Abstract <p>This study analyzes the flow behavior of the Narmada River to estimate flood levels and develop flood inundation maps for the Upper Narmada Basin Reach. Flow behavior is assessed using the open-source software HEC-RAS with a GIS interface for both 1D steady and unsteady and 2D unsteady flow approaches. The hydrodynamic models are developed for a 78 km stretch of the Narmada River, extending from Barmanghat to Sandiya. River geometry is created using a 30 m resolution SRTM digital elevation model, with cross-sections obtained at appropriate intervals. For the steady flow, Gumbel’s flood frequency analysis is conducted using the previous 30 years of annual discharge data to estimate discharge magnitude for return periods of 5, 10, 20, 50, 100, 500, and 1000 years. The model is validated with data from the Central Water Commission for 2019. The channel roughness is verified by comparing flow and stage data at downstream gauging sites. Hydraulic parameters such as water depth, velocity, water surface elevation, and inundation area are analyzed through 1D and 2D simulations and presented as maps. The results are utilized to create flood inundation probability maps. The results demonstrate that using DEM-based river geometry enhances the efficiency of hydraulic modelling, and variations in cross-section spacing influence the simulation outcomes. The model validation under extreme discharge conditions proved effective for flood prediction, and the inundation maps revealed the vulnerability of the floodplain, providing valuable insights for flood risk management. The methodology aids in flood inundation simulation using HEC-RAS aiding flood mitigation and management practices.</p>