Optimising Size of Natural Diversion Channels for Sustainable Flood Mitigation in Floodplain Townships of Regional Australia

Natural diversion channels offer a promising solution for flood mitigation within the context of natural flood management; however, their design must balance hydraulic performance and intervention scale. This study presents an integrated framework for the optimal design of modified natural diversion channels by coupling two-dimensional hydrodynamic modelling with surrogate modelling and multi-objective optimisation. A Hazard Estimation Index (HEI) is derived from 2D flood hazard maps by aggregating the spatial distribution of flood hazard within the township and agricultural floodplain areas, providing a quantitative measure in which higher values indicate greater flood hazard. Multiple Linear Regression (MLR) models are developed to approximate HEI as a function of flood peak discharge and channel cross-sectional area. Separate HEI formulations are defined for township and floodplain areas to reflect differing mitigation priorities. The surrogate models are integrated within a Multi-Objective Particle Swarm Optimisation (MOPSO) framework to minimise flood hazard in both domains and channel size, with channel cross-sectional area used as a geometric proxy for excavation cost rather than as a direct estimate of construction cost. Three independent optimisation systems are developed for minor-to-moderate, major, and very major flood regimes based on flood-frequency analysis. Application to Moree Plains, Australia, shows that minor-to-moderate floods can be addressed with an optimal channel size of approximately 425 m2, achieving HEI values of approximately 0.18 in the township and 0.07 in the floodplain. For major floods, a larger channel of approximately 1016 m2 results in HEI values of approximately 0.35 and 0.26, respectively. For very major floods, a substantially larger channel of approximately 1620 m2 is required, while township HEI remains relatively high (>0.5), indicating substantial residual risk. The results demonstrate diminishing hazard-reduction benefits with increasing channel size for larger floods and highlight the value of a regime-specific optimisation approach for supporting risk-informed natural flood management planning.