Dredging impacts on morphological equilibrium and flood propagation in the Clyde estuary

Low-lying coastal estuaries, home for ~60% of the world’s population, are often dredged for navigation. Dredging deepens the estuary channel, hypothetically disturbing the hydro- and morpho-dynamics of an estuary. However, the impact of this activity on the morphological equilibrium state (i.e. balanced state between erosion and deposition) and flood propagation of the estuary leaves a substantial knowledge gap. As such, addressing this gap is fundamental to understanding the tendency of the estuary to bounce back to its morphological equilibrium state (i.e. which part severely needs more sediment), along with hydrodynamic impact on tidal and fluvial floods, especially under current sea-level rise (SLR) projections.

Here, high-resolution bathymetric data from 2016 to 2025 are used to detect morphological changes and to run 2D hydrodynamic simulations under varying tidal and fluvial flood conditions, added with 2100 SLR projection in the Clyde estuary, United Kingdom. In tidal prism theory, scaling between the tidal prism (P) and cross section area (A) represents a dynamic equilibrium state of feedback between tidal channel morphology and hydrodynamics. It is used here to understand the current equilibrium state of the uppermost, anthropogenically constrained part and unconfined, downstream part of the estuary.

In an equilibrium state, erosion and deposition in the system are in balance. Instead model shows that the Clyde estuary has been in a morphological disequilibrium state, during the entire study period. Dredging activity, type of flood, sea-level rise and channel confinement collectively affect the morphological equilibrium state of the Clyde estuary, with the channel confinement showing the most pronounced impact.  Dredging and flood type have a minor impact on the downstream morphological equilibrium of the Clyde. In contrast, dredging significantly disrupts the equilibrium of the fully confined upstream section. Although net accretion occurs throughout the system—most strongly in the upstream region—the annual sedimentation rate decreases by approximately a factor of sixteen from 2016-2025. This is because the current disequilibrium state tends to make the system to erode itself. What is the impact of this on flooding? Increased accretion upstream restricts the upstream propagation of the tidal flood, thereby reducing the overall extent of tidal inundation. However, the associated reduction in upstream cross-sectional area increases the extent of fluvial flooding. Under the 2100 SLR projection, assuming the same levels of dredging and estuary shape as today, the estuary gets closer to its morphological equilibrium state. As the sea-level rise delivers more water into the estuary, it increases both the tidal prism volume and the cross-sectional area. Lastly, channel confinement by riverbanks to protect the infrastructure and people from flooding, impacts morphological disequilibrium the most. Channel confinement laterally reduces the cross-section area, making the space available for the flowing water laterally limited and consequently the system adjusts vertically, making it more erosive and increases flood risk. Here we show that tidal prism theory could aid us in understanding the morphological equilibrium state of a dredged estuary, providing us with a useful guideline on sustainable sediment management and flood mitigation under the projected sea-level rise.