POLICY BRIEF: Hydrodynamics Survey Along Embankments of Kumirmari Island, Gosaba, Indian Sundarban
By Cai Ladd, Sumana Banerjee, Thorsten Balke, Abhra Chanda, and Tuhin Ghosh
We conducted a Hydrodynamics Survey along the embankments of Kumirmari Island in the Indian Sundarban. The West Bengal Irrigation and Waterways Department has identified the island as being at high risk of embankment failure due to coastal erosion and previous incidents of overtopping and breaching during cyclones Amphan and Yaas.
Our study quantifies the exposure of the island's embankments to current and wave forcing and has gathered community perceptions of the causes of coastal erosion and flooding. Using innovative methods, including the use of Mini Buoys, the study offers a preliminary understanding of the hydrodynamics around Kumirmari Island and highlights the need for year-long hydrodynamic surveys to better inform coastal protection design.
Key findings show that mud embankments experienced greater exposure to currents and waves compared to concrete embankments. At the peak of south-west Kumirmari, where the mud embankments are without vegetation are, there was the greatest level of exposure. As Southern Kumirmari has suffered the highest rates of coastal erosion compared to other areas on the island throughout the last decade it calls for special attention when developing coastal flood risk mitigation strategies. The study also found that sparse mangroves and saltmarsh vegetation supplied very little or no reduction to the force of currents and waves.
We suggest that potential solutions for coastal erosion risk management may include:
creating space for mangrove establishment by realigning existing embankments further inland, allowing the land in front to flood, creating a ‘double buffer’ effect
targeted embankment breaching to form tidal beels
using other novel approaches, such as sediment arrestors and wave breaks
increasing sediment accretion to build coastal habitats seaward
adopting a ‘hard engineering’ approach, to increase the extent of concrete embankments.
long-term hydrodynamic monitoring to select suitable management interventions.
There are, of course, pros and cons to each potential solution which we explore further within this policy brief.