Analyses of Flux Measurements in the River Mersey
The present analyses forms part of a major program to improve water quality in the River Mersey and the adjacent Liverpool Bay. Since similar clean-up programmes are planned for many tidal estuaries, the measurements undertaken in the Mersey are examined: (i) to determine their efficacy in calculating contaminant fluxes within the river and (ii) more generally, to consider possible alternative future strategies.
Measurements considered here are limited to currents, suspended sediment concentrations, salinity, temperature and oxygen at a particular section in the Mersey Narrows. Continuous long term measurements were made, both at the surface and at mid-depth, from a moored-buoy. These data were supplemented by 4 cross-sectional surveys, with measurements over depth taken from 5 stations at a section close to the buoy. Each of these surveys extended over a full semidiurnal tidal cycle.
Tidal analyses of the buoy data established the relative accuracy and reliability of the data sets. The degree of consistency between the major tidal constituents derived from successive monthly observations indicated the predominance for each parameter of the tidal component. A simplified tidal analysis was applied to each of the cross-sectional surveys, these results indicated the cross-sectional variation in various tidal parameters and, thereby, the required link with the tidal results from the buoy measurements.
Analyses of the vertical variation in residual currents revealed a pronounced ‘classical’ estuarine structure with denser (saltier) sea water flowing landwards at the bed, with a compensating seaward flow of fresher river water at the surface. Analysis of the suspended sediment concentrations showed the nature of both the vertical and lateral variability. Likewise, the variation in cross-sectional mean concentrations with varying tidal amplitudes indicated that suspended sediment concentrations were proportional to tidal velocity cubed. By using this latter result, simple analytical derivations indicate the sensitivity of the ned sediment flux to: (i) spring/neap tidal conditions, (ii) net cross-sectional residual velocity, (iii) lateral and vertical variations in residual velocity and (iv) vertical variations in sediment concentration.
KeywordsSediment Concentration Suspend Sediment Concentration Neap Tide Sediment Flux Residual Current
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