Optimised and fast dimensioning of decentralised urban water infrastructures based on design events

The optimal dimensioning of low-impact development (LID) systems remains a challenge in urban drainage design. Despite numerous guidelines and manuals, there is no universally accepted standardised method to determine the required area and storage depth to prevent overflow. Existing methods rely on simplified assumptions that often lead to over-dimensioning or on complex hydrological models that assess the water balance of pre-dimensioned LIDs. However, as these models do not directly determine the optimal dimensioning, planning multiple LIDs at the urban catchment scale is time-consuming. This study introduces a computationally efficient methodology for the optimal dimensioning of LIDs receiving runoff directly from adjacent impervious areas. The method is useful for surface-storage systems, such as bioretention cells. The approach achieves dimensioning results within approximately ±5% of those obtained through more complex recursive water-balance evaluations within the LID control in storm water management model software. Our study also provides guidance on the optimal storage depth and drainage rate required to empty the system within regulatory time limits. The proposed method supports context-specific design by using local hyetographs. Overall, it offers a robust, practical tool for engineers and planners to optimally dimension LID systems, eliminating the need for complex hydrological simulations or overly conservative assumptions.