How can hydrological connectivity inform catchment scale stormwater flood management?

Landscape-based stormwater solutions can attenuate runoff locally, yet translating hydrological benefits into catchment-scale flood mitigation often remains difficult. This shortfall reflects a current site-focused paradigm that overlooks the role of hydrological connectivity (HC) in organising flood inundation. HC has long informed process understanding in fluvial hydrology, but it has been less frequently operationalised as a spatial diagnostic for urban stormwater flooding. This study applies the spatially explicit, event-aware Index of Hydrological Connectivity (IHC) to diagnose connectivity controls on urban flooding at the catchment scale and to inform strategic planning. Flood inundation patterns were generated using a validated 2D hydrodynamic model under 10-, 30-, and 100-year design storms. An interpretable machine-learning method (XGBoost) was then trained on these simulated flood scenarios to evaluate the explanatory power of multi-scale IHC features. Results show that spatial aggregation of IHC features markedly improved the representation of the spatial patterns of flooding, with the best performance at a neighbourhood scale of 240 m. The analysis reveals a depth-dependent regime shift. Shallow flooding is storage-limited and governed by local heterogeneity, whereas hazardous depths are conveyance-limited, driven by slope energised inflow interacting with corridor continuity and bottlenecks. The inferred dependence is nonlinear and interaction-rich, with hazard amplified when slope-energised inflow coincides with neighbourhood bottlenecks and weak or highly heterogeneous connectivity, while strong corridor continuity can produce threshold-like reductions in hazard. Overall, IHC offers a diagnostic basis for delineating flood conveyance corridors, connectivity bottlenecks, and slope energised inflow feeders that govern where hazardous inundation initiates, concentrates, and propagates. It therefore helps prioritise intervention locations, enabling site-scale measures to operate as a coherent, catchment-scale system.