Assam Flood, Hill Erosions and Downstream Silt Hazards … Let's Work towards a Lasting Solution!!

I was pleasantly surprised to find a report saying that Majuli, the world’s biggest river island has reclaimed around 22.08 sq. km. land during the period from 2004 to 2016. Satellite Imageries confirm, from 1988 to 2015, total deposition in Brahmaputra Valley is 208 sq. km. against erosion of 798 sq. km. Frequently, we hear about bank erosion, but not about regaining land. Some part of this deposition is through natural processes and the remaining is due to ongoing efforts of government agencies namely Brahmaputra Board and Water Resources Department of Govt. of Assam, achieved with the help of implementation of anti-erosion and bank protection measures by deploying permeable RCC porcupine screens/spurs/dampeners etc. Though, the reclaimed land cannot be used immediately for agriculture purposes, as it’ll take time to redevelop the topsoil, however, it is heartening to know about regaining the lost land by redeposition.

It’s widely accepted belief that the flood in Brahmaputra Valley cannot be controlled entirely, and solutions offered by different experts are largely divided. There is an urgent need of prioritizing our approach, a workable consensus to move ahead for a lasting solution. Brahmaputra is third largest river in the world in terms of its annual discharge, therefore, any measures towards flood mitigation is bound to be cost intensive.?

To understand the flooding of Brahmaputra valley, we must know the weather condition and its influence on the river system. Brahmaputra starts as snow fed river from Angsi Glacier and travels 1625 km through Tibet prior to entering Arunachal Pradesh. Tibetan plateau is very dry and devoid of rainfall due to barriers created by the mountain ranges. However, south slope of the mountains of Himalayas falling in Arunachal Pradesh and Bhutan experience considerable rainfall and are part of Indian subcontinent climate system. Total length of Brahmaputra in India is 918 km, out of which 278 km in Arunachal Pradesh and 640 km in Assam. Brahmaputra has 26 major tributaries on the north bank and 13 tributaries in south bank.?

Amongst the north bank tributaries, Subansiri starts from Tibet, Jiabharali from Arunachal Pradesh and Bhutan and Manas-Beki-Aie from Bhutan. These are longest rivers of north bank and contribute maximum amount of silt deposits compared to south bank tributaries.

Brahmaputra carries highest silt load amongst river of its size. Question is where from these silts originated?

Sand, silt and clay, the fine sediments, are the final product in a sedimentation cycle, known as alluvium. It starts with weathering at the source area, then transportation and ultimately deposition.

Let me explain in detail…...?

Weathering is the process of breaking down (physical) or dissolving (chemical) of rocks of any type. Means these rocks may be igneous, metamorphic or earlier formed sedimentary rocks present on the earth’s surface. In icy region, physical weathering is more pronounced due to higher intensity of expansion and contraction of minerals within rocks. The process accelerated in the presence of rain waters. As we know, water when becomes ice, its volume increases by about 10%. Thus, the ice in the cracks of a rock tend to expand and creates bigger fractures. Simultaneously, chemical weathering follows as many minerals are susceptible to chemical reaction. Physical and chemical weathering also have a biological component.?

Simplistic explanation is - a boulder is broken off of a mountain, then, the boulder is gradually weathered down to a cobble, pebble, gravel during erosion and transportation and ultimately settles into sand, silt and clay along the course of the river. Here, clay particles are finest and deposited farthest from the source and carried up to the river mouth and ultimately into the sea. For ease of explanation, deposition of sand and silt together we can refer as silting.

It is clear that the weathering and first erosion occurs exclusively in hills and mountains, and in case of north bank tributaries along with Dibang (Siang), Dihang and Lohit rivers, hills within Arunachal Pradesh, Bhutan and Tibet are their source of origin or more precisely, provenance of the sediments. The ongoing erosions were farther aggravated due to frequent occurrence of earthquakes in the region. Earthquakes of 1897 (8.0 Magnitude) and 1950 (8.6 Magnitude) and many more minor earthquakes indicate the Brahmaputra Valley, and its surrounding areas fall within tectonically active zone and disturbances occur due to continental plates colliding at two converging plate boundaries, namely Indian Plate and Eurasian Plate. Upliftment of underlying beds are a continuous process, though severity may vary. ?Even Himalayas is rising 5 mm per year.?

Transportation and silting (deposition) - how different sized rock particles, after weathering and erosion, transported by a river system:

During transportation by a river system, the weathered and eroded rock particles gradually break into smaller sizes.??Large size boulders and pebbles are rolled along the riverbed, the process called Traction; next is Saltation where small size pebbles, cobbles and gravels are bounced along the riverbed. Smallest particles like silt and clay are carried by Suspension. Also, minerals which are dissolved in water are transported through Solution.

The energy in a river determines the capacity of a river. Higher the energy of the river water, greater the transportation capacity and carries the weathered particles farthest. Again, energy is responsible for erosion too. Erosion makes the riverbeds and banks deeper, wider and longer. V-shaped vertical erosion makes a channel deeper in the hilly section or upper stage of the river. Lateral erosion makes the river wider and can be seen towards the middle and final stages. When a river loss its energy, the deposition starts.

The north bank tributaries are high in energy as they flow down from high altitudes. Slope or gradients of these rivers are also high and some of them are fed by snow peaked mountains of Himalayas.?The steeper the river’s gradient, the faster the flow of water. Heavy rainfall in the area also contributes towards higher discharge.

Now we could understand where from these significant amounts of silt, also can be called as bed load, have been deposited in the Brahmaputra River system. This siltation led to braided nature of Brahmaputra. The upliftment (shallowing) of Brahmaputra riverbed is also a contributing factor towards severe flooding in the valley.

To break the intensity of these siltation or to reduce the kinetic energy of the flowing rivers, we have to create barriers towards the upstream section of Brahmaputra and its tributaries. The discharge, gradient and average velocity all determine the amount of kinetic energy the river possesses at a certain point. As a first step, the high energy north bank tributaries along with Dihang (Siang), Dibang and Lohit must be taken up on urgent basis.

Government of India has approved 49 Master Plans submitted by Brahmaputra Board till date. These Master Plans are prepared for controlling flood in Brahmaputra and its tributaries along with arresting bank erosions and to improve flow of river waters. Also, these plans will help in effective utilization of available water resources of Brahmaputra Valley like irrigation, electricity generation, use of water ways for trade and commerce.?

Present engagement is more towards implementation of “Drainage Development Schemes”, which are part of above plans. These short-term measures are very much required, as they take care of raising and strengthening of embankments, construction of sluices, re-sectioning of channels and construction of new embankments. Generally speaking, existing embankments are more than 40 years old and poorly maintained. It’s like “plugging the holes” - you plug one here, there will be couple of new holes nearby. One needs to understand, we have to implement the long-term measures, like arresting the origin of siltation process at the source. Otherwise, same things are going to happen again and again, like for last so many decades.

Dredging can be a part of the solution, but it is not the only one! Dredging will be required routinely for removal of deposits from riverbeds, reclamation work and clearing the channels and water ways. The primary challenge is how to reduce the siltation.

The aforesaid Master Plans talk about “Storage Reservoirs”, which is also known as “Flood Control Reservoirs”, to be constructed on the upstream sections. However, the status of their implementation is not known.

To break the energy of aforesaid hill rivers, similar approach needs to be adopted like installation of RCC barriers / porcupine screens, etc. If they can be successful in Majuli and other reclamation projects in low slope areas, why not along the course of the rivers on the upstream sections. I think construction of multipurpose dams in Manas-Beki-Aei, Dihang (Siang) are also in the approved plan, but progress seems to be slow, and status is unknown.

The long-term plans are cost intensive, however, for a permanent solution, they need to be implemented soon.?It’s time to build a common consensus, prioritize the river projects to be undertaken first and move ahead with definitive plan giving priority to long term solution to Assam floods.