Lost water bodies and a flooded city – Counterfactual scenarios of the extreme Chennai flood highlight the potential of nature-based solutions

In rapidly growing cities, unregulated urban expansion may encroach water bodies and floodplains, leading to frequent flooding. Chennai's traditional water bodies, known as ‘tanks’, act as nature-based solutions (NbS) for flood mitigation. Since the early 1900s, urbanization has led to the loss of 13.6 million m³ of tank storage within the city. The remaining tanks, holding 174.7 million m³ of water outside the city, are now at risk. We construct two counterfactual scenarios – what if (1) the lost tanks from the early 1900s had been protected during the urbanization (with tanks), and (2) the existing tanks upstream of the city were also lost (u/s no tanks - u/s denotes upstream). These scenarios are analyzed against the current situation (baseline), using the extreme 2015 flooding. The analysis reveals that in u/s no tanks, the potential flood damages rose by 44 % compared to the baseline. Conversely, flood damages decreased by 17 % in the with tanks. The population at risk increases by 40.5 % in u/s no tanks, while it decreases by 25.3 % in with tanks compared to the baseline. Thus, this study highlights the multi-dimensional impact of water bodies in flood control by examining the case of a rapidly expanding city.