an overall decreasing trend from 1976 to 2010. The change in SI may be attributed to change in flow rate owing to change in rainfall pattern. Maximum change in the value is noticed between 1976 and 1979 (3.8%) followed by between 1979 and 1988 (2.44%). Both, Meander belt (M B) and Meander length (M L) exhibits a decreasing trend (Table-2); the trend is more prominent in former. Overall from 1976 to 2010 the meander ratio exhibits little change (Fig. 4) and ranges between 0.75% and 1.41%, although in 1999 it exhibits an increase of about 4.97%.

Fig. 3: Sinuosity index (SI) of Barak meander around Silchar town during 1976 to 2010; numbers in parentheses indicates the percentage change in SI.

Table-2: Planform geometrical parameters of Barak meander around Silchar town during 1976 to 2010.

Planform parameters (distance in kilometers)

Year Observed Expected Meander Belt Meander Length (O L) Length (E L) (M B) Length (M L) 1976 12.00 5.06 2.77 2.59 1979 12.43 5.06 2.73 2.54 1988 12.12 5.06 2.74 2.57 1999 12.19 5.06 2.71 2.42 2003 11.97 5.06 2.68 2.51 2010 12.12 5.06 2.68 2.55

There are three locations within the meander where shifting of channel has been observed (Fig. 2). The distance from river banks from these three points to Ground Control Points (GCPs) (Fig. 5) indicates a shift in last 34 years. Highest shift is observed at western arm of the bend from GCP 2. From GCP 2 the river bank has moved towards northwest direction about 5.1 m between 1976 and 1979, 28.02 m between 1979 and 1988, 45.85 m

Study of Barak River Meander and Associated Hazard around Silchar Town, Assam, using Remote Sensing and GIS: Das

between 1988 and 1999, 33.12 m between 1999 and 2003, and 20.38 m between 2003 and 2010 (Fig. 5). Overall the river has shifted about 132.45 m in last 34 years. The average rate of shift has increased from 1.7 m/year between 1976 and 1979 to 8.3 m/year during 1999- 2003 and then decreased to 3.4 m/year between 2003 and 2010. River bank at eastern arm towards north direction exhibits another shifting point. From GCP 3 the river bank has shifted about 28.23 m between 1976 and 1979, 10.27 m between 1979 and 1988, 10.23 m between 1988 and 1999, 7.73 m between 1999 and 2003, and 5.14 m between 2003 and 2010; overall shift being 61.59 m. The average rate of shift exhibits a reduction from 9.4 m/year during 1976-1979 to 0.9 m/year during 2003-2010. GCP 1 indicates a shift of 9.88 m during 1999- 2003 at southern part of eastern arm of meander bend; after wards the bank seems to be stable due to bank reinforcement in later years.

Fig . 4 : Meander ratio of Barak meander around Silchar town during 1976 to 2010.

Fig. 5: Distances of Ground Control Points (GCPs) from river bank during 1976-2010. Open access e-Journal Earth Science India, eISSN: 0974 – 8350 Vol. 5(II), April, 2012, pp. 51-59 http://www.earthscienceindia.info/

SI is observed to be quite high comparatively (e.g. Aswathy et al ., 2008; Panda and Bandyopadhyay, 2011). In the present study the SI showed a change of about 1.27% over a time period of 34 years (1976 to 2010). Based on Schumm’s (1963) classification, Barak meander around Silchar town can be categorised as ‘tortuous’ (SI >2.3). The river Rio Grande de Añasco in tropical climate was found to exhibit similar sinuosity index (2.4) (Alvarez, 2005). In contrast, another example of a humid tropical river, the Angabunga, in Central Congo, Africa, showed a sinuosity of 1.60 in 1970 (Chitale, 1970). Sinuosity changes of 18% were measured in the River Culm, 16% in the River Creedy, 20% in the River Otter, and 28% in the River Yarty from 1903 to 1953 (Hooke, 1977). As sinuosity increases, the probability of cut-off increases (Bridge, 2003), the sinuosity of Barak meander around Silchar town exhibits a decreasing trend. Continuous increase in meander ratio also increases the chances of developing cut-off meanders rapidly. Majumdar (2008) found that the meander ratio gradually increased in river Ganga from 1929 to 2002 in West Bengal indicating towards a future probability of development of cut-off, which does not seems to be the case in Barak river meander. In the present study the range of meander migration rate is observed to be from 0.9m/year to 9.4m/year. Migration rates for temperate rivers have been documented from less than 0.1 m/year (Hooke, 1980) to more than 7.26 m/year (Nanson and Hickin, 1983). In tropical condition Alvarez (2005) found the meander migration rate ranging from 1.93 m/year to 2.81 m/year. Overall, the meander shift in the present study seems to be towards north-west direction within the study segment (Fig. 2). The shifts in the river channel of the order of 200 m to more than 500 m between 1955 and 1999 have been observed in river Barak at various places by Das et al . (2007). Also there are cases of reduction of intensity of meandering in some parts of the river (Das et al ., 2007). Mention has been made about incidence of earthquake during 1984 which has tilted the region towards north direction and hence may be contributing towards the northward shifts of river (Das et al ., 2007). In the present study the migration pattern indicates towards systematic downstream, down-valley meander migration as discussed by Thorne (1997) and Knighton (1998).

Ground verification of bank erosion sites (Fig. 6) revealed that in sites E1 and E2 the erosion rate is very high. After discussing with village elder people it was known that many families (about 50 and 25 families in E1 and E2 respectively) have been displaced by the bank erosion activity in last more than thirty years. These families are either rehabilitated by government in the opposite bank (in site E1) or were bound to take rented house in the town. Some families have opted to buy new land in the village itself at a larger distance from the river bank. Noticeably in both the sites government project of bank strengthening using boulders have not reached yet. Use of boulders seems to retard the erosion rates in nearby areas (Fig. 7). In site E3 the erosion rate is comparatively very low and the banks are strengthened with boulders. Conclusion

The present study illustrates the geospatial application in studying the past channel shifts and changes in sinuosity vis-à-vis its consequences. It can be concluded from the study that the Barak river meander around Silchar town is shifting and the overall shifting is towards north-west direction. Three locations could be identified in the meander where the shifting is prominent. The western arm of the meander bend is shifting at a faster rate than the eastern arm of the bend. From 1976 to 2010, the former has shifted about 132.45 m, and latter has shifted about 61.59 m and 9.88 m at northern and southern parts respectively. The sinuosity index and meander ratio exhibits a decreasing trend in last thirty-four years. The shifting of the river channel has led to wide spread bank erosion and destruction of houses

Study of Barak River Meander and Associated Hazard around Silchar Town, Assam, using Remote Sensing and GIS: Das

and private properties. People are displaced from the river banks and are forced to live in marginal conditions within town and other nearby villages.

As suggested by Ahmed and Fawzi (2011) retardation blocks can be used at the Barak river bends around Silchar town to weaken the secondary currents by the river bends. Continuous monitoring of the river banks should be done and rates of erosion and deposition should be measured, and protection of banks should be completed as early as possible to stop the erosion.

Fig. 6: Locations of two bank erosion sites; western meander arm (E1) and eastern meander arm (E2).

Fig. 7: River bend near location E2 (A), and boulder protection of river bank near location E1 (B).

References

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