Explained: Why Assam is prone to floods, and what the solution is

A look at why Assam has traditionally been flood-prone, what measures have been taken over the years, and what has been proposed as a long-term solution.

Assam is in the grip of yet another flood, with 57 lakh people affected across all 33 districts, and 36 people killed besides hundreds of animals. This is the first wave of floods this monsoon, and flood control experts expect at least two more. A look at why Assam has traditionally been flood-prone, what measures have been taken over the years, and what has been proposed as a long-term solution:

Why are floods so destructive in Assam?

Apart from incessant rainfall during the monsoon, there are many contributory factors, natural and man-made. At the crux is the very nature of the river Brahmaputra —dynamic and unstable. Its 580,000 sq km basin spreads over four countries: China, India, Bangladesh and Bhutan, with diverse environments.

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), according to data from a 2008 research paper by retired Gauhati University professor Dulal Chandra Goswami, an environmentalist acknowledged as an authority on the Brahmaputra. 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 these characteristics of the river 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,” said Dhrubajyoti Borgohain, a retired chief engineer of the Brahmaputra Board, a central government body functioning under the Jal Shakti Ministry’s Department of Water Resources, River Development and Ganga Rejuvenation, and whose responsibilities include flood control in the Brahmaputra basin.

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,” said Borgohain. 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.

This article was originally published in The Indian Express in July 2019. Full article here.