Salinity ingress and embankment choice on a Sundarbans island
On an island in the Indian Sundarbans, a mud embankment runs along a tidal creek. Records show a spring tidal range of 4.6 m and a neap range of 2.1 m. Since the Farakka Barrage began diverting water, dry-season freshwater discharge reaching the estuary has fallen, and creek salinity has risen from about 12 ppt in 1990 to 24 ppt today. Paddy yields on the island have dropped and drinking-water ponds turn brackish in April–May. The embankment breached during a cyclone and was rebuilt with local mud. A proposal before the panchayat is to replace it with a 4 m concrete wall along the same alignment. An alternative is to move the embankment 150 m inland, restore a belt of mangrove and mudflat in front, and fit a sluice gate in the new embankment. About 60 households farm the land between the present alignment and the proposed setback.
1. Explain how reduced river discharge and tidal action together raise creek salinity, and why the problem peaks in the dry season and at spring tides.
Read the worked answer
An estuary's salt balance depends on seaward freshwater flow opposing landward tidal transport. When Farakka divertes water, the dry-season river flow reaching the creek falls, so the residence time of water in the estuary lengthens and salt is not flushed out. Flood tides carry saline water upstream; with a weaker ebb flow, less salt is returned to sea. Density differences also create a salt wedge that moves upstream along the bed, and tidal pumping moves salt into creeks and ultimately into groundwater and ponds. Salinity peaks in April–May because river flow is lowest and evaporation is highest, concentrating salts in shallow water. It peaks at spring tides because the tidal range of 4.6 m drives a larger tidal prism and stronger flood currents than the 2.1 m neap range, pushing the salt front further inland. The rising trend from 12 to 24 ppt is therefore a combined effect of reduced flushing and tidal import, not a local rainfall failure alone.
2. Compare the concrete wall and the set-back mangrove option as coastal protection. Which would you recommend, and under what conditions?
Read the worked answer
A concrete wall is hard engineering: it reflects wave energy but can scour at its toe and ends, cuts off sediment supply to the fronting mudflat, and fails catastrophically when overtopped by a cyclone surge. It is expensive to build and maintain on soft mud and gives no ecological benefit. A set-back embankment with a mangrove and mudflat belt is soft engineering: vegetation dissipates wave energy, traps sediment, encourages vertical accretion and provides a storm buffer and nursery habitat. It requires land and time, and may not keep pace with rapid sea-level rise or low sediment supply. My recommendation is the set-back option, with a sluice gate managed to admit controlled tidal water and sediment, provided the 60 households are compensated and involved and sediment supply is monitored. A hybrid is realistic: set-back and mangrove for most of the alignment, with a well-founded concrete section only where a critical asset cannot be moved.
3. List four measurements you would collect over one year before the panchayat votes, and state what each would decide.
Read the worked answer
First, water level and salinity at 15-minute intervals at two creek stations across spring–neap cycles: this shows how far salt moves upstream and when, and decides sluice operation and whether the set-back can be flushed. Second, suspended sediment concentration and bed-level surveys: this decides whether the mudflat can accrete fast enough for mangroves to establish, or whether the set-back will simply drown. Third, embankment profile, breach history and soil strength: this decides the alignment and whether a mud embankment can be maintained without a concrete section. Fourth, land use, tenure and household income for the 60 households: this decides compensation, feasibility and whether the community will accept the setback. Together these convert a choice between structures into a testable decision about sediment supply, salinity and social cost.