Estimating Stream Piracy in the Lower Ganga Plain of a Quaternary Geological Site in West Bengal, India Applying Sedimentological Bank Facies, Log and Geospatial Techniques

River Bhagirathi is a distributary channel of River Ganga, and Babla is one of its right-bank tributaries in the Ganga delta plain. The Bhagirathi flows in a meandering pattern through the New Quaternary alluvium plain, whereas the Babla flows through the Old Quaternary deposits of yellowish-brown sand, silt and clay. As the Bhagirathi is a distributary of River Ganga, it brings in million tonnes of sediments annually. Mid-channel bar formation, bar dynamics and non-cohesive nature of bank materials promote bank failure and rapid shifting of the channels in the Newer Quaternary sites. As the Bhagirathi migrates towards River Babla, the interfluve zone is being progressively engulfed by the course of the former. To estimate the stream piracy or capture, bank materials, internal organization of mid-channel bar, flow parameters (velocity, channel depth, discharge, etc.) have been taken into consideration. This study documents changes in channel morphology and hydrological parameters, dynamic morphology and hydrological regime of the fluvial system from 1852 to 2017. It predicts the actual time when the Bhagirathi will capture River Babla near village Natungram. The study also discusses why in recent years the rate of lateral migration has increased for a particular portion on the right bank of River Bhagirathi near Natungram.