The application of historical data and computational methods for investigating causes of long-term morphological change in estuaries : a case study of the Mersey Estuary, UK
Long-term morphological change in estuaries, of the order of 100 years, has developed into an area of significant research interest as a result of increased regulation and management of estuarine environments. The long-term behaviour of estuary morphology results from the net effects of perturbations induced by tidal, seasonal and episodic events, averaged over a longer period. Theoretically a dynamic equilibrium may exist between deposition and erosion when considered over a time period that is sufficiently long to encompass the cyclic variability that exists within an estuarine system. However the assemblage of physical processes required for a stable state to exist, and the causes of deviation from a stable state, are not well understood. The interaction of physical processes of tidal and wave action, and the influence of sea level rise and anthropogenic activity, with estuarine ecology and geology are largely responsible for the evolving state of an estuary. Although the physical processes of tidal movement and wave action are well known and documented, the interaction of these processes with factors controlling estuarine evolution over long time periods is less well understood. This thesis evaluates approaches to analysing historical data and applying computational methods to examine the interaction between factors forcing long-term estuary morphology. Historical data is of considerable value to analysis of long-term morphological change in estuaries, and forms a pre-requisite for developing understanding of the nature and causes of the long-term evolution of estuary morphology. However few data sets exist which cover a period of sufficient duration with sufficient detail to identify the processes forcing morphological change, so recourse to computational methods is required for the purpose of developing understanding of estuary behaviour. Several techniques are employed, including analysis of bathymetric data, calculation of analytical parameters and computational hydrodynamic simulations, to develop a case study of processes causing morphological change in the Mersey estuary over the last century. A major requirement for the approach adopted in this thesis is the identification and reduction of uncertainty. Areas of uncertainty are identified, and the results arising from various computational techniques employing different assumptions are examined within a framework enabling evaluation of the uncertainty arising from analysis and assumptions upon which it is reliant. Volumetric analysis demonstrates that morphological change is dominated by a trend of significant accretion between 1906-1977, with tidal volume reducing by approximately 10% (70Mm3). Previous research has identified the construction of training walls, between 1906-36 to stabilise the position of the low water channel in Liverpool Bay outside the estuary, as a probable cause of perturbation. Changes to tidal flow and related sediment transport patterns outside the estuary resulting from training wall construction are examined with regard to the stability of the estuary system. The results from computational hydrodynamic models representing 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 tidal flow characteristics that cause sedimentation but also the existence of salinity induced gravitational circulation within the estuary and the 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 1977-1997. The establishment of a steady state is attributed to a reduction in the calculated transport of sediment, from west to east, across Liverpool Bay reducing the supply of sediment to the estuary mouth.