Compound Flood Modeling: Coupling ADCIRC and HEC-RAS for Enhanced Risk Assessment along East Coast of India

Coastal flooding triggered by tropical cyclones is a frequent and devastating threat in low-lying coastal areas. The risk of inundation escalates when storm tides interact with river systems and are compounded by intense rainfall during the cyclone. This vulnerability is further heightened when cyclones land near estuaries, river deltas, or adjacent rivers along the coast. Consequently, understanding these interactions and accurately quantifying their contributions to coastal inundation is crucial for effective inland flood mapping and disaster management. ADCIRC model is one of the practical tools in computing coastal inundation, but it needs to consider precipitation, which plays a major role during flooding. The HEC-RAS model is coupled with ADCIRC to solve this issue to provide realistic coastal flooding. Validation of inundation is a very tough task during storm surge events due to the unavailability of an inundation map at the time of landfall. In our experiment, we used coupled ADCIRC and HEC RAS over the significant river estuaries (Hooghly, Mahanadi, Krishna, and Godavari) along the east coast of India since these regions are very vulnerable to storm surges. Significant cyclone landfalling over or near these river systems is selected for computing inundation. To calculate the inundation, storm tides from the ADCIRC model are used as input to the HEC-RAS model. Other parameters like river discharge and gridded precipitation are also incorporated.

Further model capability is enhanced by adding land cover, soil, and infiltration data over these river systems. Fani cyclone is one of the devastating cyclones that significantly impacted the Mahanadi basin. Inundation from the model is validated with the satellite map, which was available two days after the landfall. Model inundation is adjusted by altering the precipitation factor depending on the observed value. The same factor is used for other river basins. Results show that the model is validated reasonably well with the observation and is best suitable for assessing compound flooding along the river basin over east coast of India.