Spatiotemporal changes of geomorphic environment in the Muriganga–Saptamukhi estuarine interfluve of Indian Sundarban in the context of climate change: a case study

The surface of the earth has been continuously sculptured by the exogenetic processes primarily by the climatic hydrological attributes. Over the sequential phases of evolution of earth, the climatic attributes have altered. The alternative processes such as glaciation and declaciation, fluctuations of sea level and variability in rainfall pattern have changed the surface configuration through maneuvering the operations of the geographical processes. The issue of climate change has become a matter of scientific concern all over the world as the human communities are facing severe problems manifested in the form of flood, drought and submergence of low-lying coastal regions, estuaries and deltas. The present study is an attempt to understand the nature and extent of climate change (1890–2012) and related sea level rise (1970s–2010) in the deltaic Sundarban region of India since the last century and how far they are impacting the erosion–depositional processes (1925, 1926–2014). The changing trend of mean annual temperature, rainfall pattern and frequencies of tropical cyclones has been studied in detail, and spatiotemporal changes of geomorphic environment in different reaches of the study area have also been monitored with available multi-temporal geospatial data. The observation reveals that the flat sandy coastal belt and low-lying island in the extreme southern part have been severely affected by wave dash and seasonal storm surges which are mostly responsible for vigorous erosion. On the other hand, high tidal velocity is observed along the exposed concave bank in the upper–middle reaches of the Muriganga estuary and exposed outer bank in lower–middle reaches of Saptamukhi estuary. The thalweg line, which is very close to the river bank in those reaches of estuary, intensifies the hydraulic pressure along marginal parts of river bank which leads to erosion. Depositional activities are mostly observed along the sheltered inner reaches and convex river banks of the estuaries due to less effective tidal rushes, shallow channel depth, marginal flow velocity and gentle channel slope. Therefore, erosional and depositional processes cannot be explain by the relative mean sea level rise, because increasing mean sea level will affect the erosional processes uniformly in the entire estuary. In this present study area, both the erosional and depositional processes are simultaneously active depending on geographical exposure, sinuous channel pattern, channel depth and lithological compositions.