Brahmaputra River System
The Brahmaputra (/ˌbrɑːməˈpʊtrə/), also known as the Yarlung Tsangpo in Tibet, the Siang/Dihang River in Arunachal Pradesh, and Luit in Assamese, is a trans-boundary river which flows through Tibet, India, and Bangladesh. It is the 9th largest river in the world by discharge, and the 15th longest.
From Sanskrit ब्रह्मपुत्र (brahmaputra, “son of Brahma”)
The Brahmaputra River originates in a great glacier mass in Kailas range of the Himalayas (elevation 5300 m) and flows through China, India and Bangladesh for a total distance of 2880 km before emptying into the Bay of Bengal jointly with the Ganges (Figure 1). It drains a combined international watershed of approximately 580,000 sq. km located in China, India and Bangladesh. It is the fourth largest river in the world in terms of average discharge at the mouth and second only to the Yellow River in China in quantities of sediment transported per unit drainage area.
In India, the Brahmaputra River flows southerly and westerly through the states of Arunachal Pradesh and Assam over a distance of approximately 800 km. In the Himalayan range, before entering India, the river is known as the Tsangpo River flowing west to east, then south through the eastern Himalayas as the Dihang River. In Assam, the Dihang River is joined by other tributaries to form the main stem of the Brahmaputra. Near the western boundary of Assam, the river turns south to enter Bangladesh changing its name to Jamuna till its confluence with the Ganges from where both the Jamuna and Ganges form the Padma flowing into the Bay of Bengal. The total length of the river in Bangladesh is approximately 240 km.
The river has a gradient of 0.09 to 0.17 m/km near Dibrugarh, Assam at the head of the Valley and it reduces to about 0.1 m/km near Guwahati. Through Assam, the long-term average discharge increases from 8,500 to 17,000 cubic meters per second as flows are augmented by inflows from several large tributaries. The width of the river varies from one km on an individual channel to as much as 10 km in some reaches with multiple braided channels.
An example of the typical braided channels of the Brahmaputra near Dibrugarh, Assam
Almost through its entire length in Assam, the river has three to six channels separated by islands and mid-channel bars under low flow conditions. These bars and islands become submerged during major floods. The pattern of the channels changes frequently under flood conditions accompanied by extensive erosion of banks and disposition of sediment forming sand bars.
The valley and its adjoining highlands constitute an extremely unstable seismic region. Two geological factors are ruling in determining the morphological character of the river in Assam. Firstly, the Himalayan ranges to the north are uplifting at a rate of the order of one meter per century. Secondly, the whole region is subject to frequent seismic movements and periodic major earthquakes.
The recent earthquakes of 1897 and 1950 in Assam, both of magnitude 8.7 on the Richter scale, were among the severest within historical experience anywhere in the world. The earthquake of 1950 caused widespread floods and erosions seriously across the entire territory of Assam.
The Problem: Flood
Typical of the subtropical humid climate with abundant orographic precipitation from southwesterly monsoon, the Brahmaputra watershed is susceptible to rainfall of large intensity and duration leading to frequent floods. Historically, the causes of the flooding have been from high flows in the Brahmaputra River, synchronized with high flows from many tributaries, and from heavy rainfall on the floodplains combined with insufficient slope in the low lying areas to drain by gravity to the river.
The frequency and magnitude of floods have increased in recent years. Inadequate flood control measures with haphazardly placed embankments in the entire network of rivers and tributaries led to raising of river beds and subsequent reduction of the flood conveyance capacity of the rivers. The average area flooded during the last decade is about 20,000 square km or about 25% of the land area of Assam. The social disruption and costs associated with flooding are monumental.
The Problem: Riverbank Erosion
The unique fluvial processes of erosion-sedimentation and transport of sediments leading to the formation and recession of riverbanks of the Brahmaputra system are unique. The fine sand and silty nature of the bank material with high moisture contents and unstable bank lines create a highly favorable environment to induce shear failures of the upper bank material. Undercutting of the upper bank material by strong river currents during the high flows or by over steepening of the bank slopes produce cantilever blocks leading to eventual bank failure. Due to braided channel characteristics, the main stem of the river transforms to a number of channels and sand bars which change their locations and sizes each year.
During floods, as the change of river hydraulics (depth, velocity, and shear stress) induces variable sediment transport rates and erosive forces, the channel starts shifting at the vulnerable reaches. The saga of riverbank erosion has devoured over 4000 square km land area of Assam in five decades at a rate of 80 square km per year (WRD, 2008). The pace of bank erosion in Majuli island accelerated after the earthquake of 1950. The rates of progression of riverbank erosion in the Island during three periods (1917-72, 1972-96 and 1996-2001) of the last century have been estimated to be 1.77, 1.84 and 6.42 sq. km per year (Sarma and Phukan, 2006). The human miseries associated with such large rate of annual landmass losses of homes, croplands, public infrastructures and natural resources of the island continue to create havoc to the economy of the region.