Simulating the role of tides and sediment characteristics on tidal flat sorting and bedding dynamics |
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Authors: | Zeng Zhou Qian Liu Daidu Fan Giovanni Coco Zheng Gong Iris Möller Fan Xu Ian Townend Changkuan Zhang |
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Institution: | 1. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, 210098 China;2. College of Harbour, Coastal and Offshore Engineering, Hohai University, Nanjing, 210098 China;3. State Key Laboratory of Marne Geology, Tongji University, Shanghai, 200092 China;4. Faculty of Science, University of Auckland, Private Bag, Auckland, 92019 New Zealand;5. State Key Laboratory of Hydrology-Water Resources and Hydraulic Engineering, Hohai University, Nanjing, 210098 China
College of Harbour, Coastal and Offshore Engineering, Hohai University, Nanjing, 210098 China;6. Cambridge Coastal Research Unit, Department of Geography, University of Cambridge, Cambridge, CB2 3EN UK;7. State Key Laboratory of Estuarine and Coastal Research, East China Normal University, Shanghai, 200062 China;8. Ocean and Earth Sciences, University of Southampton, Southampton, SO17 1BJ UK |
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Abstract: | Understanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand-dominated layers (SDLs) and mud-dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood–ebb variations. A larger tidal level variation during a spring–neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types. Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding. |
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Keywords: | morphodynamic modelling sediment sorting sedimentary structure tidal bedding tidal flats tidal rhythmite |
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