Climate-induced hydraulic stress and geomorphological changes: an assessment of vulnerability of critical river infrastructure in the data-scarce basin, India.

Climate non-stationarity increasingly threatens river infrastructure, yet its quantified impact on bridge pier scour in data-scarce tropical basins remains poorly constrained. The study evaluates the climate-induced modification of the river hydrology, hydraulics and channel behaviour of the data-scarce Rupnarayan river basin in the peri-urban zone of eastern India using a case study of the Kolaghat Bridge. A high-resolution model, the Max Planck Institute Earth System Model version 1.2 Higher Resolution (MPI-ESM1-2-HR), was downscaled, bias-corrected, and coupled with a Soil and Water Assessment Tool (SWAT) model giving good skills (NSE = 0.90; R2 = 0.86) for discharge well. Three Shared Socioeconomic Pathways (SSP245, SSP370, SSP585) were used to project flow and sediment series which were used as input to a Hydrologic Engineering Center River Analysis System (HEC-RAS) 2D model that computed water-surface elevation (WSE), energy gradient and bed shear stress for 2- to 100-year return periods. The observed record (1960-2020) has no significant trend, but each pathway exhibits a large difference: mean annual temperature and rainfall increase by + 1.31 °C and + 22.1% by 2100 under SSP585. The three-model comparison shows an increase of 11-18% in critical shear stress and energy gradient at this bridge (~ 15% for MPI-ESM1-2-HR model). The HEC-18 analysis results indicate that total pier scour increased from ~ 6.07 m to ~ 10.93 m (~ 80%), which would diminish foundation safety margins, while the reduced high-frequency variability in WSE suggests increased sedimentation, channel narrowing, and overall channel sinuosity (1.08 to 1.67). In summary, under the high-emission SSP585 pathway by 2100 the critical shear stress at the Kolaghat Bridge rises by 11 to 18 per cent and total pier scour increases by about 80 per cent, from about 6.07 to about 10.93 m, which erodes the foundation safety margin below the code threshold and makes a clear case for climate-adjusted hydraulic design loads together with regular bathymetric monitoring and planned dredging.