Impact of land surface characteristics on coastal compound flooding using a coupled hydrodynamic-hydraulic modelling framework.

Compound flooding (CF) in any deltaic region is caused by complex interactions between storm tides, river discharge, and precipitation, in which the impact of land-surface parameters is often undetermined. A coupled hydraulic HEC-RAS and hydrodynamic ADCIRC model is used to examine how land surface characteristics affect CF during the Yaas cyclone. The model includes Brahmani, Baitarani, Subarnarekha, and Mahanadi river systems along the east coast of India. CF is estimated using IMDAA, ERA5, GPM precipitation, evaporation, along soil infiltration. Validation with satellite imagery suggests that precipitation is the leading cause of CF. Sensitivity analysis reveals that decreasing the minimum soil infiltration rate and maximizing soil service curve number (SCN) leads to more CF due to reduced water penetration and increased runoff. Experiments are conducted to calibrate district-wise inundation with observations. By considering suitable minimum infiltration rates and SCN values, 50% increase in CF is achieved compared to the default configuration. Calibration with Sentinel-1 SAR enhances agreement on inundation, with a correlation of 0.97. Additional simulations with two more historical cyclones demonstrate that incorporating precipitation results in twice the inundation compared to river discharge and storm tide forcing. The present study is the first-of-its-kind to utilize a coupled ADCIRC-HEC-RAS with soil infiltration for CF in Indian context, highlighting importance of key drivers, uncertainties involved, and potential solution to improve model performance.