Sediment dynamics of saltmarshes and intertidal flats, Southern and Eastern England

Abstract

Saltmarshes and intertidal flats at Freiston Shore (The Wash) and from Lymington to Keyhaven (West Solent) in eastern and southern England, respectively, are studied in terms of water and sediment movement and their interactions. Based upon tidal current measurements, wave monitoring, suspended sediment concentration and seabed sediment sampling/analyses, the sediment dynamics over the saltmarshes and intertidal flats of The Wash are identified as accretional. The area is characterised by a standing tidal wave, flood-tidal dominance, and suspended sediment supply from offshore. In contrast, the West Solent is erosional; it is characterised by a progressive tidal wave, ebb-tidal dominance, and the net suspended sediment offshore export. In the accretional intertidal and saltmarsh environments: (i) maximum tidal current speeds and suspended sediment concentrations occur at the beginning of the flood and end of ebb phases; and (ii) there is always a net landward sediment transport, due to the tidal asymmetry (flood dominant) throughout each tidal cycle. Velocity gradient rigs were deployed over the saltmarshes and intertidal flats of the two regions. Boundary layer flow parameters were obtained and their patterns established throughout tidal cycles. This part of the study illustrate the distinctive differences in the boundary layer flows between saltmarshes and intertidal flats, and sandflats and mudflats of the intertidal zone. These differences are due mainly to: the different roughness elements; and the inter-relationship between the directions of the tidal currents and the orientation of the bedforms. Comparing with the 'traditional' model elsewhere, wave action is generally effective over the intertidal flat environments in the U.K.. Interaction between the wave- and tidally-induced currents is an essential condition for transport over the intertidal flats and sediment supply to (and deposition over) the saltmarshes of The Wash. Likewise, waves are the dominant process causing erosion along the edge of the saltmarshes in the West Solent.
Based upon the results of the present study, a new dynamic classification of saltmarshes and intertidal flats is proposed: (i) tidally-dominated; (ii) wave-influenced/tidally-dominated; (iii) tidally-influenced/wave-dominated; and (iv) wave- and tidally-(alternatively) dominated.

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