Hydrological-hydrodynamic nexus for predicting flood plain inundation: a case study in Pagladiya-Putimari catchment, India

Alluvial catchments are susceptible to frequent flooding due to complex geomorphologic characteristics and upstream rainfall. For the implementation of flood mitigation measures, it is essential to have a robust inundation modeling and mapping framework that efficiently capture the hydrological-hydrodynamic nexus of the river and catchment for disaster risk reduction and development of management strategies. Adequate rainfall data, a customized hydrological model for runoff estimation, and a two-dimensional hydrodynamic model for lateral wave diffusion are the major components that effectively model flood inundation. In this work, the HEC HMS model has been customized in a quasi-distributed set-up for runoff prediction in Pagladiya-Putimari basin, Assam, India, for a particular event. The model training, testing, and accuracy assessment are carried out with the observed discharge at the outlet. The computed hydrograph from HEC-HMS is used as an upstream boundary condition in the HEC-RAS 2D model for flood inundation simulation and mapping. The inundation map from the hydrodynamic model was then compared with the flood hazard/occurrence frequency map as circulated by mandated agencies. The computed results indicate three revenue circles that come under the moderate- to high-flood-hazard zone. However, a high velocity of 2.77–3.15 m/s is found in regions close to the embankment breach locations. The present study and outcomes will be useful for effective management and monitoring of floods, especially in flood-prone regions.