River Restoration in the Western Ganges Delta towards Improving Freshwater Availability in Indian Sundarbans

Abstract:

The Indian Sundarbans is a part of the world’s largest mangrove ecosystem and located in the south western part of the Ganges delta. It supports over 4.3 million population. The region is crisscrossed by the distributary network of the river Ganges. However, most of the distributaries have been disconnected from the Ganges system and decayed, therefor do not get freshwater flow from upstreams. The decayed river channels have been converted into ponds, agricultural lands or aquaculture farms and have also been encroached by settlements. Unavailability of freshwater flow restricts downstream ecosystem services of the rivers. Restoration of these decayed river channels may revitalize the river network and enhance the freshwater flow-dependent ecosystem services. In this perspective, the present paper tries to find out the potential of river restoration and reconnection to augment freshwater flow in the Indian Sundarbans. The SWAT model has been used to estimate the present freshwater availability within the system. The HEC-RAS model has been used to reconnect the disconnected rivers hypothetically. The model has been run in the present and in the hypothetical restored scenario to estimate the present water availability in the river channels and water availability after restoration. The study estimates that during lean period Sundarbans receives 10 to 30 cumecs of flow which is not sufficient to nourish the ecosystem. The eastern sector of the Sundarbans receives more freshwater flow during monsoon months than the western and central sectors as the catchment area of the eastern zone is comparatively large. Around 470 to 490 cumecs of flow can internally be managed during lean period through river reconnection and rejuvenation. The estimated flow is mostly adequate to nurture the ecosystem of Sundarbans by reducing the salinity level of the system. To augment additional flow during dry season an international collaboration with Bangladesh is required.