As home to ∼60% of the global population, low-lying coastal estuaries are increasingly vulnerable to storm tide flooding, especially under accelerating anthropogenic sea-level rise. Conventional approaches in estuarine flood mitigation have involved building and heightening hard-engineering structures. Recent evidence from small- to medium-sized estuaries highlights that managed realignment (MR)—relocating flood defences landward—is potentially more sustainable. However, the effectiveness of such MR on estuary-scale flood hazard reduction in large, urbanised estuaries where space for large-scale MR is limited remains poorly understood. To address this gap, we applied a calibrated 2D hydrodynamic simulation of the large (120 km2) urban Clyde estuary on Scotland’s west coast and compared it with existing simulations from eight other UK estuaries of varying size. Our simulations of varying MR sizes (0.15–3.3 km2) along the Clyde resulted in minimal estuary-scale reductions in peak water levels, flood extent, and tidal prism volume. In contrast, similar realignment measures applied to much smaller UK estuaries (3–108 km2) yielded more significant estuary-scale flood attenuation. Our results show that MR exhibits scale-dependent behaviour: the effectiveness of estuary-scale flood risk reduction is proportional to the size of MR relative to estuary size. Larger estuaries require significantly more realignment space to achieve meaningful estuary-scale flood hazard reduction. Conversely, smaller estuaries achieve flood hazard reduction benefits with comparatively smaller interventions as they are more sensitive to geometric changes. The scale-dependent behaviour identified here requires consideration when designing tidal flood risk alleviation strategies for estuaries around the globe, particularly where existing land uses and plans in highly urbanised estuaries may limit available space and/or increase the cost of realignment. Moreover, it also provides evidence that avoiding buildings (re)-development in the tidal floodplains of large estuaries is the best means of minimising future flood risk in a rapidly warming world.

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