Search

Abstract: Low tide rainfall may represent an important but little studied process affecting sediment fluxes on intertidal mudflats. In this study, we simulated rainfall events on an intertidal mudflat (median grain size=18.4μm) of low slope (1 in 300) then quantified effects on sediment erodibility. Treatments consisted of a high (4.1mmmin−1 for 6min) and low (0.36mmmin−1 for 60min) rain intensity, chosen to match naturally occurring events and experiments were conducted seasonally (May and August) to encompass variations in ambient sediment stability. Changes in bed elevation due to rainfall were estimated using marked rods and sediment erodibility parameters (mass of sediment eroded at a flow velocity of 0.3ms−1 (ME-30, gm−2) and critical erosion velocity (U crit, ms−1)) were determined in annular flumes (bed area=0.17m2). Ambient/control sediment erodibility in May (ME-30=211gm−2, U crit=0.18ms−1) was higher than in August (ME-30=30gm−2, U crit=0.26ms−1) and was correlated with changes in biological activity. In May, surface sediment was influenced by high densities of the bioturbating snail Hydrobia ulvae (1736ind.m−2) and low biomass of the sediment stabilising microphytobenthos (5.7μgchlorophyll a cm−2). In contrast, in August H. ulvae densities were low (52ind.m−2) and microphytobenthic biomass higher (9.2μgchlorophyll a cm−2). The high rain treatment caused a decrease in bed elevation of between 1.5mm (May) and 4.4mm (August) and significantly reduced sediment organic content and microphytobenthic biomass. Rainfall increased sediment erodibility; compared to ambient sediments ME-30 increased by a factor of 1.4× in May and 8.8× in August and caused a 10–30% decline in U crit. The seasonal difference in treatment effect was due to the change in ambient sediment stability. The low rain treatment in August had no effect on bed elevation, microphytobenthic biomass or sediment erodibility. In May, the same treatment caused a reduction in bed elevation (0.5mm) and microphytobenthic biomass but counter-intuitively, a decrease in sediment erodibility (ME-30 was reduced by 40%, U crit increased by 5%) compared to controls. We attribute this result to removal by rainfall of easily eroded surface flocs and biogenic roughness which resulted in an underlying sediment with a smoother surface and greater resistant to erosion. Results suggest that high intensity rain events may destabilise intertidal sediments making them more susceptible to erosion by returning tidal currents and that the sediment eroded during such events may represent a considerable fraction (up to 25%) of the seasonal variation in shore elevation. The impact of natural rain events are likely to vary considerably due to variations in droplet size, intensity and duration and the interaction with ambient sediment stability. [Copyright &y& Elsevier]

Abstract: The stabilising effects of natural benthic diatom and bacterial assemblages on cohesive sediments were compared with those caused by physico-chemical binding alone. Cohesive sediment beds were reconstructed in 4 annular laboratory miniflumes, using sediment collected at 5–6m water depth from a local fjord. The sediment was left to stabilise (consolidate) for 1, 2, 5 and 10 days, before being fully resuspended in a series of erosion experiments. The flumes were aerated and subjected to different light/dark conditions; antibiotics were used to isolate diatom from bacteria effects. During consolidation, a constant current velocity was maintained, at a speed well below erosion threshold. ‘Natural’ sediment regained 27% of its original stability after 1 day, and 85% after 5 days. Complete ‘natural’ sediment stability was regained within 10 days. Benthic diatoms (mainly Nitzchia sp., Gyrosigma sp. and Pennales sp.) were responsible for about 80% of the biostabilisation, whilst bacteria contributed ∼12%, indicating the importance of light as a controlling factor for surface sediment stability. Relative to physico-chemical binding, the increase in erosion threshold induced by benthic diatoms was 120%, and by bacteria 20%. Where the assemblage consisted of both diatoms and bacteria, the increase was 150%, indicating that natural bed values are not a simple summation of the two effects, i.e. diatoms plus bacteria. Using the results, a first generation empirical relationship describing the relative contributions of microbenthic and physico-chemical factors in the development of erosion threshold is presented. Empirical relationships containing biostabilisation coefficients are important for better computer model predictions of sediment transport. [Copyright &y& Elsevier]

Abstract: 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 5cm. 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. [Copyright &y& Elsevier]