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Do earthquakes trigger floods?
By Rina Mukherji



Experts say a direct link exists between earthquakes, landslides and the severity of floods experienced in the Brahmaputra basin.

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Great bend of Brahmaputra as it flows down

Recent earthquake that left Sikkim, Nepal and a large part of northeastern India and north Bengal devastated, has led to demands that plans for hydropower projects in India’s northeast should be revisited. Especially since, the Government of India is keen to harness the hydropower potential of the northeast with plans for building 70 big dams on the various tributaries of the Brahmaputra.

The Brahmaputra basin lies in a highly seismic zone, (classified as Zone V in terms of seismicity). Several large tributaries drain themselves into the river, making it one of the biggest in terms of the volume of water carried.

Climatologist Dr Partha Jyoti Das, Programme Head, Water, Climate and Hazard Programme of Aaranyak, has already pointed out a direct link between earthquakes, landslides and the severity of floods experienced in the Brahmaputra basin.

If one dwells on the history of earthquakes in Assam /Tibet region through which the Brahmaputra flows, and eventually enters Bangladesh, one finds that there is a direct correlation between the two major types of natural disasters that plague the region.

The earliest recorded history of a devastating earthquake goes as far back as 1897, when an earthquake of intensity 8.1 on the Richter scale, with its epicenter on the Shillong plateau caused widespread destruction all over the Assam and northeastern parts of the then British Raj India.

The impact of the earthquake and resulting landslides added to the sediment load in the Brahmaputra and so raised the river bed that in the years since then, floods have become an annual feature every monsoon all along the Brahmaputra basin.

In 1950, an earthquake with its origins in Rima in Tibet resulted in landslides and earthquakes in what is now Arunachal Pradesh. The landslides blocked the Subanseri river, which is a major tributary of the Brahmaputra. This natural block broke off eight days later, and resulted in huge waves that flooded many districts of Assam. The earthquake, which was calculated to be 8.6 on the Richter scale, was of such incredible proportions that tremors were felt as far off as the fjords of Norway in Europe.

The impact of the earthquake and resulting landslides added to the sediment load in the Brahmaputra and so raised the river bed that in the years since then, floods have become an annual feature every monsoon all along the Brahmaputra basin. The increased sediment load and the raised river bed is evident in the widening of the river and changes in its flow over the years. The lowest water level, recorded as 320 feet in 1940-50 rose to 332 feet in 1980. This points to the river getting wider due to a shallow basin, and heavy sediment load, as Dr Das mentions. Studies conducted in the past by several experts show that the sediment load in the Brahmaputra steadily increases from 100 metric tonnes per square km in Arunachal Pradesh to 804 metric tonnes per square km in Assam, to 1128 metric tonnes per square km in Bangladesh.

The problem is compounded by the fact that the Brahmaputra, in terms of sediment load, ranks the second highest of the Himalayan rivers (next only to the Kosi). Besides, the Brahmaputra, known as the Yarlung Zangbo in Tibet, experiences a great fall as it moves out of the 3500 metre high Tsela Dzong to enter the plains at the 155 metre Pasighat in Assam. The drop in height is also responsible for the large sediment load in the river.

Shripad Dharmadhikary who has been studying big dams in South Asia, also warns of the formation of “quake dams” – temporary dams created from earthquake loosened debris – which could pose risks of catastrophic failure, when they eventually give way. “The floods resulting from such dams could have a cascading impact on the man-made dams with disastrous results”, he says.


The Brahmaputra as it flows down from Tibet at an average 4000 metres
height following the suture line betwen the Eurasian and Indian plate

The effect of sediment and debris on floods downstream is borne out by recent history, when a landslide in the Zhamulongba stream caused 300 million cubic metres of displaced debris, soil and ice to dam the Yilongzangbo river, which is a tributary of the Yarlung Zangbo east of Tibet. This caused flash floods in Siang in Arunachal Pradesh, destroying property worth a billion rupees, with several deaths and people missing. The Dhemaji district of Assam was also heavily affected by this natural disaster.

All over the northeast, 65 per cent of the annual rain is received during the monsoon. With global warming and climate change, the number of rainy days has dropped, although the total rainfall has remained stable. At the same time, the number of extreme events of rainfall has increased severalfold. This is borne out by data recorded in Cherrapunji in Meghalaya, one of the wettest places on earth, in connection with a study done by the Polish Academy of Sciences a few years ago. This means that rains have become heavier, and in parts, more localized.

Dams and reservoirs are known to increase seismicity all over the world. In India, the Koyna dam in Maharashtra, the Tehri Dam in Uttarakhand, and many others triggered earthquakes that left thousands dead.

Similarly, stormy weather has become the norm in Bangladesh and Assam during the summer months. Heavy localized monsoon rains, and a heavy sediment load is hence, a recipe of disaster where the Brahmaputra basin is concerned. The regular floods in Assam and in downstream Bangladesh, have come to mean untold hardships to the people in these parts, with the loss of lives and property every year. A recently-conducted International Centre for Integrated Mountain Development (ICIMOD ) study on vulnerability assessment found Assam to have the largest flood-prone area in the country, that is, 3.2 million hectare, or 40 per cent of its total area. Every year, 2000 villages get affected with crop damages estimated at Rs 2500 million affecting 3 million people.

With global warming and climate change, the number of rainy days has dropped, although the total rainfall has remained stable. This is borne out by data recorded in Cherrapunji in Meghalaya, one of the wettest places on earth. This means that rains have become heavier, and in parts, more localized. According to the Ministry of Environment and Forests (MOEF)’s Indian Network of Climate Change Assessment (INCCA) report, the northeast is projected to have the highest rise in temperature. In fact, the rise is in the region of 1.8 degrees centigrade to 2.1 degrees centigrade with respect to the ‘70s. The minimum temperature is projected to rise between 1 to 2.5 degree centigrade and the maximum between 1 to 3.5 degree centigrade. The intensity of rainfall is projected to increase 1 to 6 mm per day, but the number of rainy days is expected to decrease by 1 to 10 days. Similarly, stormy weather has become the norm in Bangladesh and Assam during the summer months. This, of course, could be seen as a direct consequence of the increased annual mean temperatures all over India’s northeast, which results in increased precipitation. The 0.6 degree to 1.8 degree centigrade increase in annual mean temperature in the Brahmaputra valley has had far-reaching consequences, with floods and inundation impoverishing farming communities in these parts during the past decade.

Dams and reservoirs are known to increase seismicity all over the world. In India, the Koyna dam in Maharashtra, the Tehri Dam in Uttarakhand, and many others triggered earthquakes that left thousands dead. Going by the evidence presented above, every earthquake –especially in the Himalayan region- leaves us struggling to cope with landslides that only add to the sediment load in our rivers, causing flash floods all over. This means coming to terms with not one, but two natural disasters.

In view of the above, isn’t it time that we revise our plans to build dams to generate hydropower all over our northeast, and save our farmlands and people?

 
Disclaimer:
The views expressed above are personal and do not necessarily reflect the views of d-sector editorial team.
 

Rina Mukherji  |  rina.mukherji@gmail.com

Rina Mukherji is a writer, poet and journalist. She is a keen commentator on issues concerning marginalized communities, environment and development. She is based in Kolkata.

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