The Meghna estuary, central part of Bangladesh coastal region, through which most of the combined flow of the GBM system is discharged into the Bay of Bengal. A complicated interplay between the forces of the river, tide and the waves creates a complex pattern of sediment displacement in the estuary, which causes morphological changes starting from a time scales of few hours to years to decades. The Bhola, biggest island of Bangladesh, located at the western part of the estuary is subjected to numerous problems to that of the estuary. Among them, bankline shifting is the major one pertaining for a quite long time. Numerical modelling in combination with satellite images can be a strong basis to explore the bankline shifting trend as well as the hydro-morphological analysis of rivers and estuaries if the uncertainties are minimized. The objectives of this study are to enhance the knowledge of the complex morphological processes and to develop qualitative relationship between long-term observed and simulated changes around Bhola Island. Three methods, namely literature review, planform analysis and numerical modelling have been carried out to achieve the objectives. Six satellite images at an interval of about six years available during `73 to `02 have been collected from CEGIS and analysed around Bhola Island with and without considering sedimentary features. Analysis show that the east bank and part of west bank is eroding, the northwest part is accreting and the south part is almost stable. However, the erosion is greater than the accretion in all periods. Moreover, the bankline prediction for 02-08 with two simple approaches show that the east bank, especially the curved outflanking part is likely to erode and the northwest part is likely to accrete further while the south part likely to remain stable. The fate of bankline also depends on the size, shape, position, orientation and distance of the bars. The Char Gazaria has great influence at the northeast of Bhola and whenever the char moves downward, the bankline erodes further downward with greater extension in such that they maintain almost parallel bank. The major factors responsible for the morphological changes in the Meghna Estuary have been explored as part of literature review. Different long-term modelling tools, scale issues and few computational methods have also been explored as part of literature review. Morphological model, Delft3D-FLOW has been used for short-term (4 months) simulations with few characteristics events. The Lower Meghna has the tendency to erode with relatively weak events while it is reverse for extreme events. The overall simulated net changes around Bhola, especially for extreme cases; show good similarities with long-term observations except some places. Moreover, extreme event simulations using high morphological factor for bottom evolution might not be pragmatic for Meghna Estuary. The same model has been used to simulate for six years, which is considered as long-term condition for the Meghna Estuary. Three events namely the weak, average and high water level at the upstream and similar downstream water level have been used for the simulation. The trends of simulated changes are scouring in the lower part of estuary whereas the observed trend is accreting. The morphological parameters, especially the critical bed shear stress for sedimentation seem too low and due to improper boundary selection, there develop high velocity gradients and shear stresses resulting in high scouring. On the basis of two simulations, one with the existing settings and the other with changed shear stress has been considered to develop relationship with long-term observations. The measured water levels at the upstream boundary during `75 to `95 periods have been taken into consideration for simulated volume calculations. The observed changes have been obtained by changing the digitised bankline and averaged depth by some percentages. From the results, it has been found that the uncertainty ranges of the numerically simulated changes are higher than the observed changes (from satellite images) in different polygons. Moreover, the uncertainty ranges are much higher for the model parameters than the effects of upstream water level variations. The large differences of the uncertainty ranges could reduced if more field data were to be available.
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