Dredging and confinement limit estuarine geomorphological recovery and shape future flood risk in the Firth of Clyde.

Low-lying coastal estuaries support some of the world's most densely populated regions. Such estuaries yet are increasingly constrained by dredging and channel confinement due to human development and river modifications, with uncertain consequences for flood risk and long-term (centuries) geomorphic stability. Using multiyear high-resolution bathymetric surveys combined with well-calibrated flood models of the Clyde Estuary, United Kingdom, we investigate how channel confinement and dredging collectively affect flood propagation and estuarine morphodynamics. Geomorphic changes observed between 2016 and 2025 show pronounced spatial and temporal variability. The inner estuary initially experienced net deposition before transitioning to net erosion, driven by variability in fluvial sediment delivery, whilst the outer estuary maintained a balance between erosion and deposition. Despite measurable bed-level changes, modelled flood propagation remained largely unchanged, with only minor effects linked to inner-estuary deposition. Regime theory reveals that the Clyde Estuary exhibits a spatially varied equilibrium state, with the outer estuary in near geomorphic equilibrium and the inner estuary in disequilibrium due to historic channel confinement designed to deepen the channel for shipping. Channel confinement decreases channel cross-sectional area and enhances the system's capacity to erode its bed, suggesting the onset of adjustment toward a new equilibrium. This study highlights the importance of diagnosing geomorphic equilibrium for understanding potential future changes in flood dynamics and supporting sustainable flood-risk management in heavily modified estuaries under a changing climate.