Tidal Character

Abstract

Being a part of Hoogly estuary , the lower reach of the Rupnarayan River is affected by semi-diurnal tide of Bay of Bengal. Tidal range, tidal prism, tidal asymmetry and variation of tidal velocity in different phases play dominant role to control the mechanism and rate of sedimentation in the lower reach. Tidal gauge data have been collected at an interval of one hour at different gauge stations. Tidal range increases from upstream to downstream (2.8 m at Kolaghat, 3.75 m at Soyadighi, 3.85 m at Anantapur, 4.05 m at Pyratungi, 4.3 m at Dhanipur and 4.4 m at Geonkhali). Extensive intertidal areas towards downstream receive more water and sediment during high tide than areas towards upstream and enhances the sedimentation rate towards downstream. Mid-tide water velocity is more (1.31–2.15 m/s) in all the places than full-tide water velocity (1.25–2.08 m/s). The mudflats and marshy areas which lie above mid-tide level are flooded over 700 times every year during flood tide and getting sedimented. High tide duration is shorter by 2–6 h than that of low tide and this tidal asymmetry results swifter flow with greater energy during high tide leading to landward transport of sediment. The sluggish low tide discharge over longer duration (8–9 h) allows sufficient opportunity for settlement of sediments.