Historical morphological change in the Mersey Estuary

Several techniques including analysis of bathymetric data, calculation of analytical parameters and computational hydrodynamic simulations are combined in this study to develop a conceptual understanding of processes causing morphological change in the Mersey Estuary between 1871 and 1997. Volumetric analysis demonstrates that morphological change is dominated by a trend of significant accretion between 1906 and 1977, with estuary volume reducing by ≈10% (70 Mm3), followed by a relatively small increase in volume between 1977 and 1997. Previous research identified the construction of training walls between 1906 and 1936 to stabilise the position of the low water channel in Liverpool Bay outside the estuary as a probable cause of perturbation. per examines the hypothesis that sedimentation in the estuary was controlled by changes to hydrodynamic flow and related sediment transport patterns outside the estuary resulting from training wall construction, and that the estuary has now evolved towards a stable state. The results from computational hydrodynamic models for the years 1906, 1936 and 1977 quantifying potential changes in sediment transport pathways from outside the estuary indicate a significant increase in potential sediment supply to the mouth of the estuary during the period of peak accretion. However, these changes cannot be solely attributed to construction of the training walls, but result from the combined effect of training wall construction and dredging activity in the sea approach channels. Furthermore, it is not simply changes in hydrodynamic flow characteristics that cause sedimentation but also the existence of salinity-induced gravitational circulation within the estuary and wider Liverpool Bay system that acts as an important mechanism for importing sediment into the estuary. Evidence for evolution towards a stable estuary state is provided by derivation of a sediment budget demonstrating a negligible net flux of sediment into the estuary between 1977 and 1997. The establishment of a stable state is attributed to a reduction in the calculated transport of sediment across Liverpool Bay reducing the supply of sediment to the estuary mouth.