Right size, right place: scale-dependency of managed realignment in an urban estuary

Managed realignment, the landward relocation of primary flood defences, is increasingly recognised as a sustainable approach to mitigating tidal flood risk in estuaries. However, the effectiveness of realignment relative to the size and location of intervention, and in relation to estuary size, remains poorly understood. This knowledge gap is critical, especially for urban estuaries where space for large-scale, nature-based interventions is limited. This study explores the scale-dependency of managed realignment using a 2D TUFLOW hydraulic flood model of the Clyde estuary, a large, meso-tidal urban estuary on Scotland’s west coast. Analytical solutions and existing flood models from eight other UK estuaries complement this analysis to facilitate comparisons between estuaries of a range of sizes. Our findings reveal that managed realignment exhibits scale-dependent behaviour: the effectiveness of managed realignment to reduce tidal flood risk is linearly proportional to the ratio of the size of the managed realignment to the estuary size. Larger estuaries, like the Clyde, require significantly more extensive realignment to achieve meaningful tidal flood risk reduction. Conversely, smaller estuaries achieve similar benefits with comparatively smaller interventions as they are more sensitive to geometric changes. Additionally for the Clyde, we also found that reconnecting a previously plugged palaeo-channel is more effective at reducing tidal flood risk than relocating primary flood defences. The results imply that a well-chosen location and size of realignment are needed to have a positive impact on reducing tidal flood risk in an estuary; this can be challenging due to existing land uses in highly urbanised estuaries. Hydrodynamic modelling will provide powerful tools to aid decision-makers and avoid risks of maladaptation, supported by long-term monitoring. Given the growing global adoption of managed realignment, this study offers critical insights into the scale-dependent behaviour of this strategy, helping to refine its implementation in diverse estuarine contexts.