Modelling large-scale estuarine morphodynamics using equilibrium concepts : responses to anthropogenic disturbance and climatic change

Abstract

This thesis explores the application of equilibrium concepts in the modelling of large-scale estuarine morphodynamics. Empirical relationships have been observed between a number of morphological measures and forcing parameters in estuaries and this suggests that estuaries tend towards a morphology that is in equilibrium with forcing. A geographical information system was used to analyse historical bathymetry charts for eight English estuaries and to construct a time series of morphological change for each estuary. Equilibrium relationships in this data set, and for a larger selection of English and Welsh estuaries, were analysed. A behaviour-oriented numerical model based on equilibrium concepts, ASMITA (Stive et al., 1997), was used to further investigate the nature of equilibrium in the study estuaries. ASMITA represents estuaries schematically, as aggregate systems containing a number of distinct morphological elements. These estuary schematisations provide a useful structure for analysing the large scale changes in morphology and also allow the equilibrium state of each element to be examined. New schematisations were used where necessary to represent the wide range of estuary morphologies found in the study sample, and in UK estuaries as a whole. The study estuaries show a variety of morphological behaviours: those showing the largest changes were the Ribble Estuary, Southampton Water and Portsmouth Harbour. In all these cases, the morphological evolution of the estuaries was driven by human disturbances to the.natural regime. In general, ASMITA was able to reproduce the evolution of the estuaries studied in line with observed data. Analysis of the data and application of ASMITA indicated that the equilibrium volume of the channel (water volume below mean low water) could be defined as a linear function of the tidal prism. Equilibrium volume of the flats (sediment volume above mean low water) could also be denned, but was sensitive to changes in basin area, such as those caused by land reclamation, making it less useful for modelling application. Generic equilibrium relationships, based on a larger sample of estuaries, were found to be poor predictors of equilibrium in the eight study estuaries and it was concluded that estuary specific relationships are much better for for modelling applications. Equilibrium theory is useful in modelling the large-scale behaviour of estuaries and has the potential to be a useful tool for long-term estuary management, especially if improved relationships can be found to define flat equilibrium volume.

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