Vertical current structure in a macro-tidal, well mixed Sundarban ecosystem, India
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Vertical current structure in a macro-tidal, well mixed Sundarban ecosystem, India J. Selvin Pitchaikani 1,2 Received: 23 April 2020 / Revised: 17 September 2020 / Accepted: 24 September 2020 # Springer Nature B.V. 2020
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. Keywords Sundarbans . Jharkhali . Tidal constituents . Current profile . Tidal current
Introduction Sundarbans is the largest deltaic tidal halophytic mangrove forest in the World (Blasco 1977), situated at the confluence of two major Himalayan rivers Ganges and Brahmaputra. This region is dominated by the monsoon, tidal currents, and often battered by cyclonic winds, floods, storms, and submergence (Ganguly et al. 2006). The geographic position and geographic conditions of the Sundarbans have made them susceptible
* J. Selvin Pitchaikani [email protected] 1
Present address: National Centre for Sustainable Coastal Management (NCSCM), Ministry of Environment, Forest and Climate Change (MoEF& CC), Anna University Campus, Chennai, Tamil Nadu 600025, India
2
ICZM Project, State Project Management Unit, Institute of Environmental Studies & Wetland Management, DD-24, Sector-I, Salt Lake City, Kolkata, West Bengal 700 064, India
to river and rainwater flooding, tropical cyclones accompanied by storm surges, tornados, nor’westers, drought and riverba
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