A thesis submitted in partial fulfillment of the requirements of the degree of Master of Engineering, School of Engineering and Technology

Thesis (M.Eng.) - Asian Institute of Technology, 2001

In the present study, Interaction of tide and river flows in Lower Meghna is investigated using the method of harmonic analysis and analytical solutions. As the records of water level along the river are contributed by various constituents of tide so harmonic analysis has been applied to decompose the complicated interaction of the fresh water discharge with various constituents of tides into its individual interaction with each constituent. Four main constituents are included in this study. Results of water level obtained by applying the analytical model to stations along the Lower Meghna are compared very well with the observed values. In analytical model, the governing equations are solved using the approximation method of perturbation to investigate the interaction of tide and fresh water discharge of lower Meghna river of which the cross-section and width varies exponentially. The zeroth order solution is obtained by direct integration and the IPPEN's solution is adopted for the first order solution. The relationship of important dimensionless parameters of the tide with fresh water discharge, especially the dimensionless damping modulus, has been established. These informations are required to calculate the salinity intrusion in estuaiy. Profiles of maximum and mean salinity as well as the intrusion length along the Lower Meghna river has been investigated. The Lower Meghna River shows a period of low flow in AprilMay. During this period saline water from the sea, under the actions of tide and diffusion, intrudes into the lower reach of the river, causing salinity pollution. Tidal convection of salinity profile has been studied. Flow conditions at different stations during periods of low flow has been investigated. Similar to the interaction of tide and river flow, analytical solutions have been described in decomposing the steady state component of salinity from the unsteady component of tidal convection of salinity. Comparison between computed and observed average salinity along the length from downstream boundary has been found.