The impact of changing sediment budgets on an industrialised estuary: a case study of Southampton Water, UK

Abstract

Approximately 80 of the 170 commercial ports in the UK are either in or near areas protected under the European Habitats Directive. These areas are often also focal points for related industries, urbanisation and recreation. This brings in to question what impact these developments have had on these protected habitats and should future developments be given permission to occur. Pressures on estuaries were found to impact upon the dynamic equilibrium of an estuary through changes in the balance of the sediment budget, energy dispersal and estuary morphology. It was the aim of this thesis to show that through understanding the historical relationship between various pressures and the impact on these components that management of these environments could be improved to protect their environmental interest while allowing sustainable development to occur. Also by understanding the state of equilibrium it could be determined whether historic projects should be included in the cumulative and in-combination impact assessment for new plans or projects under the Conservation of Habitats and Species Regulations 2010. Southampton Water is an industrialised estuary on the South coast of England, the European protected salt marsh and mudflats lining its shores have been declining so this was an ideal case study site to investigate the relationship between pressures on estuaries and their impacts. A geographical information system was used to analyse historic data for Southampton Water between 1783 and 2008 to create 3D model for Southampton Water over time. From this model the volume changes of estuary components, the sediment budget and the change in dynamic equilibrium were quantified for the entire estuary. In novel work the changes were related to historic events which had occurred in the estuary. Importantly the errors associated with the sediment budget and morphological changes were quantified to provide a degree of confidence to these findings. This thesis found that the biggest influences on the sediment budget and equilibrium state of the estuary between 1783 and 2008 were dredging, land claim and Spartina growth/dieback. Land claim alone immobilized 255.4(+/-62) x106m3 of sediment. The estuary has moved from equilibrium state between 1783 and 1894 to one which is sediment starved due to a combination of these pressures.1,525 (+/-280) x103m3/yr of sediment was needed between 2001 and 2008 to balance the budget. Due to continued pressures the basin area is too large for the intertidal area the estuary supports. Error analysis was important in providing confidence to these results. The Main Channel and Test Estuary are still adjusting to historic changes this means that future plans/ projects should take these in to account when assessing the cumulative impact for Habitat Risk Assessments. This thesis showed that future management of the estuary should focus on securing sediment supplies to the estuary and on stabilising the intertidal zone. Subject to data constraints the methods used in this thesis could be applied to any estuary to assess the impacts of drivers and pressures on protected habitats and the management regimes which could be used to ensure their survival.

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