Bathymetric evolution of the Mersey Estuary, UK, 1906–1997: causes and effects

Abstract

This paper examines how a continuous series of measurements and model studies have developed our understanding of the evolving interactions between tidal dynamics, sediment regime and bathymetry of the Mersey Estuary—among the most intensively monitored worldwide. This includes familiar assessments of the capabilities of fine-resolution 3-D models but is here extended to examine historical changes in tides, sediments and estuarine bathymetries. A century of bathymetric surveys indicates a net loss of estuarine volume of about 0.1%, or 1 million cubic metres, per year. In contrast, sea level rise of 1.2 mm a −1 represents only a 0.02% annual increase. This relative stability persists in a highly dynamic regime with suspended sediment concentrations exceeding 2000 mg l −1 and spring tide fluxes of order 200 000 t. Detailed analyses of the bathymetric sequences indicate most significant changes occur in the upper estuary and in inter-tidal zones. A long period, up to 63 years, of tidal elevation records (in the lower estuary) shows almost no changes to the predominant M 2 and S 2 constituents. The 3-D numerical model accurately reproduces both these elevation constituents and corresponding cross-sectional distributions of currents. Simulation accuracies reduce upstream due to cumulative errors associated with relatively unstable bathymetries and increasing sensitivities to the bed stress coefficients. Impacts from increases in river flows of up to two or three times the long-term mean value can only be detected in the upper estuary. A detailed examination of sediment fluxes utilised a numerical simulation of conditions over a cross-section in the ‘Narrows’, the 10 km by 1.5 km prismatic entrance channel. The limited mobility of coarse sediments was contrasted with the near-continuously suspended nature of the finest clay. A sensible match between the net sedimentation rates indicated by the model and the net observed deposition rate was found to occur for silty sand corresponding directly with evidence from dredging records and from direct sampling. While the model indicated sedimentation rates might increase by up to a factor of ten for much finer particles, such occurrences are likely to be restricted by the limited availability of such material in the adjacent coastal zone. The controlling mechanism for this import of fine sediments is via tidal pumping, the nature of which depends critically on the phase lead of tidal currents relative to elevation. However, since this phase lead is determined by the large scale tidal energy balance of the estuary, the conclusion is that stability of the overall tidal dynamics, the associated sediment regime and bathymetry are likely to co-exist and evolve slowly as indicated by the observed annual decrease in net volume. Suggestions are included for monitoring strategies in other estuaries to provide insight into such dynamics-sediment-bathymetry inter-relationships.