A consequence-based safe-fail approach for decentralised urban stormwater management for flood mitigation

Urban flooding continues to intensify globally due to the combined effects of climate change–driven extremes, unplanned settlement, and rapid urbanisation. Conventional approaches for the design of urban stormwater management structures rely on fixed design storms and fail to integrate flood consequences. In densely settled areas, there is little scope to augment existing designs to cope with climate change, demanding innovative decentralised solutions.

In this study, we extend a safe-fail, consequence-based design framework by explicitly integrating decentralised urban water management strategies within a sponge city paradigm. The proposed framework shifts the design objective from flood prevention to controlled failure with minimised flood severity, accounting for both centralised drainage networks and distributed blue infrastructure. An event-based simulation framework is developed to evaluate a wide range of extreme rainfall scenarios under present and future climate conditions, along with potential decentralised house-level water management strategies.

The method was applied to 100 cities in India that are part of the Government of India’s Smart Cities programme. Three decentralised water storage scenarios—(1) full-store, (2) constant release, and (3) smart (capacity-aware) release—were tested across all cities. The results indicate that, on average, a storage capacity sufficient to capture 10–15 mm of rainfall per unit area of the urban environment can reduce nearly 75% of the flood volume under the capacity-aware scenario. Corresponding values were 25–30 mm and 30–40 mm for the constant release and full-store scenarios, respectively.

The results highlight the potential of decentralised solutions for flood mitigation in urban areas and suggest the need for careful policy and governance interventions.