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Why so flooded?

  • Published at 06:43 pm August 26th, 2017
Why so flooded?
I am often asked by journalists and some colleagues about our country’s recent flood situation -- their line of questioning is usually whether we can do anything to prevent it. Before addressing the issue of managing floods of such enormous scale, it is important to discuss the causes. Common sense dictates that prevention is always better than cure, the scale of the devastation caused by these floods can be reduced at some stage if we can work on some of the major human-induced causes. Geographically, Bangladesh is located in a delta of the three major rivers: Ganges, Brahmaputra, and Meghna -- commonly known as the GBM basin. Some 57 trans-boundary rivers carry flood-water from four upstream countries: China, Nepal, Bhutan, and India. If heavy rainfall occurs in the upper parts of the catchment for a few days straight, direct run-off generated by excess rainfall will be carried out by these trans-boundary rivers towards our country. However, when normal monsoon floods exceed the danger level, it would cause damage to property, infrastructure, agriculture, livestock, and even risk lives. In the recent past, we have observed major floods in Bangladesh during the monsoon seasons of 1987, 1988, 1998, 2004, 2007, and 2016. In 2017, the peak water level crossed the highest recorded water levels in Brahmaputra, Teesta, Dharala, and Jamuneswari rivers. We may not able to blame nature only for such high flood levels, as there are many human-induced reasons behind them.
Human interventions in the natural river systems, and the changes in the land-use pattern of their catchments, make the hill slope steeper, causing frequent, and destructive, floods
Human interventions in the natural river systems, and the changes in the land-use pattern of their catchments make the hill slope steeper, and our rivers are now carrying large amounts of sediments than ever before, causing frequent and destructive floods. Urbanisation 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 further exacerbate the flood conditions. 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 with breaches in the embankments and flood inundation in the protected sides. Many centres such as National Centres for Atmospheric Research in the US provide long-term seasonal outlooks of precipitation which can be modelled to provide long-term outlooks for our flooding. Dredging of rivers and canals can benefit from reducing flood peaks and also act a storage reservoir of water during the dry season. It can also help in improving navigability, recharging groundwater, ensuring sustainability of the eco-system, enhancing fisheries, etc. Due to global warming, the monsoon has become more damaging in recent decades. But, as a result, our water-holding capacity has increased. The impact of climate change on hydrological cycles and extreme events is still an active field of research. Many regional climate modelling results showed us that the mean monsoon flow will be increased by more than 20% in the Brahmaputra basin by the end of this century. Moreover, the sea level rise will be expected to provide additional pressures due to the backwater effect on flood flows. A lot of studies show that the intensity of rainfall, and overall precipitation, in a year will only increase in South Asia going forward. If all the countries sharing the common river basin act together and eliminate these man-made causes of floods, it will be possible to reduce the severity of the floods up to a certain extent. Otherwise, we have to face such catastrophic disasters much more frequently in the future. AKM Saiful Islam is a Professor at the Institute of Water and Flood Management (IWFM) at the Bangladesh University of Engineering and Technology (BUET).