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Modelling the effects of macrofauna on sediment transport and bed elevation: application over a cross-shore mudflat profile and model validation

Abstract : The effects of 2 functional groups of bioturbators have been predicted in terms of long-term impact on erodability: (1) one superficial mobile deposit-feeder, the gastropod Hydrobia ulvae; and (2) one endobenthic deposit-feeder, the bivalve Scrobicularia plana. Different scenarios of morphodynamical cross-shore 1DH/1DV model were performed to simulate the equilibrium profile of an intertidal mudflat under tide and wave forcings. This process-based model for erosion is able to simulate multiphasic sequential resuspension, by discriminating various erosion behaviour like benthos-generated fluff-layer erosion (BGFL) and general bed loosening and burrowing activity in deep layers. The results were analysed and compared to examine the long-term effect of macrofauna after 14 years. It reveals that the impact of the bivalve S. plana is very significant after only 4 years of simulation while the effect of the gastropod H. ulvae is negligible in terms of sediment transport even after 14 years. More generally, this reveals the strong impact of stationary endobenthic bioturbators that induces a high downward shift of the upper shore while the effects of superficial motile bioturbators remain very low. This impact is mainly due to the effect of endobenthic species in deep layers associated to burrowing activities and their consequences on the bed erosion, but the production of a fluff layer by surface grazer like H. ulvae at the sediment surface can be neglected. The importance of macrofauna mediation of bed erodability is discussed in this study by comparing the activities of the two functional groups of bioturbation on the general functioning of intertidal mudflats. The model outcomes (transferred in a 1DV framework) were in close agreement with the measured results of flume data at 3 different bathymetric levels of the mudflat over the cross-shore profile. This validation step revealed that model of sediment transport under influence of biota effects does not need further refinements at the upper shore where S. plana dominates the species assemblage, whereas there is still a need to include further formulations of biota effects to simulate the erosion experimental results at the lower shores where other molluscs and Annelids significantly contribute to the species assemblage.
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Orvain Francis, Pierre Le Hir, Pierre-Guy Sauriau, Sébastien Lefebvre. Modelling the effects of macrofauna on sediment transport and bed elevation: application over a cross-shore mudflat profile and model validation. Estuarine, Coastal and Shelf Science, Elsevier, 2012, 108 (1), pp.64-75. ⟨10.1016/j.ecss.2011.12.036⟩. ⟨hal-01444601⟩



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