Brahmaputra River : Channel processes and sedimentation

Abstract The Ganges and Brahmaputra rivers combined have formed one of the largest deltas in the world, comprising some 23,000 sq. miles. These rivers originate within the Himalayan Mountains and drain an enormous land area before entering East Pakistan. Individually, each of the rivers discharges in excess of 2.5 million cusecs of water during flood, and combined they carry nearly 6 million cusecs of water to the Bay of Bengal, nearly three times the amount borne by the Mississippi River. Having such a large drainage area, the rivers are also heavily charged with sediment, transporting approximately 13 million tons of suspended sediment per day during flood. The large discharge and heavy sediment load cause the rivers to be extremely unstable, and the channels are constantly migrating laterally. Within Recent times both rivers have occupied and abandoned numerous river courses. The Brahmaputra followed a route some 60 miles to the east of its present course only 200 years ago. The long-term patterns of river migration indicate that the Ganges has been migrating eastward, whereas the preferred migration of the Brahmaputra is westward. These movements are obviously controlled by major faults or fractures in the earth's crust. The Brahmaputra River displays a braided pattern in plan view, and short-term channel migration is quite drastic, with rates of movement as high as 2,600 ft. a year being common. The rate of rise and fall of the river, the number and position of major channels active during flood, the formation and movement of large bedforms, cohesion and variability in composition of bank material, and intensity of bank slumping are some of the factors responsible for controlling the bankline configuration and movement. The most significant bankline modifications take place during falling-river stage, when excess sediment is deposited as bars within the channel, causing a change in local flow direction and migration of the thalweg. Studies of bedform patterns and migration indicate that during a single flood cycle the bed configuration undergoes a definite sequence of changes. During low-river stage small bedforms (WH = 1–5 ft.) are present and migrate downstream at an average rate of 400ft./24 h. As discharge increases, the bedforms grow in size (WH = 5–20 ft.), and surface turbulence assumes a regular pattern on the water surface. During maximum-flood period some of the bedforms attain gigantic size, with heights up to 50 ft., and migrate downstream at rates as high as 2,000 ft./24 h. At other positions in the channel a plane bed exists, and surface turbulence patterns become oriented parallel to the channel axis. Although quite different in scale, this sequence is similar to that reported in flume studies. During low-river stage trenches on the various exposed bars revealed the type of bedding formed by migration of the various bedforms. Crossbedding measurements at these stations were recorded and compared to sand body trends. The combined Bengal Basin rivers deliver some 1 billion tons of suspended sediment a year to the Bay of Bengal, yet map and photo comparisons indicate that the shoreline has remained quite stable. Most of the sediment brought to the bay bypasses the bar and continues on into deeper water through a canyon called the Swatch of No Ground. Thus in deep water a subaqueous delta is being formed that dwarfs the subaerial delta, one of the largest in the world.