INTERNATIONAL JOURNAL OF GEOMATICS AND GEOSCIENCES Volume 2, No 2, 2011 Copyright 2010 All rights reserved Integrated Publishing services Research article ISSN 0976 4380 Changing trends of channel pattern of the Ganges-Padma river Department of Geography and Environment, Jahangirnagar University, Dhaka, Bangladesh aleayeasmin@gmail.com ABSTRACT This study analyzes the changing trends of channel pattern of the Ganges-Padma river. A time series of satellite images in the period of 1973-2006 are compiled for analysis. It is believed that the Ganges-Padma river is a meandering river. But lately the Ganges-Padma becomes braided river for high sediment transportation by Jamuna and deposition of Ganges- Padma river bed. For the purposes of study, the sinuosity ratio and braiding index were calculated in different time series. The analysis shows that the sinuosity ratio is increased over time. In 1973, it was 1.31 and in 1984, 1996, 2006 it was 1.33, 1.37, and 1.43 respectively. In 2006, though it was sinuous but it is very close to 1.5 which indicates meandering. In case of braiding index, it has been observed that in 1973, the braiding index was 1.3 and in 1984, 1996, 2006 it was 1.43, 1.62, and 1.92 respectively. The analysis also shows that in 1984, the braiding index was very close to 1.5. After 1996, it was exceeded 1.5 and continuously increasing. By analyzing the sinuosity ratio and braiding index, it has been observed that the changing rate of braiding index is more than sinuosity ratio. Keywords: Braiding Index, Changing Rate, Meandering River and Sinuosity Ratio, 1. Introduction Bangladesh is a riverine country. The main rivers of Bangladesh are the Padma, Meghna, Jamuna and Brahamaputra. There are also small sized river across the country. The Ganges- Padma river is one of the three major river systems of Bangladesh. It has a length of approximately 2,200 km from its source in the Himalayas in Nepal to its confluence with the Jamuna river in Bangladesh. The lowermost reach of the Ganges at the downstream from the Farakka Barrage to the Jamuna confluence at Aricha, is referred to here as the Ganges River. Locally, the river in this reach is known as a part of the Padma river. In fact, the Padma river can be divided into two distinct reaches, which are hydrologically and morphologically quite different from each other. The upstream reach is from the Farakka Barrage to Aricha. The downstream reach extends from Aricha to Chandpur and carries the combined flow of the Padma and Jamuna rivers. The platform of the Padma river started to change in the shifting of the main course of the Brahmaputra River towards the Jamuna River due to the input of huge sediments and extra discharge (FAP 24, 1996a).For the major parts of the Ganges delta by releasing the sediment through it distributaries network. Increasing significance of the Jamuna River over Padma River is very high and for this reason there are so many environmental impacts is found. Gradually Padma become braiding river for high sediment transportation by jamuna and deposition in Padma river bed. Depositional models of braided river are necessary for rational interpretation of ancient deposit, and to aid the characterization of subsurface deposits. Braiding is a complex and highly dynamic process, the evolution which is at present hard to predict, even on short time scales. In order to improve the understanding and modeling of Braided river, there is a need to gain a better inside into the processes of channel bifurcations, Submitted on October 2011 published on November 2011 669
which is widely recognized has been important for the onset of Braiding. As the Ganges is predominantly a meandering river, bank erosion is strongly associated with the development and migration of meander bends. In earlier studies, the Hickin and Nanson (1984) method of predicting bank erosion in meander bends was applied along the Jamuna River given that each of the braided channels of the Jamuna River acts as a meandering channel (EGIS 2002; Islam, 2006). Rivers are a dynamic and increasingly important part of the physical environment. Their behavior is of interest to a wide variety of concerns, ranging form flood control, navigation and water resource development to recreation. Rivers are essentially agents of erosion and transportation, removing the water and sediment supplied to them from the land surface to ocean. Channel pattern is the configuration of a river. This is the trace of a channel in plan, as shown, for instance, on vertical air photographs or as represented on maps. Channel pattern is used to describe the plan view of a reach of river as seen from an airplane and includes meandering, braiding or relatively straight channels (Leopold and Wolman, 1957). Meandering patterns and braided patterns occur widely. Habit in this sense can be substituted for patterns, as when a stream is said to have a meandering or a braided habit. For purposes of definition we have used the ratio of channel length to downvalley distance as criterion. These ratio, called sinuosity, varies in rivers from a value of unity to a value of 4 or more; rivers having a sinuosity of 1.5 or greater we have called meandering, and below 1.5 straight of sinuous these definition is somewhat limited, and it might be well to require that meandering channels also have some degree of symmetry in their curvature. Rivers are usually described as being straight, meandering or braided. There is in fact a great range of channel patterns from straight through meandering to braid. Straight and meandering channels are described by sinuosity which is the ratio of channel length to valley length or the ratio of valley slope or channel gradient as measured over the same length of valley (Schumm, 1977).A total sinuosity ratio of 1.50 was selected by Leopold and Wolman to differentiate sinuous from meandering. Rivers having a sinuosity of 1.5 or greater refers meandering, and below 1.5 straight or sinuous. Table 1: Measurement of Sinuosity Ratio Sinuosity Ratio Thalweg length Valley length Channel length Length of Meander belt axis Stream length Valley length Source Leopold and Wolman, 1957 Brice, 1964 Schumm, 1963 The planform properties of braided rivers have received considerable attention, especially of their braiding intensity (e.g.brice, 1964; Howard et al., 1970; Engelund & Skovgaard, 1973; Rust, 1978a;Hong&davies, 1979; Mosley, 1981; Richards, 1985; Fujita, 1989; Friend & Sinha, 1993;Robertson-Rntoul&Richards, 1993; Islam, 2006). Usage of a suitable braiding parameter is an important measure towards better interpretation of braided river(rust, 1978a; Islam, 2006) as the intensity of braiding varies greatly from river to river (Friend & Sinha, 1993, ; Islam, 2006 and even from reach to reach along the coarse of a particular river under different flow stages (EGIS, 1997; Islam, 2006) Therefore, an important first step in this 670
study was to select an appropriate method of measuring the degree of braiding in the Ganges- Padma river using satellite images. The most appro priate measures that would allow the continuum of the Ganges-Padma river braiding to be quantified is the braiding index introduced by Brice 1964 and Islam, 2006. This has often been used to measure the braiding intensity of the largest braided rivers (e.g.gowsami, 1985; Jiongxin, 1997; Gowsami et al., 1999; Islam, 2006).Brice s (1964) braiding index (BI) is defined asfollows: BI=2( L i )/L r Where L i is the total length of bars and (or) islands in the reach, and L r is the length of the reachmeasured mid way beteen the banks. A total braiding index of 1.50 was selected by Brice to differentiate braided from non braided reaches. The objectives of the present study are as follows: 1. To calculate sinuosity ratio and braiding index 2. To find out the relationship between the comparison of sinuosity ratio and braiding index. 2. Data Source and Methodology A substantial portion of the geostatistical database for this study was derived from a time series of Landsat images. The Landsat images consist of 4 images frames for 1973, 1984, 1996 and 2006. The Landsat images were obtained from CEGIS (Centre for Environmental & Geographic Information Services). GIS software was used to visualize the change of channel pattern of Ganges-Padma river. ArcView GIS 3.3, Microsoft Excel were used to analyze the collected image from CEGIS. At first, number and length of bars of selected years are findout from image by ArcView tools and also overlapped bank line of different periods of Ganges- Padma river image. Stream length and Valley length are also measured for calculating Sinuosity ratio. Braiding Index is calculated from total length of bars and length of rivers. A total sinuosity ratio of 1.50 was selected by Leopold and Wolman to differentiate sinuous from meandering. Rivers having a sinuosity of 1.5 or greater refers meandering, and below 1.5 straight or sinuous. When the value of braiding index is less than 1.50 then the river indicates non-braided river and the value is equal or greater than 1.50 than it indicates braided river. The following formula is used for calculating Sinuosity ratio and Braiding Index in present study. Stream length Sinuosity Ratio= Valley length 3. Result and Discussion For purpose of calculating sinuosity ratio, the Table 2 is given below: Table 2: Measurement of Sinuosity Ratio of the Ganges-Padma Rive during 1973-2006 Year Stream Length (Km) Valley Length (Km) Sinuosity Ratio 1973 275.81 209.18 1.31 1984 279.90 209.72 1.33 671
1996 287.52 208.63 1.37 2006 298.11 208.98 1.43 The analysis shows that the sinuosity has increased over time. In 1973, it was 1.31 and in 1984, 1996, 2006 it was 1.33, 1.37 and 1.43 respectively. In 2006 it was very close to 1.5 (Table-2) which indicates meandering river. The total and stream length of bars is tabulated in Table 3. This total length of bars and length of rivers are calculated for measuring Braiding index by using Brice (1964) formula. Table 3: Measurement of Braiding Index of Ganges-Padma Rive during 1973-2006 Year Total Length of Bars (Km) Length of Rivers (Km) Braiding Index 1973 136.08 209.18 1.3 1984 150.74 209.72 1.43 1996 169.11 208.63 1.62 2006 200.81 208.98 1.92 The analysis shows that braiding index was 1.3 in 1973 which indicated the Ganges-Padma river was non-braided. In 1984 it was 1.43 which also indicated non-braided but it has been observed that the braiding index is increased and also closed to 1.5. In 1996 and 2006 the braiding index was 1.62 and 1.92 respectively. This value of braiding index indicates that it exceeds 1.5. If the braiding index exceeds 1.5 then the river is going to be braided. Sinuosity ratio and Braiding index are measured for understanding the present conditions of Ganges- Padma river. Sinuosity ratio and braiding index of different years are given in figure-1. Sinuosity Ratio and Braiding Index of the Ganges-Padma River during 1973-2006 Frequency 2 1.9 1.8 1.7 1.6 1.5 1.4 1.3 1.2 1970 1980 1990 2000 2010 Year Braiding Index Sinuosity Ratio Figure 1: Braiding Index of the Ganges-Padma River during 1973-2006 In case of Sinuosity ratio, the analysis shows that the sinuosity is increased over time. In 1973, it was 1.31 and in 1984, 1996, 2006 it was 1.33, 1.37 and 1.43 respectively. In 2006 it was very close to 1.5 (Figure 1) which indicates meandering river.the analysis shows that braiding index was 1.3 in 1973 which indicates the Ganges-Padma river was non-braided. In 1984 it was 1.43 which also indicated non-braided but it has been observed that the braiding index is increased and also closed to 1.5.In 1996 and 2006 the braiding index was 1.62 and 1.92 respectively. This value of braiding index indicates that it closed 1.5. If the braiding 672
index exceeds 1.5 then the river becomes non-braided to braided. The analysis reveals that the Ganges-Padma river became braided river from 1996. In compare between sinuosity ratio and braiding index, it has been analyzed that the changing rate of braiding index is more than sinuosity ratio. Figure 1: Image of the Ganges-Padma river in 1973 Figure 2: Image of the Ganges-Padma river in 1984 Figure 3: Image of the Ganges-Padma river in 1996 Figure 4: Image of the Ganges-Padma river in 2006 673
4. Conclusion A time-series of satellite images was compiled to document historical changes in morphology and to provide the basis for an empirical analysis of changing trend of Ganges-Padma river. The Ganges-Padma river is one of the major river systems in Bangladesh. In the Ganges- Padma River, the sinuosity is increased over time. In compare to 1973 banklines sinuosity ratio is increased from 1.31 to 1.33 in 1984. sinuosity ratio is increased from 1.33 to 1.37 in 1996. In compare to 1973 and 1996, sinuosity ratio is increased from 1.37 to 1.43 in 2006. The analysis also shows that the braiding index is increased over time. The number of bars was decreased but the length of bars was increased in compare to 1973 and 1984. On the otherhand, the Braiding index is increased from 1.3 to 1.43 in 1984.In compare to 1973 and 1984, the number of bars was increased but decreased than 1996. The length of bars was increased from the year of 1973, 1984 and 1996. On the otherhand, the Braiding index is increased from 1973, 1984 and 1996 respectively. The braiding index was 1.3 in 1973 which indicates the Ganges-Padma River was nonbraided. In 1984 it was 1.43 which also indicated non-braided but it has been observed that the braiding index is increased and also exceeds to 1.5. In 1996 and 2006 the braiding index was 1.62 and 1.92 respectively. This value of braiding index indicates that it exceeds 1.5. If the braiding index exceeds 1.5 then the river is going to be braided. 5. References 1. Brice, J.C., (1964), Channel patterns and terraces of the Loup Rivers in Nebraska. U.S. Geologicall Survey Professional papers, 422-D, pp 1-41. 2. EGIS, (1997), Morphological Dynamics of the Brahmaputra-Jamuna River, Research Report, Environmental and GIS Support Project, Banani, Dhaka, 1213, p 76. 3. Engelund, F. and Skovgaard,O., (1973), On the origin of meandering and braiding in alluvial streams. Journal of Fluid Mechanics, 57, pp 289-302. 4. FAP 24., (1996a), River survey project: morphological characteristics. Final Report- Annex 5. Flood Action Plan, Water Resources Planning Organization, Dhaka. 5. Friend, P. F. and Sinha, R., (1993), Braiding and meandering parameters. In: Best, J.L and Bristow, C. W (eds.) Braided Rivers, Geological Society Special Publication No. 75, London, pp 105-111. 6. Fujita, Y., (1989), Bar and Channel formation in braided streams. In:Ikeda, S. and Parker, G. (eds.), River Meandering, American Geophysical Union, Water Resources Monographs, 12, pp 417-462. 7. Goswami, U., Sarma, J.N. and Patgiri, A. D., (1999), River channel changes of the subansiri in Assam, India. Geomorphology, 30, pp 227-244. 8. Hong, L.B. and Davies, T.R.H., (1979), A study of stream braiding Geological Society of Amerrican Bulletin, 79, pp 391-394. 9. Howard, A. D., Keetch, M.E. and Vincent, C. L., (1970), Topological and geometrical properties of braided rivers. Water Resources Research, 6(6), pp 1674-1688. 674
10. Islam, M.N., (2006), Braiding Morphodynamics of the Brahmaputra-jamuna River, A H Development Publishing House, Dhaka, pp 11-47 11. Jiongixn, X., (1997), Study of sedimentation zones in a large sand-bed braided river: an example from the Hanjiang River of China. Geomorphology, 21, pp 153-165. 12. Leopold, L.B. and Wolman, M.G., (1957), River channel pattern: braided, meandering and straight. U.S. Geological Survey Professional Papers, 282, pp 1-84. 13. Mosley, M. P., (1981), Semi-determinate hydraulic geometry of river channels, south Island, new Zealand. Earth Surface Processes and landforms, 6, pp 127-137. 14. Richards, W.R.R. and Thorne, C.R., (1985), River: Forms and Processes in Alluvial Channels. Methuen, London, pp 361. 15. Roberts, G., France,M., Johnson, R.C.and Law, J.T., (1993), The analysis of sensed image of the Balquhidder catchment for the estimation of land covers types. Journal of Hydrology, 145, pp 259-265. 16. Rust, B.R., (1978a), A classification of alluvial channel systems.in: Miall, A.D. (ed), Fluvial Sedimentology. Canadian society of petroleum Geologiests Momoirs, 5, pp 221-245. 17. Schumm, S.A., (1977), The fluvial System. New York: John Wiley & Sons. 675