The North-eastern Indian state of Assam is naturally the most flood-prone state of the country as the monsoon winds
blowing from Bay of Bengal hits the sub-Himalayan hills of Arunachal Pradesh to its north lashing the plains with heavy
precipitation during the summer. According to the Rastriya Barh Ayog (RBA)—the National Flood Commission, 31.05 lakh
hectares of the total 78.523 lakh hectares area of the state is prone to frequent floods. Secondly, the Brahmaputra—the
lifeline of the state originates from a glacier at Tibet that carry additional amount of water due to snow-melt in the
summer makes the entire valley flooded. There are reasons for the annual havoc of flood and river bank erosion affecting
Assam annually, but one more phenomenon has been troubling the region. This is the increasing air pollution felt in the
area in last few years.
The Brahmaputra factor:
The incessant rainfall during the monsoon and the very nature of the river Brahmaputra which is very dynamic as well as
unstable at the same time are at the crux of the problem of flood in Assam. On its 580,000 sq km basin spreading 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, the Congo and the Yangtze (Goswami, 2008).
By the time the river enters Assam it deposits vast amounts of this silt, leading to erosion and floods in the plains
surrounded by hills. The Brahmaputra losses its velocity suddenly as it falls to a flat plain from a high slope making
it to unload the sediments. The river’s channels prove inadequate amid this siltation, leading to floods. The massive
earthquake of 1950 left the river to acquire a stable character rising its bed up considerably (highest two meters in
Dibrugarh).
Moreover man-made factors like habitation, deforestation, population growth in catchment areas (including in China) have
led to higher sedimentation creating temporary sandbars or river islands (locally called Chars) where people have been
settling in in the last five decades or so. The sand bars or Chars restrict the space the river has to flow. As rainfall
becomes heavy, it combines with all these factors and leads to destructive floods—annually.
Air pollution may be behind intense rainfall:
Research conducted by Indian Institute of Technology (IIT), Kanpur found a link between air pollution and intense
rainfall recently. Analysing long-term satellite data between 2002 and 2016 and modelling to understand the association
between aerosols and cloud properties, the study found that air pollution particles lead to change in the physical
properties of monsoon clouds, which can lead to more intense rainfall, flash floods and unusual gaps in the progression
of monsoon rainfall (Nature). “Not all extreme rain events can be linked to aerosols as other meteorological factors
also play a role, but cases of extreme and erratic monsoon rainfall in inland areas affected by air pollution may be
linked to this phenomenon,” says the study.
Aerosols, particularly black carbon and dust particles, induce “cloud invigoration”, which means cloud cover and
thickness increases because of reduction in the size of cloud droplets and other structural modifications during cloud
formation. Enhanced cloudiness reflects more sunlight back to space, leading to a cooling effect of the clouds’ surface.
Apart from cooling, air pollution-induced cloud properties can also be linked to intense rainfall. It also facilitates
thunderstorm-type clouds that have an umbrella type structure. This umbrella structure stops solar radiation and creates
cooling, which causes a delay in the arrival of the next thunderstorm. This leads to episodes of intense rain followed
by breaks in progression, the study say. Aerosols, whose main sources are fossil and bio-fuels combustion from
residential and industrial sector, biomass burning absorb solar radiation, such as soot, cause these disturbances.
Air Pollution Makes Floods Worse
Air pollution can affects health and the World Health Organization estimates that it caused 3.7 million premature deaths
worldwide in 2012 alone. But now another recent study shows that air pollution can trigger floods as well. In July 2013,
heavy rainfall resulted in a catastrophic flood in Sichuan Basin, a mountainous area of China. Nearly 2.5 feet of rain
fell in just five days which led to the findings that air quality played a role in the valley’s disaster. It was found
that in some geographic regions, aerosols absorb heat from the sun, suppressing the chance of rain during the day and
stabilizing the atmosphere. At night the warmed, rising air is transported to mountainous areas before it heads higher
up into the atmosphere. This causes massive precipitation that can actually make flooding worse (Fan, J., et al. (2015).
Air pollution from soot and aerosol emissions is also making cyclones over the Middle East and South Asia more
destructive. Natural differences in wind speed and direction over different heights in the atmosphere, known as 'wind
shear', normally keep cyclones in check—effectively tearing the storms apart before they reach a certain size. But
emissions from sources such as biomass burning and diesel vehicles have interfered with wind patterns, reducing wind
shear and enabling cyclones to grow twice as intense, according to a study published in Nature last week (3 November).
Cyclones that occurred between 1997 and 2010 were up to three times more intense than those between 1979 and 1996. It
may be recalled that five of the strongest storms during the period occurred after 1998. Aerosol emissions in the region
have also grown six-fold since the 1930s, creating a three kilometre-thick layer of pollution over the Indian Ocean,
known as the South Asian atmospheric brown cloud, which absorbs sunlight, causing the ocean to cool and affecting wind
circulation.
Air Pollution and flood in Assam:
According to a report by Pollution Control Board of Assam (PCBA) in 2015, Guwahati has one of the highest Black Carbon
pollution levels in the world. The city’s PM2.5 concentration is more than eight times the WHO standards. Transport and
dust are the primary contributors. It is due to the rapid urbanization and poor environment quality control in Guwahati
which is giving rise to such high Black Carbon levels. Though Assam does not have heavy industries the entire state is
moderately polluted in air. On July 21, 2019, leading doctors of Assam and other North-Eastern States join the movement
for launch of Doctors for Clean Air—a campaign for raise awareness about the increasing air pollution and its health
impacts in the region. Besides health risks, air pollution is the likely cause of abnormal monsoon and heavy rains and
thunderstorms compounding the already existing flood problem of Assam.
In May this year, ahead of the onset of the monsoon at least 23 people lost their lives in storms and lightning strikes.
The worst-hit districts are Golaghat, Sivasagar, Dhubri, Sontipur and Cachar. The two natural disasters have also
affected around 22,801 families in 18 districts of the state in the current year (Assam State Disaster Management
Authority-ASDMA). In May 11, 2018 eight people were killed and several others injured in lightning strikes and
thunderstorms in the state. That time two people died in Karbi Anglong district and six others died in lower Assam’s
Kamrup, Barpeta and Kokrajhar districts (ASDMA). According to ASDMA Project Officer Kripal Jyoti Mazumdar 64 people died
due to lightning and storms till May that year. The spate of severe thunderstorms in these two early summers may be
attributable to such aerosol-induced changes as found by IIT-Kanpur’s study and those in Sichuan Basin, China.
Pollutants reduction for flood mitigation:
The studies on atmospheric simulations in Sichuan Basin show that if current industrial emissions are reduced to a level
before the China’s economic boom, less precipitation (up to 60% lower) could be predicted. This is due to weakening of
the “aerosol-enhanced conditional instability” mechanism. This suggests that air pollutants reduction can effectively
mitigated floods in that area. This discovery is also called “aerosol-enhanced conditional instability,” and its
flipside is that reducing pollution in places like Sichuan can actually reduce floods. As pollution particles worsen
flooding, it could help future weather forecasters predict floods.
In Guwahati industrial emission contributes only 5.29% of PM2.5 concentration in the air. The major contributor has been
the transport (36.5%) and dust (27%). To reduce the presence of aerosols in the atmosphere vehicular emissions require a
considerable control. A new and effective vehicle policy is very essential to mitigate the increased threat from flood
caused by air pollution. Green technology in the transport sector, mostly the adoption of electric vehicles should be
seriously considered to control the aerosol induced precipitation but also to make Assam a climate resilient state.
Opting to public transport from private vehicles and shifting of public transport to electric power from fossil fuel
could contribute effectively in achieving the desired goals to mitigate flood and climate change. (Farhana Ahmed is a journalist based in Assam, India. E-mail: farhana.ahmed777@gmail.com)