Drivers of tidal flow variability in the Pussur fluvial estuary: A numerical study by HEC-RAS.

This paper aims at characterizing the longitudinal and temporal variability of tidal flows in the Gorai-Pussur River system of Bangladesh, which spans about 158 km, starting from Bordia upstream to Akram Point downstream. Considering the upstream fresh water discharge and the downstream tide level as the drivers of tidal flow variability, the spatiotemporal change in hydrodynamics of the Pussur Fluvial Estuary was simulated using HEC-RAS software at the neap-spring and seasonal scales. The model was calibrated by comparing the simulated discharge and water levels with available measured data at an intermediate station. During the dry season, the tidal effect from the sea reaches Bordia, while in the monsoon season, the tide reaches about 15 km less (up to Kalia) due to the increased flow of freshwater. The Pussur river experiences tidal amplification up to Chalna, located approximately 90 km from its mouth. Beyond Chalna, tidal dumping covers a distance of 100-170 km upto Bordia. The tidal range ratio between Akram Point and Mongla is 1.28. The phase shift of the high water is found approximately 1.0 h over a distance of 50 km. It is found that due to 50% increase of discharge in Bordia, the increase of discharges at Kalia and Chalna were found as 43% and 7%, respectively. For a 0.5 m increase in water level at downstream (Akram Point), the high and low water levels at Chalna are increased by 0.15 and 0.69 m, while for 1.0 m increase in water level at downstream, the high and low water levels at Chalna are increased by 0.67 and 1.20 m, respectively.