The Brahmaputra-Jamuna River, Bangladesh

The Brahmaputra-Jamuna River, Bangladesh, is one of the world's great rivers, ranking in the top three in terms of both sediment and water discharge. The high water and sediment discharges are generated by the monsoon-dominated floods and the tectonic setting, which provides abundant sediment from Himalayan uplift into the subsiding Bay of Bengal. Structural, climatic and autocyclic fluvial processes have produced a large multi-channel river in Bangladesh with individual channels up to 5 km wide that can scour down to 50 m. The Brahmaputra-Jamuna has experienced large-scale avulsion at the end of the 18th century, recent widening and westerly migration and exhibits rapid bank erosion in response to large floods, the most notable being during the recent floods in 1987, 1988, 1998 and 2004. This chapter presents a synthesis of: i) the broad setting of the Brahmaputra-Jamuna River in Bangladesh, ii) channel morphology and its recent historical change, iii) the major bedforms and their dynamics, iv) the nature and importance of channel bifurcations, offtakes and confluences, v) floodplain sedimentation, vi) the sedimentology of the Jamuna River, and vii) examples of applied geomorphology and engineering within some of the largest and most dynamic channels. Data and discussion are drawn both from published papers and recent multi-national studies on the Brahmaputra-Jamuna that have allowed the most detailed study ever conducted on this mighty river. This chapter concludes with an outline for future areas of research needed to understand better and manage one of the world's most majestic and strategically important large rivers. 1. Background Bangladesh is dominated by three great rivers - the Brahmaputra-Jamuna, Ganges and Meghna - that combine to feed sediment into one of the world's largest deltas in the Bay of Bengal (Fig. 1). Bangladesh has been shaped by, and is dependent upon, its rivers, which provide fertilie soils and a diverse flora and aquaculture but also bring significant flood hazard and risk to infrastructure for a large and growing population. The people of Bangladesh have adapted their lifestyle for centuries to live with river flooding - frequently moving their temporary bankside homes, planting on newly emergent river bars, and sometimes raising their homesteads above water level in flood periods (Paul, 1997). However, a growing population, coupled with the expansion of infrastructure and economic development, has resulted in an increase in the intensity of flood damage (FPCO, 1995; Paul, 1997; CPD, 2004). The lives of many millions of Bangladeshi citizens is thus reliant on these rivers, with up to 600,000 people living on the riverine islands and bars alone (Sarker et al., 2003). Bangladesh's rural economy relies upon annual 'normal' floods to bring moisture and fresh sediments to the floodplain soils (Paul, 1997): for instance, two of the three rice varieties (aus and aman) cannot survive without floodwater and the fish caught both on the floodplain during flood season and from the many floodplain ponds ('beels') provide the main source of protein for many rural populations (Chowdhury, 1994; Paul, 1997; de Graff, 2003; Shankar et al., 2004). However, the effect of 'abnormal' floods can be devastating and result in appreciable damage to crops and houses, severe bank erosion with consequent loss of homesteads, schools and land, and loss of human lives, livestock and fisheries (BDER, 2004; Shankar et al., 2004). For example, in the 1998 flood, over 70% of the land area of Bangladesh was inundated, affecting 31 million people and 1 million homesteads (Chowdhury, 2000). The 1998 flood, which had an unusually long duration from July to September, claimed 918 human lives and was responsible for damaging 16000 and 6000 km of roads and embankments respectively, and affecting 6000 km2 of standing crops (Chowdhury, 2000). In the recent 2004 floods over 25% of the population of Bangladesh, or 36 million people, was affected by the floods; 800 lives were lost; 952,000 houses were destroyed and 1.4 million badly damaged; 24,000 educational institutions were affected, including the destruction of 1200 primary schools; 2 million government and private tubewells were affected, and over 3 million latrines were damaged or washed away, this increasing the risks of diarrhoea, cholera and other waterborne diseases; 1.1 M ha. of rice crop was submerged and lost before it could be harvested, with 7% of the yearly aus (early season) rice crop lost; 270,000 ha. of grazing land was affected, 5600 livestock perished together with 254,000 poultry and 63 MT of lost fish production (BDER, 2004; CPD, 2004). In the districts that are dominated by the Jamuna River, the 2004 flood damage to infrastructure (homes, roads, culverts), tubewells and latrines, with ensuing unemployment of many of the population, were some of the areas of critical impact. Hence, due to the nature of these devastating floods, and their recent occurrence in 1987, 1988, 1998 and 2004, the possible influences on catastrophic flooding, such as the role of Himalayan deforestation (Kattelman, 1990; Mirza et al., 2001), the type of intervention and infrastructural response to flooding (e.g. the Brahmaputra Right Embankment (BRE) from the Teesta to Hurasagar confluences, see Fig. 1) and the nature, scope and need for the Flood Action Plan (e.g. Haggart et al., 1994; Paul, 1997) have thus received much attention and debate over recent years (see for instance, Boyce, 1990; Hossain, 1993, Haggart et al., 1994; Reavill and Rahman, 1995; Paul, 1997; Islan, 2001). Additionally, political debates concerning water usage and construction of dams have become a major issue for Bangladesh (e.g. Patel, 1996; Wood, 1999; Mirza, 2004), since over 90% of the catchments of these three great rivers lies outside the boundaries of the country. The Jamuna and Ganges rivers combine to form the Padma River, which carries the third greatest water discharge of all the world's rivers but is often ranked the highest in terms of sediment discharge (Schumm and Winkley, 1994). The Jamuna is the local name given to the river for its entire length in Bangladesh to the Ganges junction (Fig. 1). The Brahmaputra-Jamuna has one principal tributary input, the Teesta River in the north-west, and two major offtakes on the left bank that are the Old Brahmaputra (see below) and the Dhaleswari (Fig. 1). The Brahmaputra/Jamuna River contributes ~51% of the water discharge and 38% of the sediment yield to the Padma (Schumm and Winkley, 1994), with the sediment yield being estimated at 590 MT yr-1 and the sand fraction contributing 34% of this total (Sarker, 1996). The Jamuna can have a braidplain width up to 15 km in flood and scour depths of up to 40 m have been recorded (Klaassen and Vermeer, 1988). Thus, by any definition, the Brahmaputra-Jamuna is one of the world's truly great rivers, and has a direct and daily influence on the prosperity of its population and the country's economic growth and political stability. Since the seminal work of Coleman (1969), much research has been conducted on these rivers, especially in the last fifteen years as part of the Bangladesh Flood Action Plan (e.g. Thorne and Thiagarajah, 1994; Haggart et al., 1994; FAP24, 1996a-h; Paul, 1997), together with work from organisations such as the Center for Environmental and Geographic Information Services (CEGIS) and Water Resources Planning Organization (WARPO) in Bangladesh, and this has allowed a dramatic increase in our knowledge of the behaviour of the Brahmaputra-Jamuna River. This work, aided by major advances in monitoring techniques, such as frequent, all-year satellite imagery and whole-flow depth monitoring within the main channels at even the highest discharges, has resulted in the river being characterised in more detail than ever before. Recent attempts to predict morphological change have also met with some success (EGIS, 2002; CEGIS, 2003; Mosselman, in press), and together with numerical modelling (Jagers, 2003) offer some hope to both understand and predict river channel movement, and establishing management plans for such change. Additionally, development of large-scale infrastructure within Bangladesh, such as construction of the Bangabandhu Multipurpose Bridge across the Jamuna, and numerous bank-protection works (Mosselman, in press), has demanded an increased quantification of the alluvial channel processes and prediction of future channel change. This chapter seeks to provide a synthesis on aspects of the geomorphology and sedimentology of the Brahmaputra-Jamuna River within Bangladesh, between the northern Bangladesh border and its junction with the Ganges some 240 km to the south (Fig. 1), and examines issues of applied geomorphology in response to the flooding and migration of this huge and largely untamed river. Details of the Brahmaputra River upstream of the Bangladesh border are given by Singh et al. (chapter X , this volume), and a recent synthesis on the water resources of the Brahmaputra basin in India is provided by Singh et al. (2004), including a chapter on fluvial geomorphology by Bora (2004).