Abstract:
This study examines the current status and existing estimation approaches of Standard High Water Level (SHWL) and Standard Low Water Level (SLWL), which are two important datums used in the waterways of Bangladesh that were established by Bangladesh Inland Water Transport Authority (BIWTA) in 1991 using harmonic analysis of tidal constituents for tidal waterways. The vertical clearances of structures over inland waterways are measured from the SHWL, and the SLWL provides important information about the navigational channels in terms of minimum expected water levels which is required for operating water vessels. However, since the BIWTA (1991) study has become outdated and has several technical limitations, inaccurate SHWL values are leading to incorrect estimation of the dimensions of structures. This study identifies the limitations and complexities associated with the existing harmonic approach followed by BIWTA. Moreover, due to the lack of technical expertise, contemporary practitioners only conduct traditional frequency analysis and ignore the tidal constituents for estimating the SHWL and SLWL, and hence, there is a concern regarding the compatibility between different approaches. This study addresses these issues by conducting Harmonic Analysis by the Method of Least Squares (HAMELS) using MATLAB-based HAMELS_TIDE program and using observed water level data of 5-minute and 1-hour resolutions from eight tidal gauge stations of BIWTA to generate amplitudes of ten tidal constituents. These constituents have significance in the context of Bangladesh coast and in estimation of SHWL and SLWL. Subsequently, the spatiotemporal variability in the generated amplitudes of the tidal constituents is analyzed which offers a broader understanding of the characteristics of the tidal constituents in this region, revealing disparities with certain assumptions made in the BIWTA study. This study finds a prominent spatiotemporal variability in the amplitudes of the tidal constituents. For instance, the largest amplitude of M2 constituent is found at Chattogram (1.59m) and the smallest amplitude (0.74m) at Galachipa. This study shows that ignoring the temporal variation leads to underestimation of Tidal Addition and Tidal Reduction which are the terms used in the SHWL and SLWL equations. Based on the goodness of fit tests, this study uses Pearson Type III probability distribution for frequency analysis to estimate SHWL and SLWL. Up-to-date SHWL and SLWL values estimated in this study show a noticeable increase in SHWLs after three decades for about 0.30m to 0.89m at different locations, while the SLWLs remain stable along the Bangladesh coast. The comparative study between the traditional frequency analysis approach and the harmonic analysis approach indicates that the traditional frequency analysis method generates conservative values of SHWL and SLWL. At the Chattogram station, the compatibility between the two methods is apparent. The SHWL and SLWL are found 3.88 mPWD and -1.97 m PWD, respectively using harmonic analysis, and 4.06 mPWD and -1.98 mPWD using the traditional frequency analysis method. However, at the Hiron Point station, the conservativeness of the traditional frequency analysis method is found higher. In the traditional frequency analysis method, the SHWL is found 26cm higher than the harmonic analysis method. While the use of a frequency analysis approach would be safe, it can increase the construction costs of structures. This study can help policymakers and practitioners better understand the complex issues around SHWL and SLWL estimation and make informed decisions.