The Brahmaputra features among the world’s top five rivers in terms of discharge as well as the sediment it brings. At 19,830 cubic meters per second (cumec), it ranks fourth in discharge at the mouth, behind only the Amazon (99,150 cumec), the Congo (39,660 cumec) and the Yangtze (21,800 cumec).

In terms of sediment yield, two spots along the Brahmaputa’s course were at second and third places in 2008, behind the Yellow River whose annual sediment yield is 1,403 tonnes per sq km. The Brahmaputra’s annual sediment yield was 1,128 tonnes per sq km at Bahadurabad of Bangladesh, and 804 tonnes per sq km at Pandu of Guwahati.

How do the characteristics of the river Brahmaputra relate to flooding?

The vast amount of sediment comes from Tibet, where the river originates. That region is cold, arid and lacks plantation. Glaciers melt, soil erodes and all of it results in a highly sedimented river.

By the time the river enters Assam — a state comprising primarily floodplains surrounded by hills on all sides — it deposits vast amounts of this silt, leading to erosion and floods.

As the river comes from a high slope to a flat plain, its velocity decreases suddenly and this results in the river unloading the sediment. The river’s channels prove inadequate amid this siltation, leading to floods.

Again, because of the earthquake-prone nature of the region, the river has not been able to acquire a stable character. Following the devastating earthquake of 1950, the level of the Brahmaputra rose by two metres in Dibrugarh area in eastern Assam.

Besides these natural factors are the man-made ones — habitation, deforestation, population growth in catchment areas (including in China) — which lead to higher sedimentation. For example, the sediment deposition itself creates temporary sandbars or river islands.

It is common for people to settle in such places, which restricts the space the river has to flow. When rainfall is heavy, it combines with all these factors and leads to destructive floods. This happens very frequently.

Has the government tried to address the factors that cause floods?

In its master plan on the river in 1982, the Brahmaputra Board had suggested that dams and reservoirs be built to mitigate floods. The idea of dams, however, has traditionally been a double-edged sword.

While one of their objectives is to regulate the release of flood waters, the release when it comes can sometimes be beyond the capacity of the channels downstream.

In the Brahmaputra basin, locals and environmentalists protested against dam-building plans on grounds of displacement and destruction of ecology, preventing the plans from moving forward.

The Brahmaputra Board proposed a multipurpose dam in the present Gerukamukh site where NHPC (National Hydroelectric Power Corporation) is constructing the Subansiri Lower Hydroelectric Project.

But the proposal was scrapped because of objections by the Arunachal government due to submergence [concerns], which included a few small towns as well. In fact, the Subansiri project — on which the government spent crores — too has been suspended.

As such, the government has been using only one approach towards floods: building embankments on the river. However embankments were proposed only as an interim and ad hoc measure for short-term mitigation. Their lack of durability has often been on display.

Most embankments built in the 1980s are not strong enough. Since they were temporary measures, the government did not spend on high-specification embankments. These are weak and are regularly breached.

The government also considered dredging, basically digging up the riverbed and making the river “deeper”. However, experts have strongly advised against this simply because the Brahmaputra sediment yield is among the highest in the world.

Experts believe that even if we take out all the silt this year, more silt will be deposited the following year, making the very expensive effort futile.

Thus, the government’s measures have been “piecemeal” and “short-term”.

Is there a long-term solution?

Basin-wide approach:

For a sustainable solution, there needs to be “a basin-wide approach” to the problem. An “integrated basin management” system that should ideally bring in all the basin-sharing countries on board.

Addressing the issues only in Assam when the flood strikes isn’t the solution — one needs the countries to come to an understanding about taking measures in the catchment areas. For that, interstate relationships, political cooperation and the role of the government are important.

Flood-plain zoning:

Depending on the vulnerability of the area, we divide them into categories, and accordingly ban certain activities on it: like farming, building a house etc.