Assessment of riverbank erosion and erosion probability using geospatial approach: a case study of the Subansiri River, Assam, India

Riverbank erosion is one of the key geomorphological problems encountered in the floodplains of the alluvial rivers. Many recent studies on fluvial dynamics have indicated advantages of geospatial technology over traditional techniques in terms of time, cost, and practical usability by the end-users. This study aims to assess the riverbank erosion and erosion probability in a highly dynamic and unstable stretch of the Subansiri River in Assam (India) using geospatial approach along with the Graf's model. Temporal Landsat datasets for a period of 29 years (1989 to 2017) in time step of 4-5 years are used for mapping the river channels (active floodplains) of the Subansiri River. These river channel datasets were then analyzed to spatially quantify the erosion/aggradation and identify the high riverbank erosion zones. Identification and analysis of the high riverbank erosion zones revealed a general westward shift of the Subansiri River during the studied period. The Graf's model, used for estimating the riverbank erosion probability, is implemented in geographical information system (GIS). The transition probability matrices for riverbank erosion were generated for different time spans (1989-1994, 1994-1998, 1998-2002, 2002-2006, 2006-2010, and 2010-2014) using the distance to river channel and erosion/aggradation maps prepared using remote sensing data. Flood recurrence intervals of the annual floods from 1988 to 2017 were estimated using observed discharge data. The transition matrices and flood recurrence intervals were then used to calibrate the Graf's model for estimating the probability of riverbank erosion of the Subansiri River. The results were validated with observed erosion/aggradation map of 2014-2017 time period. The study demonstrates the strength of geospatial approach for rapid assessment of riverbank erosion of alluvial channels. The calibrated Graf's model developed in this study along with understanding of the migration behavior of the Subansiri River will be useful for taking mitigation measures and planning river management strategies.