Narrow Results

The position and configuration of the coastal shelf of the northern Bay of Bengal amplify the storm surge height extraordinarily. The low-lying coast of Bangladesh is severely devastated when extreme surges hit the land. However, the influence of track of a cyclone on surge height is not well known. A calibrated numerical model is used to simulate series of storm surges for a severe cyclone by shifting its track position and angle. The cyclonic wind field has been generated using the extreme parameters adopted from historical measurements. Surge heights simulated by the two-dimensional model are compared at selected locations along the coast. It was found that the cyclone with a landfall location at Barguna, a place between the Sundarban and the Meghna Estuary, develops the highest surge of 12.0 m (PWD) at the northeast of the estuary. The magnitude is further amplified to 13.0 m (PWD) when the surge coincides with the high-tide condition; a surge of similar magnitude attacked the coast of Bangladesh in 1876. Change in track orientation shows little influence on surge heights at the eastern coast. However, relatively higher effect was found in the western coast. Surge height distribution has also been presented with respect to local land or embankment elevation for the extreme event. A flood map is produced showing inundation depth in the aftermath of the extreme surge attack. The coastal areas at the northern tip of the upper bay and the western banks of the Meghna Estuary are found to be the most vulnerable locations.

The purpose of this experimental study was to investigate the effects of a rectangular canal on the hydrodynamics of turbulent surges before and after the canal by implementing a series of physical experiments. A dam-break wave model was used to simulate the tsunami-like turbulent waves passing over a smooth and horizontal surface, in the presence and absence of a canal. Three canal depths of $d=0.05$, 0.10 and 0.15m were used to model shallow, moderate and deep conditions and three canal widths of $w=0.60$, 1.60 and 3.0m were selected to model narrow to wide canals. The front velocity of the dam-break induced surges were controlled by rapidly releasing upstream impounded set volumes of water with depths of $d_o=0.20$, 0.30 and 0.40m. The dam-break wave propagation over a horizontal, dry and smooth bed revealed four regimes describing the variations of surge height with time. The arrival time to reach the maximum surge height and the quasi steady-state regime was correlated with each impoundment depth and an empirical formulation was proposed to estimate the onset of the quasi steady-state flow. The maximum surge heights measured before and after the mitigation canal location were compared with those recorded in the corresponding tests without the presence of the canal. It was found that the peak surge height upstream of the canal could increase up to 40% compared to the test without the presence of the canal in relatively small impoundment depth and in presence of a narrow canal due to momentum dissipation. The wave height downstream of the canal increased between 10% and 50% of the wave height without the presence of the canal and the minimum change in the wave height occurred for the canal width to depth ratio of 20. The time-history of surge velocity after the mitigation canal indicated a significant decay of between 40% and 60% in the presence of a canal due to the bed friction changes and momentum dissipation.