A Multi-Objective Hydraulic Optimization Framework for the Rehabilitation of Urban Stormwater Drainage Networks

Urban stormwater drainage networks in old urban districts often suffer from insufficient drainage capacity, adverse pipe slopes, and outdated design standards, which increase the risk of urban waterlogging during heavy rainfall events. To address these issues, this study proposes a multi-objective hydraulic optimization framework for the rehabilitation of urban stormwater drainage networks by coupling SWMM hydrodynamic simulation, an improved bottleneck index method, and the NSGA-III algorithm. The proposed bottleneck index integrates the ratio of actual hydraulic gradient to pipe slope with pipe filling ratio (h/D), enabling the identification of pipes with insufficient drainage capacity and unreasonable construction conditions. The Sanjie River catchment in Fuzhou, China, was selected as the study area. The results indicated that the maximum proportion of bottleneck pipes increased from 33.28% to 41.62% as the design storm return period increased from 1 to 30 years, while the rate of increase gradually diminished under larger return periods. Multi-objective optimization results showed that increasing the maximum allowable rehabilitated pipe diameter from 1.2 m to 1.5 m yielded only limited additional improvement in drainage performance, ranging from 2.06% to 4.11%, whereas the rehabilitation cost increased by 3.10% to 4.41%. Therefore, 1.2 m was identified as the more cost-effective maximum allowable diameter. Under this condition, the rehabilitation schemes for the 3-, 5-, 10-, and 20-year design storms reduced nodal overflow volume by 36.56%, 33.67%, 31.24%, and 27.24%, respectively. The proposed framework provides a systematic and practical approach for diagnosing and rehabilitating existing urban stormwater drainage networks.