Why are we observing severe floods in Bangladesh?

Before the monsoon season, it is important to ensure that flood control infrastructures are functioning properly

I often was asked by the journalists and many colleagues that why we are facing such severe floods in Bangladesh. And then the next obvious question is that whether we can do anything about it to prevent or not? Before addressing the issue of what can be done or how we can manage floods of such an enormous scale, it is important to discuss the causes of the floods. As prevention is better than cure, the scale of the devastation of the floods can be reduced at some stage if we can work on some of the major human-induced causes. Several reasons of this severe monsoon floods in Bangladesh have been where both natural and anthropogenic activities can be equally blamed.

Geographically Bangladesh is located in a delta of the three major rivers, the Ganges, Brahmaputra, and Meghna which is commonly known as GBM basin. The total drainage area of GBM basins is about 1,761,300 square kilometers and its maximum discharge is about 200,000 cumec (m3/sec) during the flood seasons. About 93% of the catchment area of the GBM basins is outside Bangladesh which plays a significant role in the monsoon floods in Bangladesh. Fifty-seven transboundary rivers of Bangladesh carry flood water in Bangladesh from the 4 upstream countries: China, Nepal, Bhutan, and India. If heavy rainfall occurs in the upper parts of the catchment for a few consecutive days, direct runoff generated by such excess rainfall will be carried out by these transboundary rivers towards our country. As soon as any river becomes bank full stage, it spills and we are going to observe floods or normal floods in that river.

Up to a certain extent and duration flood is desirable for our country as it recharges ground water, cleaning solid waste, helping navigation, carrying nutrients to improve soil fertilities, enhance fish production, preserve ecosystems and provides many other benefits. However, when normal monsoon floods exceeding danger level (a level set by the Bangladesh Water Development Board (BWDB) considering the crest height of major infrastructures such as embankments, nearby buildings etc.), it would cause damage to properties, infrastructures, agriculture, livestock and even take human lives. However, such extreme floods are not occurring every year as it depends on the natural variability (or on the chances of occurrence) of the climatic systems. In the recent past, we have observed major floods in Bangladesh during the monsoon season of 1987, 1988, 1998, 2004, 2007, 2016. The 1988 floods were remembered for the highest floods peak having a 100-year return period and 1998 flood for its longest duration of flood inundation (more than 60 days in some places). In 2017, the peak water level crossed the historic recorded highest water levels in Brahmaputra, Teesta and Dharala, Jamuneswari rivers. We may not able to blame the nature only for such high flood levels as there are many human induced (anthropogenic) reasons behind these.

In recent decades, due to many anthropogenic activates makes floods more intense and more frequent like the encroachment of river banks and wetlands, lack of drainage, unplanned urban growth, hill cutting and deforestation. These scenarios can be found in our country and other upstream countries sharing these 57 transboundary rivers. Such human interventions in the natural river systems and the changes in the land-use patterns of their catchments make the hill slope steeper and rivers are carrying huge sediments than in the past. As a consequence, floods become more frequent and devastating than in the past. As a result, rivers like Teesta, Dharala, Dukumar, Atrai, Korotoa, Punarbhaba, Mahananda, Ichamati are observing unprecedented floods this year.

On the other hand, the process of urbanization generates more runoff while encroachment of wetlands and embankment confine flood water inside the river channel which raises the flood peak. Moreover, excess sediments raise the bed level and make the flood condition, even more, worse than in the past.  Due to many illegal interventions along the course of the old Brahmaputra river, flood water is not able to travel through this channel as fast it was in the past. This certainly benefits the greater Dhaka division as it observes floods at least 5 to 7 days later than other north-central districts of the country. However, as a consequences, huge flood water flows through the Brahmaputra from the north central to the central regions of Bangladesh. Such heavy flood flow aggravates the flooding and river bank erosion in the central parts of the country.

The highest flood peak at Bahduraba in Brahmaputra-Jamuna exceeded all the historic flood peak levels in recent time. As mentioned above that this year, flood water crossed the record high water level at Bahduraban in Jamuna, Dalia in Teesta, Kurigram at Dharala. Last year we also observed that the peak flood exceeds the previous record water level at Bahdurabad. Observing this recording breaking flood events, it can be easily understood by us that flood intensity and severity has been increased.

Before the monsoon season, it is important to ensure that flood control infrastructures are functioning properly. Inadequate and improper maintenance of the embankments and water control structures such as sluices, regulators etc. would also assist making the breaches of the embankments and flood inundation in the protected sides.

Flood Forecasting and Warning Center (FFWC) (http://ffwc.gov.bd) of BWDB doing an excellent job for providing deterministic flood forecast up to 5 days and satellite altimetry-based flood forecast up to 8 days. However, lead time of monsoon floods can be easily doubled by regional cooperation with neighboring countries.  Basin-wise flood modeling can improve the lead-time and accuracy of the flood forecasting. Moreover, it is also possible to provide long-term (beyond 10 days) seasonal forecast. Many centers such as National Center for Atmospheric Research (NCAR) in USA provides long-term seasonal outlooks of precipitation which can be modeled to provide long-term flood outlooks.  At present, dissemination of flood early warnings to the community is either absent or in a limited scale through Department of Disaster Management (DDM). Location specific flood warning is extremely important and must be incorporated by FFWC which can help reduce risks of flood disasters.

Dredging of rivers and canals can benefits reducing flood peaks and also act a storage reservoir of water during the dry season. It also improves navigability, recharging groundwater, provides sustainability of the eco-system, enhances fisheries etc. Dredging can be conducted natural ways such as applying bandaling techniques which use locally available materials like bamboos, woods etc. Other low-cost natural dredging options can be explored to improve water conveyance capacity of the river and reduce flood peaks.

Finally, due to global warming monsoon becomes active in recent decades. A recent article in Science showed that gradient between land and sea has been increasing and monsoon becomes more active. On the other hand, water holding capacity due to global warming has been increased. An impact of climate change on hydrological cycles and extreme events are still an active field of research. Many regional climate modeling results showed that the mean monsoon flow will be increased more than 20% in the Brahmaputra basin at the end of this century. Moreover, sea level rise will have expected to provide additional pressure due to the backwater effect on flood flows. Many studies showed that intensity of rainfall, the consecutive rainy days, total precipitation in a year will be increased in the future for this South Asian region. This will certainly enhance the severity of the monsoon floods in Bangladesh. Extreme climatic events like floods in this region will be more severe in the future in the warming world.

If all the countries, sharing the common river basin, act together and eliminate these anthropogenic causes of floods, it is possible to reduce the severity of the floods up to a certain extent. A basin-wise comprehensive flood risks management strategy should be prepared and implemented considering climate change and other human induced issues as mentioned above. Otherwise, we have to face such catastrophic disasters much more frequently in the future!

The writer is a Professor of the Institute of Water and Flood Management (IWFM) at the Bangladesh University of Engineering and Technology (BUET), Bangladesh. Email: [email protected]

Why are we observing severe floods in Bangladesh?

Before the monsoon season, it is important to ensure that flood control infrastructures are functioning properly

I often was asked by the journalists and many colleagues that why we are facing such severe floods in Bangladesh. And then the next obvious question is that whether we can do anything about it to prevent or not? Before addressing the issue of what can be done or how we can manage floods of such an enormous scale, it is important to discuss the causes of the floods. As prevention is better than cure, the scale of the devastation of the floods can be reduced at some stage if we can work on some of the major human-induced causes. Several reasons of this severe monsoon floods in Bangladesh have been where both natural and anthropogenic activities can be equally blamed. Geographically Bangladesh is located in a delta of the three major rivers, the Ganges, Brahmaputra, and Meghna which is commonly known as GBM basin. The total drainage area of GBM basins is about 1,761,300 square kilometers and its maximum discharge is about 200,000 cumec (m3/sec) during the flood seasons. About 93% of the catchment area of the GBM basins is outside Bangladesh which plays a significant role in the monsoon floods in Bangladesh. Fifty-seven transboundary rivers of Bangladesh carry flood water in Bangladesh from the 4 upstream countries: China, Nepal, Bhutan, and India. If heavy rainfall occurs in the upper parts of the catchment for a few consecutive days, direct runoff generated by such excess rainfall will be carried out by these transboundary rivers towards our country. As soon as any river becomes bank full stage, it spills and we are going to observe floods or normal floods in that river. Up to a certain extent and duration flood is desirable for our country as it recharges ground water, cleaning solid waste, helping navigation, carrying nutrients to improve soil fertilities, enhance fish production, preserve ecosystems and provides many other benefits. However, when normal monsoon floods exceeding danger level (a level set by the Bangladesh Water Development Board (BWDB) considering the crest height of major infrastructures such as embankments, nearby buildings etc.), it would cause damage to properties, infrastructures, agriculture, livestock and even take human lives. However, such extreme floods are not occurring every year as it depends on the natural variability (or on the chances of occurrence) of the climatic systems. In the recent past, we have observed major floods in Bangladesh during the monsoon season of 1987, 1988, 1998, 2004, 2007, 2016. The 1988 floods were remembered for the highest floods peak having a 100-year return period and 1998 flood for its longest duration of flood inundation (more than 60 days in some places). In 2017, the peak water level crossed the historic recorded highest water levels in Brahmaputra, Teesta and Dharala, Jamuneswari rivers. We may not able to blame the nature only for such high flood levels as there are many human induced (anthropogenic) reasons behind these. In recent decades, due to many anthropogenic activates makes floods more intense and more frequent like the encroachment of river banks and wetlands, lack of drainage, unplanned urban growth, hill cutting and deforestation. These scenarios can be found in our country and other upstream countries sharing these 57 transboundary rivers. Such human interventions in the natural river systems and the changes in the land-use patterns of their catchments make the hill slope steeper and rivers are carrying huge sediments than in the past. As a consequence, floods become more frequent and devastating than in the past. As a result, rivers like Teesta, Dharala, Dukumar, Atrai, Korotoa, Punarbhaba, Mahananda, Ichamati are observing unprecedented floods this year. On the other hand, the process of urbanization generates more runoff while encroachment of wetlands and embankment confine flood water inside the river channel which raises the flood peak. Moreover, excess sediments raise the bed level and make the flood condition, even more, worse than in the past. Due to many illegal interventions along the course of the old Brahmaputra river, flood water is not able to travel through this channel as fast it was in the past. This certainly benefits the greater Dhaka division as it observes floods at least 5 to 7 days later than other north-central districts of the country. However, as a consequences, huge flood water flows through the Brahmaputra from the north central to the central regions of Bangladesh. Such heavy flood flow aggravates the flooding and river bank erosion in the central parts of the country. The highest flood peak at Bahduraba in Brahmaputra-Jamuna exceeded all the historic flood peak levels in recent time. As mentioned above that this year, flood water crossed the record high water level at Bahduraban in Jamuna, Dalia in Teesta, Kurigram at Dharala. Last year we also observed that the peak flood exceeds the previous record water level at Bahdurabad. Observing this recording breaking flood events, it can be easily understood by us that flood intensity and severity has been increased. Before the monsoon season, it is important to ensure that flood control infrastructures are functioning properly. Inadequate and improper maintenance of the embankments and water control structures such as sluices, regulators etc. would also assist making the breaches of the embankments and flood inundation in the protected sides. Flood Forecasting and Warning Center (FFWC) (http://ffwc.gov.bd) of BWDB doing an excellent job for providing deterministic flood forecast up to 5 days and satellite altimetry-based flood forecast up to 8 days. However, lead time of monsoon floods can be easily doubled by regional cooperation with neighboring countries. Basin-wise flood modeling can improve the lead-time and accuracy of the flood forecasting. Moreover, it is also possible to provide long-term (beyond 10 days) seasonal forecast. Many centers such as National Center for Atmospheric Research (NCAR) in USA provides long-term seasonal outlooks of precipitation which can be modeled to provide long-term flood outlooks. At present, dissemination of flood early warnings to the community is either absent or in a limited scale through Department of Disaster Management (DDM). Location specific flood warning is extremely important and must be incorporated by FFWC which can help reduce risks of flood disasters. Dredging of rivers and canals can benefits reducing flood peaks and also act a storage reservoir of water during the dry season. It also improves navigability, recharging groundwater, provides sustainability of the eco-system, enhances fisheries etc. Dredging can be conducted natural ways such as applying bandaling techniques which use locally available materials like bamboos, woods etc. Other low-cost natural dredging options can be explored to improve water conveyance capacity of the river and reduce flood peaks. Finally, due to global warming monsoon becomes active in recent decades. A recent article in Science showed that gradient between land and sea has been increasing and monsoon becomes more active. On the other hand, water holding capacity due to global warming has been increased. An impact of climate change on hydrological cycles and extreme events are still an active field of research. Many regional climate modeling results showed that the mean monsoon flow will be increased more than 20% in the Brahmaputra basin at the end of this century. Moreover, sea level rise will have expected to provide additional pressure due to the backwater effect on flood flows. Many studies showed that intensity of rainfall, the consecutive rainy days, total precipitation in a year will be increased in the future for this South Asian region. This will certainly enhance the severity of the monsoon floods in Bangladesh. Extreme climatic events like floods in this region will be more severe in the future in the warming world. If all the countries, sharing the common river basin, act together and eliminate these anthropogenic causes of floods, it is possible to reduce the severity of the floods up to a certain extent. A basin-wise comprehensive flood risks management strategy should be prepared and implemented considering climate change and other human induced issues as mentioned above. Otherwise, we have to face such catastrophic disasters much more frequently in the future! The writer is a Professor of the Institute of Water and Flood Management (IWFM) at the Bangladesh University of Engineering and Technology (BUET), Bangladesh. Email: [email protected]