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 m 3 of tank storage within the city. The remaining tanks, holding 174.7 million m 3 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. • Novel multidimensional impact analysis to quantify water bodies role in flood control. • Economic and people-centric perspectives for easy knowledge transfer to policy makers. • Protecting water bodies could reduce damages by 17 % and population at risk by 25.3 %. • Existing water bodies if lost increases damages (44 %) and population at risk (40.5 %). • Assessment reveals water bodies play key role in flood resilience of Chennai city.