Analysis of compound flooding in the Tapi River Basin

We coupled a hydrologic model Variable Infiltration Capacity (VIC) with the Hydrologic Engineering Center River Analysis System (HEC-RAS-2D) to model the compound impact of flood drivers in the Tapi River basin, India. Our modelling framework consists of two distinct phases; firstly, we calibrate and validate the VIC simulated daily stream flow of the Tapi River using the data observed at the Sarangkheda gauge (upstream of Ukai Reservoir) during the 2005-2012 and 2013-2016, respectively. Secondly, to simulate the high and low flow events, a separate HEC-RAS 2D model is forced with flood hydrograph (Ukai dam release) and stage hydrograph (Tidal level at Hazira) as upstream and downstream boundary conditions, respectively. We calibrated this hydrodynamic model for the 2012 flood event and validated it for the 2006 and 2014 flood events with the observed discharge and water level at the five gauges (Kakrapar Weir, Ghala, Kathor, Singanpur Weir and Nehru Bridge) located along the Tapi River in the Lower Tapi Basin (LTB). We observed that the VIC simulated daily stream flow accords well with the observed in-situ measurements. The Kling-Gupta and Nash Sutcliffe Efficiency values for calibration are 0.84 and 0.86, while, for validation, the values are 0.78 and 0.71, respectively. Furthermore, the hydrodynamic model analysis indicates satisfactory performance with the Root Mean Square Error (RMSE) for discharge and water levels in the range of 300-325 m³s^-1 and 0.12–0.43 m, respectively. Finally, we prepare the flood hazard maps to provide critical insights for effective flood management and to enhance the flood resilience of the flood-prone regions of the LTB.