Plan Shape Geomorphology of Alluvial Valley in the Middle-Lower and Deltaic Courses of the Subarnarekha River Basin, India

Abstract

The middle-lower and deltaic courses of the Subarnarekha river from Jamsola (upstream) to Chaumukh (mouth) have been considered for the study of plan shape geomorphology partially along the alluvial valley floors between bank margin environment. Geologically and topographically, the depositional environments comprise with Lalgarh formation, Sijua formation, Panskura formation, Basudebpur formation, Daintikri formation, and beach ridge chenier formations of fluvial, fluvio-marine and marine depositional processes with the seaward gradient from Jamsola to Chaumukh. Distinctive sub-environments of the upstream fluvial dominance, ancient delta-fan lobe extension, and areas of sea level fluctuations with lower deltaic beach ridge chenier at downstream section are categorized based on the identified assemblages of landforms in the present study. The geospatial techniques, repeated field observations, Total Station survey and sedimentological analysis of bank margin stratigraphic sections have been considered in the study to explore the spatial diversity of plan shape geomorphology in the different sections of the studied river course. The study reveals that the course of the river sections bears diverse geometry of meander properties with discontinuous straight courses and wider valleys. The section-wise plan shape geomorphological features assemblages of 16 categories in the section of Jamsola–Ragra stretch, 18 categories in Ragra–Dantan stretch, 15 categories in Dantan–Rajghat stretch, and 24 categories in Rajghat–Chaumukh stretch in the form of instream deposition, channel fringe deposition and floodplain deposition. Distinct morphological variations of the three major mid-channel bars at the different channel positions indicate the nature of seasonal hydrodynamics and signatures of catastrophic floods. The layer-wise sedimentological analysis of the younger fill terrace and two mid-channel bars shows the trend of discharge and fluctuating flow regimes in the different flood events of single or multiple years. Identification of palaeo-shorelines, ancient delta-fan lobes and cut and fill terraces along the courses of the river valley highlights the role of dynamic marine, fluvio-marine and fluvial environments in the region.