Tidal impact leading to sedimentation at lower reach of Rupnarayan River, West Bengal, India

During the last 20 years the lower reach of the Rupnarayan River has experienced a net shoaling of 26.57 million m(3) (survey made by Kolkata Port Trust, 1992-2012) with 42.04 million m(3) shoaling and 15.47 million m(3) scouring. Bathymetric close grid survey of the study area was made using Leveling instrument and Echo sounder interfaced with Position Fixing System (GPS). Simultaneous tidal observations were available at an interval of one hour, velocity of high tide and low tide, tidal asymmetry and discharge of water etc. are measured at different Gauge Stations in the field. Swifter and stronger high tide results more available energy during high tide than as low tide. The incoming high tide brings a lot of sediments from the downstream and deposits it on the bed of the studied river in absence of any upland discharge during the non-monsoon months. During the freshet (July, August and September) the impact of high tide is felt less due to the voluminous anti-directional flow of the upland discharge. Tidal range is more near Geonkhali (4.5m) and decreases towards upstream, being lowest near Kolaghat (2.9m). Tidal asymmetry increases from downstream (2 hours at Geonkhali) to upstream (6 hours at Kolaghat). Tidal prism (volume of water that enters a tidal region during flood flow) is very high in that region.