Impact of morphological changes at the eastern part of the Meghna estuary on storm surge considering sea level rise

Abstract

This study aims to investigate the impacts of the morphological changes under different sea level rise scenarios on the storm surge height and storm surge induced inundation at the eastern part of the Meghna estuary. To investigate this, first from the freely available Landsat satellite images, the coastlines of 1991 and 2018 are extracted and newly accreted land surfaces at the eastern part of the Meghna estuary are identified. With the implementation of the Noakhali-Urir Char-Sandwip cross-dam the Eastern part of the Meghna estuary is expected to gain more land in future. Both the newly accreted lands identified from the satellite images and the expected hypothetical newly accreted lands are given 1m, 2m and 3m land elevation arbitrarily. The depth of adjoining channels and water bodies are also altered accordingly. TC-1991 is the strongest cyclone (in terms of wind speed and pressure drop) in history to hit this part of the Bangladesh coast. In total five different cyclone tracks: TC-1974, TC-1991, TC-Sidr of 2007, TC-Mahasen of 2013 and TC-Roanu of 2016 are used to generate different cyclonic scenarios with the same strength as TC-1991. With the inclusion of SLR settings, a total number of 65 scenarios are created to investigate the impact of storm surge in the study area. This study involves application of a coupled Delft3D flow and Delft Dashboard open-source tool and ArcGIS to simulate, calculate and visualize storm surge and inundation characteristics due to the newly reclaimed lands. Primary and secondary source of information are used to validate the model. Simulation results show that surge characteristics (i.e., surge height, surge duration and surge propagation speed) are strongly dependent on the cyclone tracks for cyclones with same strength. At the eastern part of the Meghna Estuary the cyclone tracks of Roanu, Mahasen and TC-1991 have shown the most significant rise of the surge height. Tidal phase has significant impacts on the surge height and duration. Surge water level that starts to increase during the rising tidal phase has produced a sharp peak and short duration and vice-versa. The cyclone tracks (e.g., Mahasen track) that first pass over the offshore islands and then moves forward to other locations produces lower surge height compared to those that hit the land directly (e.g., Roanu track). The surge height for the different scenarios of the same cyclone track (i.e., Roanu and Mahasen track) gradually increase from the outer part of the Sandwip channel towards the inner part of the channel. It is also found the time of getting the peak inundation mostly depends on the location because of the flow pattern during the cyclones. From numerical simulations of flow fields, it is observed that irrespective of the cyclone track the mass flux of the cyclonic surges always enter through the Sandwip Channel from the south-eastern part of the channel and travel towards the north-western direction towards the Noakhali mainland and then reaches the northern part of the Sandwip island. As a result, 1991, Roanu and Mahasen track induced simulations show the peak inundation first at the south-eastern part of Sandwip and then at the northern part of Sandwip Island. Impact of coastline change upon surge induced inundation shows that inundated area has increased from the 1991 coastline to 2018 coastline to the hypothetical ‘All-land’ coastline. The percentage of inundated area has increased from 28.8% in 1991 coast to 34.3% in 2018 coast and 43.7% in the ‘All-land’ coast. Also, the inundated areas with higher inundation depth increases at the accreted new lands in the offshore areas but the accreted new lands decrease the inundation depth in the mainland part of the coast. Impact of SLR on surge induced inundation show that the percentage of total inundation increases with the increase of (SLR) both at the mainland and at the offshore islands. It is found that the offshore islands and the surrounding newly accreted lands are more susceptible to the impact of SLR.