Aspects of 2016 flooding in Bangladesh

by AKM Saiful Islam
Flood

Homesteads are under water as flooding inundated vast areas in Tangbhanga char in Jamalpur on August 2. — Sanaul Haque

BANGLADESH has been suffering from devastating monsoon floods from the middle of July. Floods are not uncommon to the people living in this delta of the three mighty rivers, the Ganges, the Brahmaputra and the Meghna. Those of us living in cities protected by the embankment easily suggest that flood is a blessing as it improves the fertility of the land, recharge groundwater, clear the environment by removing solid wastes and so on. However, the poor people living in char islands or close to rivers without any protections suffer miserably during the floods. They lost their lives, cattle, valuables, crops, houses; they go through the disastrous situation during and after the floods.
AspectsAlthough Mother Nature can be blamed for the floods, the degree of flood hazards is often exacerbated by our unplanned development activities, deforestation, greed, corruption and violation of laws. The root cause of flood is the long durational heavy rainfall over the catchment. However, the height and duration of flood also depends on the land-use and land-cover pattern of the catchment areas, soil moisture conditions of the catchment and both groundwater and surface water levels of the river. Changes in the land-use pattern through deforestation, filling up wetlands, urbanisation, polderisation (embanking) and compartmentalisation of basin increase the flood peak and duration.

El Niño-Southern Oscillations and floods in Bangladesh
DETERMINISTIC forecasting of weather more than 10 days is not realistic because of the chaotic nature of the weather. Forecast often uses initial condition of atmosphere, land and ocean to derive the future conditions of weather. However, slight changes in this initial condition will make significant changes in the forecast beyond the period of 10 days. Currently our flood forecasting division provides deterministic forecast for five days and probabilistic forecast up to 10 days. However, seasonal or monthly scale predictions are possible as a number of weather components are found to be correlated on the longer time-scale.
One of such components is ENSO (El Niño-Southern Oscillation) cycle which refers to the coherent, large-scale fluctuation of ocean temperatures, rainfall, atmospheric circulation, vertical motion and air pressure across the tropical Pacific. It has been observed from the climate record that recent occurrence of major floods in Bangladesh during a moderate or strong La Niña phase. Floods in 1988, 1998 and 2007 can be explained by the La Niña ENSO cycle that occurred during that period.
During 2015–2016, a ‘Godzila’ El Niño has been observed which has similarity with the 1997–1998 El Niño. After the El Niño phase, La Niña is also expected to occur later part of this year, during August and September, which would give a boost to the south-west monsoon over India (Fig 1). Though the progression of La Niño is slower than it was predicted, significant rainfall has already occurred over the Himalayan foot hill and north-east parts of India which brings devastating floods in Bihar and Asam in India and north, central and north-east parts f Bangladesh.

Entrance and propagation of floods in Bangladesh
INTENSE heavy rainfall occurred in the foot hills of the Himalayas and over the Meghalaya basin from Uttarkhand to Arunachal during the second week of July. Continuous heavy rainfall over that regions generates huge runoff as soil became saturated and brings water level in bankful stage. Intense rainfall over the Meghalaya basin caused flash floods in the north-east part of Bangladesh and flood water enters from the hilly Jadukatha and other small rivers to the River Surma and caused floods in Sunamganj. In the Brahmaputra basin, floods come from the Teesta, Dharala, Dudkumar and Brahmaputra rivers.

Comparison of 2016 flood with other historic floods
THE 2016 flood in Bangladesh will be remembered as a flood of over 100 year return period as the water level at the Bahadurabad station crossed the highest ever recorded water level during 1988. However, the duration of 2016 flood will not exceed that of 1998 flood due to many reasons. The water level of the River Ganges was much below the danger level which creates a favourable condition for the flood water to flow through towards the Bay of Bengal much faster. Moreover, Because of the weak neap tide condition prevailing in the Bay of Bengal makes a steeper gradient for the receding flood water.

Threats of flood under climate change conditions
BECAUSE of global warming, the world temperature is continuously rising and 2015 has been the warmest year since 1880 (Fig 2). According to NASA and NOAA, June 2016 was also the warmest month compared with temperature of the 20th century. Such a continuous rise in temperature will have profound impact on the floods as the possibility of the heavy rainfall occurrence will increase in the future. Deep convection and increasing capacity of water holding in the atmosphere due to global warming will certainly increase the possibility of heavy rainfall in the future. Therefore, the chances of the occurrence of the monsoon floods in Bangladesh will also increase in the future.

Urbanisation and floods in Dhaka city
MANY of us are surprised to observe that Dhaka was not flooded up to the level as predicted. Due to major urbanisation (roads, buildings, filling up of wet lands, siltation of the river bed, etc) and narrow river course (and water conveyance capacity) of the major rivers like the Old Brahmaputra, the Bangshi and the Dhaleswari carried less water into the surrounding rivers (the Turag, the Balu, the Buriganga) of the Dhaka city. Actually, the process of rapid urbanisation of the greater Dhaka city bypassed the flood in the central parts of the country. The sufferings of the people of Manikganj, Faridpur and Shariatpur will be prolonged as much of the flood water could not pass through the old Brahmaputra river.

AKM Saiful Islam is a professor of the Institute of Water and Flood Management at the Bangladesh University of Engineering and Technology.

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