Vertical current structure in a macro-tidal, well mixed Sundarban ecosystem, India

Abstract

The present study reports on the vertical structure of the current profile of the Jharkhali of Sundarban ecosystem. The clay embankments constructed along the Sundarbans for coastal protection are facing severe erosion problem due to tidal current, extreme water levels, storm surges, overtopping and breaches in embankments which lead to widespread flooding. The understanding of the current profile of Sundarban aquatic system with respect to depth of water column is essential for suitable engineering design for embankment. It will be used for conservation of coastal resources and inhabitants of the Sundarbans from the natural disasters. The tidal range during the present study was 6.1 m and 5 tidal constituents viz. M2, S2, K1, O1, and N2 are predominant at Jharkhali. The minimum (114.55 cm/s) and maximum (134.45 cm/s) current speed were observed at bottom and mid depth respectively. The maximum speed at bottom is also due to longitudinal salinity gradients, which may induce larger velocities in the lower part of the water column. The vertical velocity profiles during flood are characterized by a near bed maximum. The maximum speed usually occurs during mid-flood and mid-ebb periods and also varying with moon phase, usually maximum and minimum speed were observed during spring tide and neap tide respectively. This trend is persisting in the entire water column from the surface to bottom. The direction of the current is controlled by orientation of the river channel and adjacent tributaries. In general, water travels towards North and WNW direction during flood tide and E & ESE direction during ebb tide. Interestingly, about 80-degree difference in current direction was noticed between surface and bottom current with clockwise spiral movement.