Sediment erodability in sediment transport modelling: Can we account for biota effects?

Sediment erosion results from hydrodynamic forcing, represented by the bottom shear stress (BSS), and from the erodability of the sediment, defined by the critical erosion shear stress and the erosion rate. Abundant literature has dealt with the effects of biological components on sediment erodability and concluded that sediment processes are highly sensitive to the biota. However, very few sediment transport models account for these effects. We provide some background on the computation of BSS, and on the classical erosion laws for fine sand and mud, followed by a brief review of biota effects with the aim of quantifying the latter into generic formulations, where applicable. The effects of macrophytes, microphytobenthos, and macrofauna are considered in succession. Marine vegetation enhances the bottom dissipation of current energy, but also reduces shear stress at the sediment-water interface, which can be significant when the shoot density is high. The microphytobenthos and secreted extracellular polymeric substances (EPS) stabilise the sediment, and an increase of up to a factor of 5 can be assigned to the erosion threshold on muddy beds. However, the consequences with respect to the erosion rate are debatable since, once the protective biofilm is eroded, the underlying sediment probably has the same erosion behaviour as bare sediment. In addition, the development of benthic diatoms tends to be seasonal, so that stabilising effects are likely to be minimal in winter. Macrofaunal effects are characterised by extreme variability. For muddy sediments, destabilisation seems to be the general trend; this can become critical when benthic communities settle on consolidated sediments that would not be eroded if they remained bare. Biodeposition and bioresuspension fluxes are mentioned, for comparison with hydrodynamically induced erosion rates. Unlike the microphytobenthos, epifaunal benthic organisms create local roughness and are likely to change the BSS generated by the flow. In this paper, we attempt to describe state-of-the-art sediment transport models accounting for biological processes. Such applications generally demonstrate a clear effect of the biota on erosion/deposition, but morphodynamic coupling is rarely achieved. In the present study, a modelling exercise of this type was run, based on a cross-shore morphodynamic model of an intertidal mudflat [Waeles, B., Le Hir, P., Silva Jacinto, R., 2004. Modélisation morphodynamique cross-shore d'un estran vaseux. Comptes Rendus Geoscience 336, 1025-1033] in which the equilibrium profile of the intertidal flat under tide and wave forcing is simulated. A seasonal presence of the microphytobenthos in late spring and summer, represented by a fourfold increase in the erosion threshold, generates sediment level changes of about 5 cm. However, these effects disappear in autumn and winter when the erosion threshold returns to its abiotic value, even when wave erosion is ignored. In contrast, the reduction of BSS in the upper flat to simulate the effect of a saltmarsh induces a spectacular seaward shift of the upper flat. The simulations show the strong, potential, long-term effect of vegetated beds, i.e. the protection of sediment from wave erosion. In contrast, local stabilisation by the microphytobenthos does not have a significant long-term effect. Some recommendations are given on the need to define experimental protocols for erosion tests and studies on biota effects. A stochastic approach is suggested to cope with the problem of patchiness and extreme variability of erodability patterns, combined with histograms of BSS.