Unveiling integrated geo-hydraulic assessment of river meandering, bank erosion and sandbar dynamics in Alluvial channels

Predictability of river bank erosion in sinuous alluvial channels requires a combined study of the planform processes, hydraulics processes, sediment transportation, and the geotechnical properties of riverbanks. The research paper provides a detailed analysis of the evolution of channels within the Nabadwip-Kalna stretch of the Bhagirathi-Hooghly River (1990-2020). This analysis combines the synthesis of remote sensing, on-field surveys, lab experiments, and numerical model analysis into a multidimensional analysis. GIS was used through the Digital Shoreline Analysis System (DSAS) to measure changes on the bank-lines using historical satellite images of the same period of time. A two-dimensional migration coefficient (MC) model was used to model spatial-temporal changes in channel centrelines, and an RVR Meander was used to develop a model that takes into consideration depth-averaged flow velocity and reach-averaged hydraulic parameters. The characterisation of cross-sectional bathymetry and near-bank hydraulics was based on ADCP. The results of the geotechnical analysis showed that stratified streambanks showed critical shear stresses of 7.1-7.7 kPa, internal frictional angles of soils less than 30°–34°, and were predominantly affected by either cantilever collapse or piping as a result of varying maximum heights of streams between 5.7 and 6.8 metres. Bank stability through both BSTEM and BEHI was assessed, whereas sediment forecasting combined with SWAT to predict overbank flow and a Genetic Algorithm (GA) to estimate the total load. DSAS analysis on bank-line displacement revealed different erosion patterns within 170 transects, showing different RMSE of 0.090 to 0.162 in predicting zone boundaries. The MC method was able to model the 24-year centreline migration patterns, recording changes in the centreline-geometry parameters. Analysis of five cross-sections instrumented found instability and a factor-of-safety ratio of 0.81-0.95, resulting in 4.07-5.85m/yr and 4.35-7.15 km²/yr, respectively, lateral retreat and the eroded areas. Mean collapse rates were 0.125 to 0.198 m/yr, and the failure angle was 81°–87°. The maximum bank-failure mass was 41.24 kg (seasonal maximum), and the calibrated toe-scour mass was 0.28 kg. The GA model was tried using ten parameterisations and demonstrated the best prediction ability with the coefficient set at ten, where R² = 0.96 and mean relative error (MRE) = 42% gave significantly better performance than the traditional regression analysis (R² = 0.87 and MRE = 40%). There were also considerable changes in the area behind sandbar dynamics, that is, Nandai-Hatsimla increased by 11.87 ha in 1990 to 19.05 ha in 2020; Media by 39.7 ha to 57.68 ha; Char Krishnabati by 82.52 ha to 81.07 ha. Land-use/land-cover (LULC) predictions for 2040 indicated settlement expansion from 13.61% (2020) to 20.19%, with validation accuracy (RMSE = 0.253) confirming model reliability. This combined model shows that the combination of remotely sensed, field, laboratory, and model data provides quantitatively sound estimations of fluvial risks and forms the basis of evidence-based management of high-suspended riverine areas. The modular design can be applied to monsoon-dominated alluvial basins throughout the globe, which will promote adaptive land-use planning and long-term infrastructure development in the vulnerable riparian societies.