Dam-break flood hazard and risk assessment of large dam for emergency preparedness: A study of Ukai Dam, India

The large dam at the upstream of the densely populated city required due attention on its failure due to catastrophic consequences in the downstream area. The densely populated city of Surat, India, is situated 100 km downstream of the large Ukai Dam having a storage capacity of 7414.2 Mm3. 3 . The potential failure of the Ukai Dam could lead to substantial damage in and around Surat city, underscoring the necessity for dam-break flood modelling and associated risks for emergency action plans. In the current study, the Ukai Dam failure is simulated to derive the dam-break flood hydrographs by integrating an empirical dam breach equation with a hydrodynamic model for piping as well as overtopping conditions. The simulations for different (36 Nos.) scenarios by considering the effects of reservoir level, inflow with an early peak, and erosion of the earthen embankment indicates the sensitivity of the considered breach parameters on the outflow hydrograph. The peak discharge resulting from the Ukai Dam failure at full reservoir level (FRL), highest flood level (HFL), and 1 m overtopping are 4.6, 5.0, and 7.5 times higher than the maximum observed flood recorded after the construction of the Ukai Dam. The effect of dam-break flood wave propagation in the lower Tapi River and Surat city is evaluated using the well-calibrated 2D HD model. The dam-break flood simulation, corresponding risk assessment revealed that the maximum area of the city, including existing critical infrastructure, was flooded with water depths more than 1.2 m. Nearly 50 % of the considered area is exposed to significant flood hazards, exhibiting threats to people, vehicles, and building safety. The socio-economic vulnerability and exposure analysis for the wards and villages within Surat Urban Development Authority (SUDA) administrative boundary revealed that, 30 % and 22 % area is under high to very vulnerable and exposure category. Ultimately, it leads to nearly 25 % area under SUDA is under high-risk category. The developed inundation, hazard, and risk maps will be useful to local and disaster management authorities to develop plans for response to recovery practices, emergency action plans (EAPs), and prioritize the wards and villages during the flood mitigation with scarce resources.