Tidal Prediction for Complex Waterways in the Bangladesh Region

Abstract

The navigational aspects of marine transport and its manoeuvrability require precise knowledge on tides. Information on time varying water levels, magnitude and direction of tidal currents is quite critical in coastal waterways and estuarine environment. Real-time water levels along with other hydrodynamic parameters are used in almost all major ports for effective operations throughout the year. Also various coastal engineering projects require precise information on site-specific water level elevations. There is a rapid expansion of coastal infrastructure in the recent times, and therefore it is very essential to have reliable water level prediction system that caters the need for coastal engineers, port and harbour activities etc. The Sea Level Processing Package (SLPR2) developed by the University of Hawaii Sea Level Center in collaboration with National Oceanographic Data Center (NODC) provides reliable estimate of location specific sea level information. SLPR2 performs three primary tasks that include tidal analysis and prediction, quality control, and filtering. Harmonic tidal analysis using linear least square produces the relevant tidal constituents of a specific region. High frequency data (preferably one-hour duration) for a minimum duration of one complete year would suffice the tidal analysis phase in SLPR2. The tide prediction algorithm used in the present study uses a maximum of 68 harmonic constituents. The region of interest is the head Bay of Bengal region located along the east coast of India. Preliminary analysis of measured sea level data indicate that tide gauges located along Bangladesh has research quality data, and therefore used in this study. Station data from seven locations viz; Hiron Point, Khepupara, Charchanga, Chittagong, Khal No.10, Cox's Bazaar, and Teknaf all located in Bangladesh were used for tidal analysis, and thereafter the predictive capability of SLPR2 for one-year period was investigated. There are exceptions at two stations, Teknaf (comprising of 87% observed data) and Khal No.10 (99% of observed data), and remaining five stations are free from data gaps. In a hydrographic perspective, the Bangladesh region has complex network of waterways, and bottom topography have several detached shoals. Tides enter the Bangladesh coast through two submarine canyons, reaching Hiron Point and Cox Bazaar at almost the same time. Amongst the principal constituents, dominant modes are exhibited by M2 and S2 whose natural oscillation periods are 12h 25min, and 12h respectively. The results from SLPR2 indicate considerable seasonal variation in water level prediction during the monsoon season, attributable to meteorological reasons and excess river discharge, at stations Cox Bazaar and Charchanga. The locations Hiron Point and Khepupara show elevated predicted tides after the onset of monsoon, indicating the presence of seasonal signature resulting from large amplitude of the annual tidal component ‘Sa’. Residual time series produce the de-tidal water level variations attributed due to meteorological effects such as wind, atmospheric pressure and river discharge. Inspection of residual can help to quality control measured data such as datum level correction, and replacement of data voids. The effective shallow nature in North-Eastern Bay produces partial reflections thereby increasing the tidal range. In addition, the seasonal effects of meteorological forcing along with non-linear shallow water interaction can result in number of higher harmonics. Finally, the correlation of tidal prediction between SLPR2 and measurement show a reasonable good match.