More research about relationship between flash floods and climate change needed


Abu Siddique

  • In northeastern Bangladesh, an early start to the monsoon and unusually heavy rains have caused massive flooding.
  • The Meghna River Basin is accustomed to these flash floods, but the scale of the disaster this year has been compounded by human encroachment and development in the watershed region, said M. Monirul Qader Mirza, a water management expert.
  • In an interview with Mongabay, Mirza emphasised the need for infrastructure planning to consider river and rainfall dynamics to mitigate flood risk, and to have an early-warning system in place to minimise damage.
  • Mirza also said that identifying the role of climate change in the problem is complex and requires extensive studies and modelling, but added it’s indisputable that rainfall patterns have become increasingly erratic.

A large part of the Ganges-Brahmaputra-Meghna Basin makes up the most of Bangladesh’s land. The northeastern part of the country, which the Meghna Basin covers, is currently experiencing highly destructive flash floods. Unusually heavy rains during the early monsoon have left around 4 million people stranded in the floodwaters.

Human encroachment and development in the watershed has compounded the problem, blocking the quick runoff of excess water.

M. Monirul Qader Mirza, a water management expert and adjunct professor at the University of Toronto, Scarborough, attributes the problem to a range of factors, from extreme rainfall to siltation of waterways as a result of mining. Mirza has written extensively on flood issues in his native Bangladesh and the wider South Asia region. His Ph.D. focused on modelling the effects of climate change on flooding in Bangladesh, and he has also contributed to several assessments of the Intergovernmental Panel on Climate Change (IPCC) as an author.

Mongabay’s Abu Siddique spoke with Mirza recently about how the monsoon patterns have changed over the years, why attempts to “control” the sheer volume of water in the three rivers are misguided, and to what extent climate change plays a role in all of this. The following interview has been lightly edited for clarity and style.

Mongabay: Is the ongoing flood in northeastern Bangladesh different from other times?

M. Monirul Qader Mirza: Yes, it is.

Let’s talk about the arrival of the monsoon. It usually arrives on the Kerala coast [in southwest India] on June 1, with seven days of standard deviation, and the average monsoon arrival date in the Meghna Basin of Bangladesh and India is June 5.

This year, the monsoon arrived five days early on the Kerala coast, on May 27, and it hit the Meghna Basin on June 3. Then the Indian part of the Meghna-Barak Valley Basin, especially in the Shillong Plateau, experienced a significant amount of rainfall.

It takes six to seven hours for the flow to reach downstream in Bangladesh from upstream Meghalaya state [in India]. In the city of Cherrapunji [in Meghalaya], on June 15, 24 hours of rainfall was recorded at around 972 millimeters [38 inches]. Cherrapunji annually receives over 11,359 mm [447 in] of rainfall. So, around 10% of the rainfall occurred in a single day.

A significant amount of rainfall occurred for more than two weeks in the first fortnight of June in the basin spread over India and Bangladesh, and the antecedent condition from another minor flash flood in May this year aggravated the flooding condition.

If the flood occurred during the harvesting season in April and May, the damage would be more. But in terms of infrastructure, it has inflicted significant damage. The enormity of the flood caused an unbelievable magnitude of misery to the people living in the region.

As it is, the average precipitation in the Meghna Basin is between 2,600 and 3,700 mm [102-146 in], which is much higher than the other two large river basins, the Ganges and the Brahmaputra. This is attributed to the topographic features of the northeastern part of India. The moisture-laden monsoon air rushes up the hills and mountains of Meghalaya and then cools down quickly, which causes a significant amount of rainfall in Meghalaya and adjoining areas of Sylhet [in Bangladesh].

For example, the world’s highest precipitation was recorded in Cherrapunji, on the south slope of the Meghalaya plateau, which is 1,200 meters [3,900 feet] above the Bay of Bengal. Such a huge amount of precipitation in the Meghna Basin triggers severe flash floods in the northeastern region of Bangladesh. Also, the monsoon continues in the Meghna Basin until October 15, which is the longest among the river basins.

The Meghna Basin usually has two types of floods: flash floods and riverine floods. Flash floods occur between mid-April-May and early June and cause significant damage to agricultural production, especially the boro season rice crop, which is almost ready for harvest at that point.

Riverine floods usually occur between August and September with somewhat similar characteristics to the Brahmaputra Basin. Problem arise when the floods peak in the two rivers at the same time. If peak arrives in all three major concurrently, it has a huge impact on the flood plains and the life and livelihood of the entire basin.

Mongabay: This flood occurred in the monsoon season but you call it a flash flood. Could you please explain why?

M. Monirul Qader Mirza: Riverine flooding takes place when the water moves at a consistent pace, and the rise and fall in water level occurs over 10 to 20 days, or more. On the other hand, flash floods are characterised by a sharp rise of water level in the rivers and high flow velocity. Its occurrence is quick, and impacts are huge.

Mongabay: Can we attribute this flood to the impacts of climate change?

M. Monirul Qader Mirza: With each degree of global warming, the moisture-holding capacity of the atmosphere increases by 7%. However, an increase in moisture-holding capacity does not necessarily translate to extreme rainfall. There are other conditions involved. However, in general, the IPCC’s latest report indicates a potential increase in extreme precipitation events due to increased warming.
The type of flood that occurred in June this year in the northeast is usually a flash flood. The timing of occurrence and duration was very short. Attributing this type of phenomenon to climate change is rather complex. Unless research is done with a climate model, with natural and greenhouse forcing, and then creating the atmospheric conditions of that region during that time, it is difficult to assess if there is any climate change impact to this kind flooding episode.

But one thing is noticeable: this kind of episode of rainfall is becoming erratic. Hence we need to find out through research whether there is a relation to climate change.

Mongabay: Is such an amount of rainfall in northeastern Bangladesh unusual? What does the historical data say?

M. Monirul Qader Mirza: Yes, this amount of rainfall is unusual.

India publishes long-term rainfall data, both monsoon and annual. According to their data, overall monsoon rainfall in India has decreased slightly between 1971 and 2020. The southwest monsoon rainfall of all of India is 869 mm [34.2 in]; it was 881 mm [34.7 in] between 1961 and 2010. This means that in the last 10 years, India experienced less monsoon rainfall.

That does not mean that extreme precipitation has decreased. This year’s extreme precipitation is the effect of La Niña, which is the cold phase of the El Niño–Southern Oscillation. As per the projection of Columbia University, this year’s La Niña is moderate, and it could continue until the early next year. In the past, La Niña was attributed to the larger flood of 1998, which inundated some 70% of Bangladesh. It continued for 68 days, the longest in the country’s history.

The India Meteorological Department has already forecast that monsoon precipitation in India could increase by 3% of the long-term average. The positive side to this is that Indian agriculture is by and large dependent on the monsoon, as 60% of the agriculture land is rain-fed. So they are expecting a bumper harvest of some of the crops.

If there is no flood in the Ganges and Brahmaputra basins this year, Bangladesh’s agriculture will also benefit. If there is a flood in Bihar, West Bengal and Assam states [in India], Bangladesh will be a loser due to its geographical location and water draining systems and mechanisms.

As this is just the beginning of the monsoon, we have to wait and see whether the other two large river basins experience floods or not.

Mongabay: Are the haor wetlands in northeastern Bangladesh losing the depth necessary to carry the high volume of water during the monsoon?

M. Monirul Qader Mirza: Yes, because of siltation of the riverbed, as a result of which, the rivers and channels are losing their capacity to drain water. The other issue that needs serious attention is excessive sand mining. It facilitates erosion of the riverbanks and adds soil to the riverbeds. Upstream, Indian sand, coal and limestone mining are also filling up the haor beds.

When the floodwater cannot drain out naturally, it floods the floodplain. Additionally, encroachment has also become a serious problem. A very recent study by Bangladesh’s Institute of Planning and Development estimated that 86% of the haor areas have been filled up in the last 30 years.

Mongabay: Has the recently built physical infrastructure in the haor region expedited the impact of the floods?

M. Monirul Qader Mirza: Development is a continuous process. For large infrastructure in Bangladesh, we have modeling capacity. And they are carried out. But we also have a lot of encroachment taking place.

In the haor basin, some pockets of roads have been built. Before undertaking construction, if proper research is not carried out, it could impede the drainage of floodwater.

I don’t have any updated information on how many roads were built there recently and how they are constructed in the greater Sylhet region. But I believe before taking on any construction process, thorough research should be conducted by taking geographical patterns and river networks into consideration.

Mongabay: In the haors of Bangladesh, instead of maintaining the natural water draining passage, there are now hundreds of sluice gates and other routes for water to pass through. Are these sufficient to drain out floodwater?

M. Monirul Qader Mirza: When engineers plan any infrastructure project, they consider the geography and pattern of floods. However, while ascertaining patterns, the length of historical records is important. When the magnitude of a recent extreme event exceeds anything in the historical records, infrastructure projects built on those records will prove to be inadequate.

There is, however, always scope for revisiting past experiences and design criteria. For example, in developed countries, their design standards are now being updated considering climate change. I strongly believe that Bangladesh should proceed in this direction. For the country, whether it is roads, culverts or bridges, factoring in climate change should be incorporated into the design criteria.

Mongabay: There’s controversy around the newly built road, called the weather road, in the haor basin, which reportedly created an impediment to floodwater passing through. What do you think?

M. Monirul Qader Mirza: I have not seen the road. I cannot make any comment as I do not know the design criteria and what kind of hydrological scenarios were taken into consideration.

Mongabay: Flood control is a popular term in Bangladesh used by government officials and experts. But are floods really controllable? Or are they simply manageable?

M. Monirul Qader Mirza: The magnitude of water arriving from the three large rivers is not controllable.

I have read some stories about how the mayor of Sylhet wishes to construct an embankment in his city. I have also heard of demands for some sort of embankment in Sunamganj. Against the backdrop of the 1988 flood, the government built the Dhaka city embankment to save the city from floodwater, which created waterlogging in some parts of the city. The authorities needed to set up pumps to pump out the water. Learning from the past could guide us in the management of embankments.

The construction of embankments for flood mitigation is common in developed countries. But the geography and water flow characteristics are very different in Bangladesh. The volume of water that passes through these rivers in Bangladesh, as well as in the haor basin, is enormous and is beyond control.

Mongabay: What are your recommendations to avoid such catastrophes in the future?

M. Monirul Qader Mirza: Firstly, northeastern Bangladesh has been experiencing such kinds of floods for a long time. The locals are aware of that. But the magnitude of this year’s flood surprised them. So flood warnings and flood forecasting should become stronger. This could happen with the cooperation of upstream India, which could provide real-time and advanced warning data so that Bangladesh can design proper forecasts and an evacuation process.

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