Flooding is a natural phenomenon which occurs every year in the low-lying areas of major rivers. However, we term it a major flood when flood inundation areas exceed more than 20% of the country or its return period is more than 2.33 years, and causes damage to property.
We have seen monsoon (riverine) floods become more frequent and more intense in this decade for the Brahmaputra basin. Four major monsoon floods were observed in the past five years.
In each flood of the last five years, the height of the water level of the peak flood exceeded the previously recorded highest level at Bahadurabad station in the Brahmaputra river.
In the Teesta river at Dalia barrage point, peak flood level was observed as 53.12m at 11am on July 13, which is the highest recorded water level at this station. We also observed severe flooding in the Meghna basin due to heavy rainfall in the Meghalaya, Barrak, and Tripura basins.
The flood this year has some remarkable characteristics considering the major floods in the recent past in the Brahmaputra river basin. The highest flood levels at the Bahadurabad station in the Brahmaputra river in 1988, 2016, 2017, and 2019 were 20.62m, 20.71m, 20.84m, and 21.16m above the PWD datum respectively.
The highest flood level was 20.79m on July 16, which was also greater than that of 1988, 1998, and 2016 floods. The duration of this flood is expected to be more than a month considering the crossing of the danger level on June 26 in Bahadurabad in the Brahmaputra.
This would be the second-longest flood since 1998, when the water level was 63 days above the danger level. However, such types of prolonged floods in the Brahmaputra-Jamuna river never started so early in the past.
This flood would have been more severe if the peak of three rivers -- Ganges, Brahmaputra, and Meghna -- synchronized as in 1998 and 1988. So far, 31% of the area of the country is flooded by the 2020 flood, which could increase further to 34%.
Warning signs
During 1988, 1998, 2007, and 2017 floods, about 61%, 68%, 42%, and 42% of the area of the country were flooded.
Due to many infrastructure developments across the flood plain and sedimentation at the mouth of many distributaries, flood waters of the Brahmputra-Jamuna river are confined within the main channel and floodplains.
This eventually increases the height of the peak flood, but reduces the spreading of the flood.
Several studies have predicted that the intensity and duration of floods in the Ganges, Brahmaputra, and Meghna (GBM) basins will be increased in the future due to climate change.
Due to global warming, a significant increase in heavy rainfall (>100 mm/day) during monsoon is expected along the foothills of the Himalayas.
As more than 93% of the basin area is located outside of our country, changes in the upper catchment will play a big role in flood-flow in the future.
A recent study by BUET also found that the percentage changes in the mean annual sediment load of the Brahmaputra basin compared to the baseline period in the middle and end of the century would be 67% and 115% respectively.
This increased sediment load will reduce the carrying capacity of the river due to siltation, and will also increase the severity of flooding in the future. Both anthropogenic climate change and human activity could be linked with the recent changes in the flooding.
A recent study lead by the University of Oxford in collaboration with Institute of Water and Flood Management of BUET suggested that the climate change signal is not strong relative to natural variability.
Other factors like changes in aerosols would also play a role in the changes in the characteristics of floods. However, we need to carry out event attribution studies to establish links between these extreme events and anthropogenic climate change.
Over the last 50 years, many structural interventions and land-use changes of the upstream catchment have taken place, which has altered floodplain functions and sediment carrying capacity of the river.
Several factors that would contribute to the changes in flood characteristics in Bangladesh are listed below
1. Rapid urbanization has changed the land cover by increasing impervious surfaces. Such land cover changes have prevented the natural infiltration of rainfall into the ground and increased direct runoff
2. Loss of natural storage by illegal encroachment in the wetlands and flood plains helps to increase flood-flow in the river system
3. Construction of embankment prevents floodwater from entering flood plains and also disrupts the role of the flood plain to attenuate flood peak
4. Deforestation and converting natural forests into agricultural land increase soil erosion and sediment load in the river
5. Structural interventions by constructing roads across the flood plains or flood control projects negatively affect the natural drainage system
6. Construction of dam and barrages of the upstream changes in the flood regimes and river morphology in many rivers
7. Failure of embankments change the spatial-temporal patterns of flood inundation.
8. The river bed has aggraded due to the siltation, and the construction of dams in the rivers would increase flood peak
The flood control measures and policies should be focused on reducing flood damage than on flood prevention. Flood management policies in Bangladesh consider both structural and non-structural measures.
The major structural measures practiced in Bangladesh are the construction of embankments parallel to river banks. The drainage capacity of the channel can also be improved by excavation and installation of regulators or sluices.
Non-structural measures of flood management such as flood forecasting and early warning dissemination, or flood-proofing by rising homestead above flood level can also reduce flood risks.
Basin-wise flood management through transboundary cooperation and flood information sharing, institutional reform, Integrated Water Resources Management (IWRM) and Adaptive Delta Management (ADM) would further reduce flood risks of the country.
A total of 7,555km of embankments, including 4,000km in the coastal areas, 7,907 hydraulic structures, including sluices, and around 1,000 river regulators, 1,082 river closures, and 3,204km of drainage channels have been constructed in Bangladesh since 1964.
Flood embankment protects a region from flooding, though it also increases flood peak by confining flood flow within the channel. Construction of structures across the flood plain is responsible for increasing flood peak and duration.
Considering the adverse impact of flood embankments, flood protection measures are suggested only for metropolitan areas, seaports and airports, and export processing zones as a matter of priority according to the National Water Policy in 2012.
In rural areas, people should be motivated to develop different flood-proofing measures such as raising the platform of homesteads, market places, educational institutions, community centres, etc.
Any construction within the flood flow zone should be avoided. Housing project in any flood flow zone or open water bodies is not permissible.
Ambitious programs like “Integrated Jamuna-Padma Rivers Stabilization and Land Reclamation Project” as proposed in the Bangladesh Delta Plan 2100, need further investigations before formal implementation as there are huge uncertainties.
The uncertainties remain due to the lack of observed flow and sediment distribution data, the unpredictability of transformation processes from braided to meandering, and changes in future flow and sediment under global warming.
Frequent failure of the embankment also creates doubts about the effectiveness of these structures. There are many causes of the failure of embankment such as breaching, overtopped, riverbank erosion, lack of proper maintenance, deviation from the design, use of unsuitable materials, incomplete repair of damaged embankments, etc.
The following should be considered for building climate-resilient infrastructures
1. Design criteria and construction standards should consider the impact of climate change
2. River training works such as groynes, revetment, and porcupine should be undertaken to protect the embankment from bank erosion
3. Regular maintenance and urgent repair of the embankment are essential. Community participation would help the maintenance of the embankments
4. Any illegal settlement on the embankments should not be allowed. People living on the flood embankments should be rehabilitated
5. Suitable construction material should be used. All weak peat soil should be replaced with clay soil
6. Construction of roads and embankments should not create drainage congestions or obstruct flood flows. Enough cross drainage structures should be provided
7. Geo-textiles can be applied to protect the slope of the embankment
According to the Ministry of Disaster Management and Relief, more than 2.8 million people have been affected, in which 1 million people are surrounded by floodwaters. As of today, more than 9.6 million people are suffering from the flood this year in South Asia, which deepens the humanitarian crisis in this region.
With an increasing population, changes in land and water interventions and climate change will make flood management more challenging in the coming decade.
Bangladesh and other South Asian countries like India, Nepal, Bhutan, and China should continue regional collaborations on basin-wise flood management and flood information-sharing to mitigate flood hazards and achieve the UN Sustainable Development Goals by 2030.
AKM Saiful Islam, PhD, is a Professor at the Bangladesh University of Engineering and Technology (BUET). He can be reached at [email protected].
