Numerical model based investigation of physical and hydraulic factors controlling sedimentation in a selected tidal basin in the south-west of Bangladesh

Abstract

Tidal basins under the Tidal River Management projects in Bangladesh exhibit complex morphological patterns consisting of substantial spatial and temporal variability of sedimentation. Sedimentation in the tidal basin is controlled by various physical and hydraulic factors. Therefore, it is indispensable to understand the dominant factors behind these morphological patterns. Flow field and sedimentation pattern in Pakhimara Beel (tidal basin), located in southwestern Bangladesh, are being investigated under the present study considering a number of selected factors (e.g., basin’s and link canal’s properties, Coriolis effects, tidal asymmetry). A total of 63 scenarios are created to investigate the impact of different physical and hydraulic factors in the sedimentation process. Delft3D model is used to simulate the constructed scenarios. Ideally shaped computation domains with varying depth and orientation, inlet canal location, canal length and width, extent of vegetation cover have been investigated to analyse their impact on sedimentation. Impact of Coriolis force has been tested changing the latitude of the computation domain. Also, the asymmetry in tide has been analysed with respect to asymmetry in the rising period and falling period and asymmetry in flood and ebb amplitude. Four hydraulic combinations of the dry and wet season; flood and ebb dominance have been studied in the present research. Results show that wetland and link canal characteristics highly influence the sedimentation pattern by providing different pathway conditions to the water and sediments flowing into the basin. It is found that with the change in the basin’s shape and size, sediment accumulation differs significantly. For larger size of the beel, less sedimentation occurred per unit area. Tilted basin with same surface area shows reduced sedimentation. In addition, the link canal situated at the d/s of the beel induces more deposition inside the beel than the link canal connected at the u/s as more sediments can enter into the beel due to high tidal velocity. However, for even sedimentation link canal located in the middle performs better than other cases where the link canals are located elsewhere. Simultaneous operation of two link canals shows reduction of sedimentation inside the beels primarily due to sedimentation in the link canals. Sedimentation increases with wider link canals but decreases with longer ones. Thus, it is found that if the link canal length is required to be increased, the width also should be increased keeping an optimal ratio. Study results indicate that vegetation cover significantly influences the uniformity as well as the volume of sedimentation. A completely vegetated beel has the most uniform sedimentation but it has the least volume of sediments, and vice versa. Moreover, large length scales of the beel, shallow water depth, and low depth-averaged velocity created by the hydraulic gradient of sediment- laden flows lead to Rossby number (defined as Ro = U/fL) <1, making the system susceptible to the effect of Coriolis force. This condition alters the flow field and, thereby, the sedimentation pattern. Furthermore, the results portray that, for all domains, the flow velocity is deflected towards the right in the northern hemisphere due to Coriolis Effect. It is found that at large peripheral areas of the beel with substantially low flow velocity the circulations are influenced by Coriolis force and higher sediment deposition occurs for higher latitudes. The results imply that the deposition patterns in the large tidal basins are significantly influenced by Coriolis Effect. The results also depict that asymmetry in flood and ebb amplitude has more significance on sedimentation than asymmetry in rising period and falling period. As a consequence, for the flood-dominated domain, both in the dry and wet season, the accumulation escalates more with the rise of the flood amplitude than the increase of rising period. Also, it is found that for the ebb-dominated domain, the accumulation remains almost the same in all the combinations of hydraulic conditions.