Evaluation of the Short-Term Processes Forcing the Monsoon River Floods in Bangladesh

Abstract

Bangladesh is a low-lying country in the Meghna delta located at the confluence of three major rivers: the Ganges, the Brahmaputra, and the Meghna. Monsoon river flooding is an annual event with river water levels rising and falling slowly during the monsoon season, though with some pulsation. Forcing of the monsoon river floods is complicated and not fully understood. A flood-causing factor often cited is the synchronization of high inflows through the Ganges and the Brahmaputra. In the present study, the driving forces of the monsoon river floods are analyzed using extensive field data and a numerical model. The main forcing factors found to cause extreme floods in Bangladesh are high inflows from upstream combined with extreme high rainfall inside the country. Near perfect synchronization between inflows through the Ganges and the Brahmaputra, the timing of spring tide in the Bay of Bengal and high pre-monsoon groundwater levels in Bangladesh are of only little importance in creating an extreme country-wide flood situation. Local rainfall is important in the northeast region of Bangladesh, where it causes local flash floods and affects the flows in the Meghna River. A higher than normal sea level in the Bay of Bengal can be of importance in worsening a flood situation in the coastal area of Bangladesh approximately 50 km inland. Potential climate changes as accepted by the Government of Bangladesh by the year 2030 (corresponds closer to the predicted conditions by IPCC [200lb] for year 2070) may worsen the flood situation dramatically in the country.