Understanding Complex Processes of Flood Inundation in a Regulated Coastal Lowland River: A Modelling Framework Integrating Hydrological, Hydraulic, Tidal and Geomorphic Data

Inundation dynamics in lowland rivers are an outcome of a complex interaction between hydrologic and geomorphic factors. The co-occurrence of river flow and tidal surge in coastal regions further complicates these interactions. The extent to which these factors mediate inundation mechanisms in regulated lowland river reaches remains challenging and has been poorly understood. This study presents a novel framework that examines the combined effect of flood drivers (riverine and high tide) and geomorphic factors in determining the inundation mechanism along the Lower Tapi Basin (LTB) in western India through an integration of hydrological, tidal, hydrodynamic, and geomorphic datasets. Using variable infiltration capacity (VIC) and Hydrologic Engineering Center River Analysis System (2D HEC-RAS) with geomorphic characteristics of river channels, we explore how spatial inundation dynamics vary across different valley settings under different flood return periods. Our integrated modelling framework accurately simulates daily streamflow and flood inundation. Incorporation of multidisciplinary data through the integrated modelling framework has advanced our understanding of flood inundation dynamics in lowland rivers. We have further shown that the model uncertainty can be reduced by incorporating field data, bathymetric measurements and rule curve of reservoir operation in the model domain. Although this modelling framework has been developed on a specific lowland reaches of the Tapi River, it can be further extended to any lowland rivers in similar settings. Such multidisciplinary models will help to analyse river response to future projected climate and sea level change.