Identification of flood vulnerability zones using hydrodynamic River modeling in the lower Godavari basin in Peninsular India

Each year, the monsoon rainfall in Peninsular India causes frequent floods and extensive damage to life and property, aggravated by rapid population growth and unplanned development. The largest river in the region is Godavari with a length of 1450 km. It is subjected to frequent flooding every year especially in the lower reach of the basin. Identification of flood vulnerable areas in this reach would enable flood plain zoning and better management of the damages associated with the floods. For this study, hydrodynamic modeling is carried out using MIKE Hydro River model in the lower reach of Godavari river from Perur (18^o35'14" N, 80^o26'10" E) to Polavaram (17^o15'07" N, 81^o38'53"E). The extreme value analysis has been conducted for the discharges at Perur to arrive at 25 year and 100 year return period flooding discharges which serve as upstream boundary conditions for the model and stage discharge rating curve at Polavaram was taken as downstream boundary condition. For the preparation of the model, water level and discharge at the various stations along the study reach were obtained from insitu Central Water Commision (CWC) Hydrological Observation (HO) data. SRTM 90m DEM data was used to extract river networks and cross-sections. The calibration and validation of the model was carried out with the help of the water level at a HO station located in the reach by tuning the Manning's Roughness parameter (n). The efficiency of validation was estimated based on model performance statistics such as R², Nash-Sutcliffe Efficiency (NSE) and RMSE values. Model outputs showed that the areas in the downstream section of the lower Godavari basin from Dummugudem (17^o51'2.88" N, 80^o53'10.32''E) to Polavaram (17^o15'07" N, 81^o38'53"E) are prone to larger inundation extents as compared to the upstream section of the study reach. Further, the blue lines and red lines corresponding to the 25 year and 100 year return period flood were identified along the river banks for planning of developmental activities in this region as well as the design of flood protection works.