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  • Ian D. Lichtman
    National Oceanography Centre, Liverpool
  • Jacobus Hugo Baas
  • Laurent O. Amoudry
    National Oceanography Centre, Liverpool
  • Peter D. Thorne
    National Oceanography Centre, Liverpool
  • Jonathan Malarkey
  • Julie A. Hope
    Institute of Marine Science, The University of Auckland
  • Jeffrey Peakall
    Leeds University
  • David M. Paterson
    Scottish Oceans Institute, School of Biology, University of St. Andrews
  • Sarah J. Bass
    School of Marine Science and Engineering, Plymouth University
  • Richard D. Cooke
    National Oceanography Centre, Liverpool
  • Andrew J. Manning
    School of Marine Science and Engineering, Plymouth University
  • Alan G. Davies
  • Daniel R. Parsons
    Department of Geography, Environment and Earth Sciences, University of Hull
  • Leiping Ye
    Department of Geography, Environment and Earth Sciences, University of Hull
Many coastal and estuarine environments are dominated by mixtures of non-cohesive sand and cohesive mud. The migration rate of bedforms, such as ripples and dunes, in these environments is important in determining bed material transport rates to inform and assess numerical models of sediment transport and geomorphology. However, these models tend to ignore parameters describing the physical and biological cohesion (resulting from clay and extracellular polymeric substances, EPS) in natural mixed sediment, largely because of a scarcity of relevant laboratory and field data. To address this gap in knowledge, data were collected on intertidal flats over a spring-neap cycle to determine the bed material transport rates of bedforms in biologically-active mixed sand-mud. Bed cohesive composition changed from below 2 volume % up to 5.4 volume % cohesive clay, as the tide progressed from spring towards neap. The amount of EPS in the bed sediment was found to vary linearly with the clay content. Using multiple linear regression, the transport rate was found to depend on the Shields stress parameter and the bed cohesive clay content. The transport rates decreased with increasing cohesive clay and EPS content, when these contents were below 2.8 vol% and 0.05 weight%, respectively. Above these limits, bedform migration and bed material transport was not detectable by the instruments in the study area. These limits are consistent with recently conducted sand-clay and sand-EPS laboratory experiments on bedform development. This work has important implications for the circumstances under which existing sand-only bedform migration transport formulae may be applied in a mixed sand-clay environment, particularly as 2.8 vol% cohesive clay is well within the commonly adopted definition of ‘clean sand’.

Keywords

  • Bedform migration, Sediment tranport, Mixed cohesive clay-sand, Physical and biological cohesion, Current and wave forcing, Tidal flats
Original languageEnglish
Pages (from-to)17-32
Number of pages66
JournalGeomorphology
Volume315
Early online date1 May 2018
DOIs
Publication statusPublished - 15 Aug 2018
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