Flood modeling can make a difference: Disaster risk-reduction and resilience-building in urban areas

Abstract. Surat, India is a coastal city with a population of approximately 4.5 million people that lies on the banks of the river Tapi and is located 100 km downstream from the Ukai dam. Given Surat's geographic location the city is repeatedly exposed to flooding caused by large emergency dam releases into the Tapi river combined with high tide water levels. Flood events of this type occur twice a decade, but their frequency and magnitude may increase due to the urbanization, encroachment in flood plain and climate change. A first step towards strengthening resilience in Surat requires a robust method for mapping flood exposure at fine spatial resolution. Here, in this study we have developed such a method for Surat using a reduced-complexity hydrodynamic model to simulate flooding, but is easily transferable to other urban locations. Our method features three distinct phases that involve: (1) modelling dam release discharge from the Ukai dam arriving at Surat, (2) modelling flooding within Surat caused by the combination of dam release and tides, and, (3) identifying Surat critical infrastructure, population, and income groups exposed to flooding. Our flood model of Surat utilizes topography produced using elevation data collected from an extensive survey. Within the city we have modelled flood scenarios that represent the uncertainty in flood peak discharge and duration resulting from possible climate change. These scenarios include catastrophic conditions that flood 50 % of the city and expose > 60 % of the population and critical infrastructure to deep flooding. Finally, we highlight how our modelling has contributed to changes in flood risk management within the city following a major flood and resulted in actions that have increased community resilience to flood hazard.