A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 2002

The management of water resources has become a critical need in Bangladesh because of growing demand for water and increasing conflict over its alternative uses. Four distinct kinds of problems related to water supplies and their management: contamination, shortage, salinity and siltation; constitute threats of different degrees to the economy and to human health. Salinity intrusion can be seen in the Southwestern coastal belt where reduced dry season river flows in the Ganges system already permit salt to penetrate both surface and groundwater. The Southwest region of Bangladesh comprises of two distinct and very different zones, the inland zone and the coastal zone. The Northern inland zone has wide spread shallow tubewell irrigation, and the dry season surface water is generally fresh. But in the coastal zone surface water salinity is widespread. The salinity condition in the tidal channels is governed by the boundary conditions at the river entrance facing the Bay of Bengal, the transport-dispersion processes, storage volumes of the upstream and local freshwater discharges and their distribution in the river system. The river system of the Southwest region starts to be affected by coastal salinewater from the month of November, reaching to a maximum in April and May following the trend of Ganges flow. The study is based on MIKE-11 HD and AD model results developed for Southwest region of Bangladesh. The tidal and salinity intrusion condition in Southwest region in different months is quantified from model results. It gives an idea of freshwater discharge condition and its effect on salinity intrusion prevailing in this region. The main routes of freshwater discharge and the major rivers discharging into the Bay are sought out. Along the biggest route from the Pussur to the Gorai tidal amplitude is seen to have increased upto a distance of 106 km. During May when the salinity condition is extreme water is not usable upto a distance of 135 Ian. Irrigation and other demands on surfacewater sources are found to have little effect on salinity intrusion condition. The condition with maximum possible flow without any structural measure through the main freshwater source shows a little improvement of the situation.