The entire Brahmaputra valley is at present reeling under a devastating flood. The peak flow of the Brahmaputra during this wave of flood at Pandu was about 39000 cubic meters per sec (cumec). Statistically, this kind of flood is quite common and happens almost every 10 years. The highest ever recorded flow at Pandu is about 72000 cumec. It is difficult to imagine the kind of devastation such a flood may cause today.
Lurking behind this flood is its twin brother – the erosion waiting to strike once the flood recedes. It is even a more serious problem than floods in the Brahmaputra valley. It causes immense damages to life and property. Farmers in the river bank are turned into paupers overnight through a permanent loss of their land.
Unlike floods, which have many benefits as well, such as enriching the soil, recharge of groundwater, increased fish population, etc, erosion has no good side and is feared by everyone living on the river banks of the valley.
Complete and permanent immunity from erosion in Brahmaputra valley is a utopian concept. This is because the entire Brahmaputra valley is formed of river deposits (alluviums) brought down by the river and its tributaries. Changes in a river course through erosion and deposition in its alluvial plain is a natural process and cannot be stopped permanently.
To assess the extent of erosion from 1988 to 2015, a bank line study of the river was carried out from satellite imagery by Brahmaputra Board in 2016. The study revealed that there has been an erosion of 798 sq km against the deposition of only 208 sq km during this period. Even this deposited land has no immediate value. Such land can be utilized for agricultural purposes only after the formation of the topsoil, which generally takes decades.
Comparative bank line studies on the Brahmaputra have also been carried out by other reputed organizations for different periods and in the Master Plan of the Brahmaputra prepared by the Brahmaputra Board from 1930 to 1967. All such studies indicate erosion to be much higher than land formation from the year 1950 onwards in the Brahmaputra valley.
This implies a trend of an increased riverine area and decreased habitable land in the valley. It also indicates an increased instability and higher erosion rate in the river with time which is a worrisome situation.
A total of 2534 villages were obliterated and 90726 families rendered homeless during the same period. Thus, the average erosion in this period by the Brahmaputra has been about 75 sq km per year with 5670 families losing their land annually. The Sadiya town was completely eroded away due to erosion after the 1950 earthquake.
In 1954, a major part of Dibrugarh and Palasbari towns were eroded by the Brahmaputra. The Dibrugarh town could be saved through the intervention of no less a person than the late Jawaharlal Nehru, the then Prime Minister of India, who visited Dibrugarh in 1954.
Severe erosions still continue in a number of locations along the banks of Brahmaputra and many of its tributaries. Large damages to private and public properties including roads, bridges, embankments, buildings, power transportation infrastructures, homesteads, and cultivated land happen every year. In addition, intangible losses in the form of loss of forest areas, loss of daily wages/wasted time due to communication breaches are a common feature in the valley.
Main causes of bank Erosion: Floods and erosion are interrelated. Floods are intricately linked to causes of erosion whereas erosion leads to floods in many cases. However, the basic reason for erosion in the Brahmaputra is due to the fact that it is an extremely braided river.
The characteristic features of a braided river configuration such as the Brahmaputra are -a wide river, unstable and poorly defined banks and shallow depth of water. The braided watercourse consists of a number of entwined channels, divided by islands, which meet, cross and separate again. During floods, the channels are combined, most of the sand bars get submerged and the river appears to be a wide straight unbroken channel.
The main causes which seem to bring about braiding of the Brahmaputra river are:
i. Steep slopes: For a river of its size, the river slope of Brahmaputra is considered by experts as steep. Enormous kinetic energy is generated while this huge mass of water flows. The energy generated is higher when the slope is steeper. To dissipate this energy, the river attacks its banks, erodes and widens. The channels meander between the banks and try to lengthen its path thereby making the slope flatter to attain a regime condition. This leads to the braiding of the river.
ii. Abundant sediment and bedload: Amongst the largest rivers in the world, Brahmaputra carries the highest sediment. Almost the entire quantity of it is carried by the river during the rainy season. The main stem Siang and numerous tributaries bring down a large amount of silt along with heavier bedload particles from the hills.
On entering the plains of the valley, the river slopes flatten immediately resulting in a drastic reduction of the velocity. The river loses kinetic energy and is unable to carry forward the heavy silt load. Initially, it starts depositing the boulders, pebbles and coarse sediments.
As the river moves downstream through the valley, the slope flattens further resulting in the settlement of even finer sediments/silts. The deposited sediments which form the bedload of the river are also gradually carried downstream by the river but at a much slower pace than the lighter suspended sediments.
Whenever there is a large flow, bedload propagation becomes more active. As soon as flow reduces, the bedload settles down which gets deposited on the river bed and blocks the flow. Unable to find its way forward, the flowing water moves in a lateral direction and attacks the banks. In the process, the river either erodes the banks or forms avulsed channels to make its way, thereby further braiding the river.
iii. Erodible banks: Except for a few nodes where the banks on either side are stable, the Brahmaputra banks are not well defined all along its length. The bank materials, other than at these nodes and hard points (rock outcrops) are made of silty or sandy loam in the top layer followed by silty clayey loam or clayey loam in the deeper portions.
These are easily erodible materials. Therefore, the river banks keep shifting due to erosion as well as deposition of sediment. The condition also encourages the formation of distributaries by eroding the high banks (levees) and entering into low lying areas in the countryside thereby braiding the river.
iv. A highly variable discharge (flow per unit time): The discharge of Brahmaputra varies from 10 to 20 times between its lean periods and normal monsoon periods. During the monsoon itself, there is a wide variation of flow. The discharge is much higher during the intermittent waves of floods in this period. Along with the heavy siltload present, this variation of flow leads to the instability and braiding of the river.
Nature adjusts its hydraulic parameters at a given cross-section of a river. It is done in a manner so that the water and sediment load coming from the upstream is transferred to the downstream in the most efficient way with minimum expenditure of energy. Given the above parameters, nature has chosen the braided river plan form for the Brahmaputra river as the most suitable and efficient pattern to execute the transport of its water and sediment.
For the same reason, many of the north bank tributaries of the Brahmaputra originating in the Himalayas such as Lohit, Dibang, Subansiri, Jiabhoreli, Manas, Aie, Sankosh, etc have a highly braided form in the valley. Therefore, all these rivers have serious bank erosion problems. But the south bank tributaries have meandering forms in the valley and are much less problematic from the point of bank erosion. This is because they have a much flatter slope and carries about one-fifth of silt load to that of Himalayan rivers for a unit area of the catchment.