Impacts of coastal realignment on intertidal sediment dynamics: Freiston Shore, the Wash

摘要

The impacts of land reclamation and managed realignment on saltmarsh and the adjacent intertidal flats at Freiston Shore (The Wash) are investigated. The hydrodynamic and sediment dynamics over the intertidal flats and a managed realignment site are discussed, with an extensive set of hydrodynamic, sediment dynamic and geomorphological data presented. The managed realignment has improved the coastal defence of the area; the site has been colonised by vegetation, accreted sediment and experienced limited wave activity. However, the channels within the breaches in the embankment were eroded, and a creek system over the adjacent intertidal flats experienced an enhancement in its development. This has allowed the natural and enhanced development of the creek system to be described and compared. These impacts were caused by the managed realignment site being at a lower elevation than the adjacent saltmarsh, causing high tidal current speeds as water flowed into the site and prolonged drainage of the site. This resulted in sheetflow over the intertidal flats, which caused an enhancement to the natural creek development. The water and sediment interaction between this creek system and the adjacent intertidal flats are discussed; the creeks were net exporters of sediment, while the intertidal flats were importers. The most rapid period of change was in the 2 months immediately after breaching the old embankment. The ‘Regime Theory’ was used to predict the equilibrium size of the channels within the breaches, and the time for this to be attained. Some 36 months after the initiation of the scheme, the adjacent intertidal flats appear to have adjusted to a new dynamic equilibrium, related to the managed realignment. The sheetflow over the intertidal flats has stopped and, consequently, the enhanced creek development has ceased and the creeks silted up. Despite the impacts of the managed realignment scheme, no obvious changes have been identified in the pattern or strength of tidal currents and sediment transport, over the intertidal flats adjacent to the managed realignment. In contrast, the previous creation of the embankment (for land reclamation purposes) led to an increase in tidal current speeds and erosion of the intertidal flats. Hydrodynamic and suspended sediment measurements show that, over the intertidal flats, the suspended sediment concentration increases exponentially with an increase in tidal current speed; similarly, linearly with wave height. The majority of wave activity over the intertidal zone is dissipated over the saltmarsh, with the mudflats causing little wave attenuation. Based upon the results of the present study, it appears that managed realignments can provide a successful future coastal defence, as long as certain guidelines for site selection are followed.