The role of environmental forcing on tidal dynamics along complex near-shore waters off Bangladesh

Abstract

In a geomorphic sense, the Bangladesh located in the head Bay region comprises of numerous tidal creeks, waterway inlets, and complex coastline geometry. This region encompassing the world׳s largest deltaic system is thickly populated and extremely low-lying, and therefore highly susceptible to coastal flooding and sea level rise. High tidal range existent in this region primarily governs the hydrodynamic behavior and coastal processes. The present study deals on a comprehensive tidal analysis, understanding the variations in tidal behavior over spatial and temporal scales in the near-shore region off Bangladesh. The harmonic tidal analysis using SLPR2 estimates the tidal constituents using linear least squares with respective nodal correction. Tide gauge observations from five different stations viz; Hiron Point, Khepupara, Charchanga, Khal No-10, and Cox׳s Bazaar were used for tidal analysis. The results from SLPR2 signify considerable seasonal variations in water level attributed due to meteorological factors and excess river discharge. Tidal pattern at all these five stations are primarily mixed semi-diurnal with M2 as the dominant constituent. The t-tide toolbox computed the equilibrium tidal amplitude for the study area. Some of the components known to be influenced by meteorological aspects such as Solar Annual component (Sa), Semi-Annual component (Ssa), and Solar Radiational component (S1) have higher amplitudes than expected, in the study area. The shallowness of the delta cause severe deviations in the tidal behavior from equilibrium tides. Interaction of tides with river discharge also dominant in this region indicates the role of hydrological forcing. Interestingly, the study also reveals the existence of compound tidal constituent Msf in this region. Monthly analysis of Msf component, using FFT, revealed peaks corresponding to the monsoon, Kal-Baisakhi months and to the seasonal changes in atmospheric pressure, thereby unfolding the seasonal influence on Msf. The funnel shaped head Bay plays an important role in amplification of tidal constituents, wherein the form factor and maximum tide levels showed an increasing trend. This study also demonstrates how tidal dynamics can be elucidated using location specific water-level observations. Investigations on the annual variations in observed and predicted water levels clearly signify the seasonal patterns. The correlation analysis show a reasonable match with skill level exceeding 93%, and the overall prediction by SLPR2 is highly satisfactory.