An increase in frequency and magnitude of urban floods is reported across the world, and the design of efficient urban storm sewers is a preliminary step in controlling pluvial flooding. A design storm approach is widely used for this purpose due to its simplicity and availability of the Intensity-Duration-Frequency relationships for many cities. However, the design storm approach accounts for the probability of failure and not the consequences. In this study, a novel safe-fail approach was developed to design the urban stormwater drainage system to occur minimal flood consequences during a failure. At first, a series of statistical and stochastic procedures were developed to identify and model the extreme rainfall events that were further used in flood-scenario simulations. The safe-fail design is a two-step approach; in the first step, the stormwater drainage system was designed considering a 2-year failure risk. In the second step, an iterative optimation was carried out to identify the optimal design return period to achieve the safe-fail criteria, where the flood consequences are minimal. A safe-fail criterion is a designer's choice of flood depth, duration, and frequency considered to be safe in a given urban area. The hydrodynamic model HEC-RAS was used to simulate flood inundation. A case study in Chennai City of India reveals that for safe-fail criteria of 10 cm depth, 30 min duration, and 15 y frequency, the design return period should be 5 years instead of the conventionally used 2 years to have minimal flood consequences. The framework is computationally efficient and can help develop a sustainable urban stormwater drainage system.
