Impacts of dynamic interaction between astronomical tides and monsoon wind on coastal flooding in Bangladesh

Abstract

Variation of the tide levels and its interaction with strong monsoon wind can increase the frequency of coastal inundation. These specific hazards are not well studied in Bangladesh coast while inundation due to storm surge and mean sea level rise are recognized in many research works and considered by policy makers. Astronomical and meteorological forcing are the two main cause of tide level modulation which can lead to devastating coastal inundation. Present study aims to study characteristics of wind and tide along Bangladesh coast, computing sea surface swelling due to strong monsoon wind and simulate the coastal inundation when the peak of long-term astronomical tide coincides with strong monsoon wind. In this study, wind with its direction along the coast are analyzed from short term and long term wind data. Modulation of long-term astronomical tide due to lunar nodal and perigean cycles is identified from the harmonic analysis fitted to sinusoidal signals. Characteristics of short-term and long-term tide level are analyzed to see the variation of tide levels along the 710km long coast by tidal constituent analysis. Sea surface swelling due to strong monsoon wind or wind setup is computed by applying an analytical model. Flow module of Delft3D modeling suit is applied to simulate the coastal inundation due to extreme combination of astronomical tide and strong monsoon wind. Areal extent of inundation, average inundation depth and likely scenarios of polder overtopping are computed in union scale for all the 19 coastal districts of Bangladesh. This study finds that the coast experiences the strongest wind along the Chittagong coast and the weakest wind along the Khulna coast. Along Bangladesh coast, 18.61-year lunar nodal cycle has no significant influence but 4.425-year lunar perigean cycle are clearly visible. M2 (Principal lunar semidiurnal constituent) is the dominant tidal constituent along the coast. Both M2 and HAT (Highest Astronomical Tide) show an increasing trend which is a likely evidence of SLR in Bangladesh coast. Wind speed of 10 m/s is identified as the representative wind that can be treated as the strong monsoon wind without cyclone. Wind setup or sea surface swelling is found to be high in the mouth of Meghna estuary and low in the mouth of Pasur-Shibsa estuary. During the extreme combination of astronomical tide and strong monsoon wind, 153 unions of Bangladesh coastal zone are likely to be inundated by overtopping 16 coastal polders.