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Natural coastal mechanisms - flume and field experiments on links between biology, sediments and flow. Preface
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Day-night variation of intertidal flat sediment properties in relation to sediment stability
No full textThe majority of investigations that have measured sediment properties related to intertidal sediment stability have been undertaken during daylight subaerial exposure periods. As a consequence, models based upon such data represent only partially the intertidal flat surface conditions within any 24 h period.In this contribution, a comparison is made between surface sediment properties related to sediment stability measured during six consecutive (day/night), semi-diurnal subaerial exposure periods, at three stations on an intertidal sand flat in late March 1999. The study site was selected on the basis of its suitability for sampling and data collection at night, with special regard to safety and logistics. Seawater temperatures ranged from 4.1 to 9.6 [deg]C, and salinities from 33.9 to 34.8.Eleven parameters related to intertidal flat sediment stability were measured, or derived. These variables included the critical erosion shear stress ([tau]c), chlorophyll a, phaeopigment, and colloidal carbohydrate content, mean grain size and settling velocity of the surface (0-1 mm) sediment fraction. Bed elevation was described using an accretion/erosion parameter (AEP) (West and West 1991), whilst additional physical terms included ambient seawater salinity and temperature, as well as tidal range and wind speed, during the preceding immersion periods. One-way ANOVA was used to detect significant differences between day- and night-time emersion periods; similarly, principal components analysis (PCA) was applied to detect continuous variation between properties.The results show a high degree of temporal and spatial variability between day- and night-time intertidal flat variables, the PCA differentiating clearly between day and night conditions. Surface sediments across the intertidal flat exhibited varying degrees of biostabilisation. The maximum biostabilisation coefficient (18) was recorded at night in high microalgal biomass areas; the minimum (5) occurred during both day and night, in areas with lower microalgal biomass. All surface sediment parameters varied rhythmically between day- and night-time. Significant differences were found between day- and night-time biostabilisation coefficients, however, differences between day- and night-time [tau]c values were not detected. It is suggested that sediment stability at night is enhanced in high microalgal biomass areas as a result of degradation products from bound extra-cellular polysaccharides (EPS) not easily detected using standard extraction procedure
Day-night variation of cohesive sediment stability
No full textSurface sediment properties related to cohesive sediment stability were measured over 8 consecutive day- and night-time emersion periods at three upper intertidal sites on a mudflat in August 2003, during the transition from spring to neap tides. Significant differences between day- and night-time critical erosion shear stress (τc) and chlorophyll a were found. A high degree of temporal and spatial variability existed between the sediment properties. During the first half of the study period, a rhythmic day–night variation occurred between τc, chl a, colloidal-S- and EDTA-extracted carbohydrate. During the second part of the study, the magnitude of variation of these parameters diminished. Results showed that sediments were more stable during the day than at night. Differences between day- and night-time sediment stability were related not only to diatom migration, but also to wave energy during preceding immersion periods. No significant relationships existed between τc and either chl a, or colloidal-S- or EDTA-extracted carbohydrate sediment content. It is suggested that tidal phasing, in terms of both the time during the day at which low water spring and neap tides occur, as well as the duration of the emersion period, control the biomass dynamics. The tidal phasing effect is expected to be more pronounced on a cohesive intertidal flat where low water spring tides occur at noon and midnight. The results of this study will be of use in time-dependent estuarine models
Sediment transport pathways in a dredged ria system, southwest England
No full textThe Fowey Ria system, southwest England, comprises the River Fowey catchment, the Fowey estuary, the cliffs and bays adjacent to the ria mouth, and part of the inner continental shelf of the English Channel. Previously, large quantities of sediment were introduced into the upper ria by ore mining activity. Today, in common with other rias, the Fowey receives a low riverine sediment input. Material from maintenance dredging in the lower ria is dumped in a spoil ground outside the ria mouth. The sediments of the system are investigated using an integrated approach to determine sediment distribution and sediment transport pathways. Surface sediments are analysed for grain size and mineralogy. Grain size trend analysis is used to examine sediment dispersal patterns away from the locus of deposition in the spoil ground. Archived data are used to investigate the seabed morphology and to determine long-term (100 year) bathymetric changes.Within the ria, mixing of sediment from several sources occurs. In the upper reaches, riverine and locally-eroded sediment is transported seawards towards the main area of commercial activity. Sand and finer-grained material moves into the ria from offshore. The bed of the inner continental shelf comprises interfluves covered by a thin veneer of sediment, with a natural composition of locally-derived lithic fragments and biogenic material. The area is mainly low/non-depositional in character, except within the partially-infilled palaeovalley and its tributaries.Sediments dumped in the spoil ground disperse in a complex pattern: coarse-grained material is moved by the action of waves and tidal currents towards the southwest and northeast; fine-grained material is transported either to the east or the west, depending upon the prevailing wave and tidal current regime. Because of its geomorphology, the lower ria acts as an efficient sediment trap, retaining (a) riverine material and sediment eroded from the upper reaches; and (b) sediment entering the ria from offshore. Despite being subjected to major anthropogenic disturbance from past mining and present-day dredging activities, the Fowey Ria conforms to the general sediment model for southwest England rias [Castaing, P., Guilcher, A., 1995. Geomorphology and sedimentology of rias. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries – Developments in Sedimentology, No. 53. Elsevier Science, Amsterdam, pp. 69–111]. A conceptual model of sediment transport pathways for the Fowey Ria system is presented as the basis for further investigations
Estuarine mud flocculation properties determined using an annular mini-flume and the LabSFLOC system
No full textMost entrained estuarine sediment mass occurs as flocs. Parameterising flocculation has proven difficult as it is a dynamically active process dependent on a set of complex interactions between the sediment, fluid and the flow. However the natural variability in an estuary makes it difficult to study the factors that influence the behaviour of flocculation in a systematic manner. This paper presents preliminary results from a laboratory study that examined how floc properties of a natural estuarine mud from the Medway (UK), evolved in response to varying levels of suspended sediment concentration and induced turbulent shearing. The experiments utilised the LabSFLOC floc video camera system, in combination with an annular mini-flume to shear the suspended sediment slurries. The flows created in the mini-flume produced average shear stresses, at the floc sampling height, ranging from 0.01 N m?2 to a peak of 1.03 N m?2. Nominal suspended particulate matter concentrations of 100, 600 and 2000 mg l?1 were introduced into the flume. The experimental runs produced individual flocs ranging in size from microflocs of 22.2 ?m to macroflocs 583.7 ?m in diameter. Average settling velocities ranged from 0.01 to 26.1 mm s?1, whilst floc effective densities varied from 3.5 up to 2000 kg m?3. Low concentration and low shear stress were seen to produce an even distribution of floc mass between the macrofloc (>160 ?m) and microfloc (<160 ?m) fractions. As both concentration and stress rose, the proportion of macrofloc mass increased, until they represented over 80% of the suspended matter. A maximum average macrofloc settling velocity of 3.3 mm s?1 was attained at a shear stress of 0.45 N m?2. Peak turbulence conditions resulted in deflocculation, limiting the macrofloc fall velocity to only 1.1 mm s?1 and placing over 60% of the mass in the microfloc size range. A statistical analysis of the data suggests that the combined influence of both suspended concentration and turbulent shear controls the settling velocity of the fragile, low density macroflocs
Microalgal mediation of ripple mobility
No full textThe interaction between physical and biological factors responsible for the cessation of ripple migration on a sandy intertidal flat was examined during a microalgal bloom period in late winter/early spring, as part of a wider study into the biostabilisation of intertidal sediments. Ripple positions and ripple geometry were monitored, and surface sediment was sampled, at weekly intervals over a 5-week period. Ripples remained in the same position for at least 4 weeks, during which time there was a progressive reduction in bedform height (smoothing) and deposition of some 1.5 cm sediment, mainly in the ripple troughs (surface levelling). The mean chlorophyll a (chl a) sediment content was 6.0 µg gDW?1 (DW: dry weight) (0–1 mm depth fraction), with a maximum value of 7.4 µg gDW?1 half way through the bloom. Mean colloidal-S carbohydrate (S: saline extraction) content was 131 µg GE gDW?1 (GE: glucose equivalent) (0–1 mm), with a maximum of 261 µg GE gDW?1 towards the end of the bloom. Important accessory pigments were peridinin (indicative of dinophytes) and fucoxanthin (diatoms). Stepwise multiple regression showed that peridinin was the best predictor of chl a. For the first time, in situ evidence for the mediation of (wave) ripple migration by microalgae is provided. Results indicate that diatoms, and quite possibly dinophytes, can have a significant effect on intertidal flat ripple mobility on a temporal scale of weeks. In addition, microalgal effects appear capable of effecting a reduction in bed roughness on a spatial scale of up to 10?2 m, with a subsequent reduction in bottom stress and bed erodability. It is suggested that a unique combination of environmental conditions, in conjunction with the microalgal bloom(s), promoted the initial cessation of ripple movement, and that stationary-phase, diatom-derived extracellular polymeric substances (EPS) (and possibly dinophyte-derived EPS) may have prolonged the condition. It is reasonable to suppose that ripple stabilisation by similar processes may have contributed to ripple mark preservation in the geological record. A conceptual model of sandy intertidal flat processes is presented, illustrating two conditions: (i) a low EPS/microalgae sediment content with low ripple stabilisation and preservation potential; and (ii) a high EPS/microalgae content with higher preservation potential.<br/
The relative contributions of physical and microbiological factors to cohesive sediment stability
No full textThe 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 5–6 m 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. <br/
Transport, sloughing and settling rates of estuarine macrophytes: mechanisms and ecological implications
No full textThe study of plant-bound nutrient transport has been largely neglected in estuaries. Lately however, it has been shown that nutrients bound to macroalgae and seagrasses can constitute a major part of the nutrient transport in shallow tidal estuaries. Organic detritus in estuaries comes from various sources. This paper looks into the source of detritus from sloughing, and the transport behaviour of plant detritus under unidirectional flows. In order to determine the extent of the sloughing of macrophytes, the threshold current velocities for the traction and resuspension of the most common submersed macrophytes in Venice Lagoon (Ulva lactuca, Enteromorpha sp., Ceramium rubrum, Cladophora sp., and Chaetomorpha linum) were studied in a laboratory flume. It was found that all macrophytes subjected to flows of 1.5–3.0 cm s?1 move initially as bed load. The threshold for suspension of the macrophyte tissue was at current speeds >3 cm s?1. The exception was the filamentous macroalgae, C. linum, which moved as bed load at all current speeds. This implies that the advection of plant-bound nutrients in Venice Lagoon is widespread and takes place over virtually all stages of the tide.Initial experiments were carried out on unattached macroalgae. A second study focused on the sloughing of attached macroalgae by steady currents. The threshold current speeds at which sloughing commenced varied between different types of algae, and sloughing rate was related largely to current speed. The resuspension rates were uniform between the different macrophyte groups.Our results help explain why plant matter has been trapped in nets close to the bed of Venice Lagoon on ebbing tides. It shows that a major component of the bedload is organic in origin. The results verify that a large proportion of the net nutrient export from estuaries is bound in macrophyte tissue. These findings need to be included in future ecological models that describe the resuspension, sloughing and settling of macrophytes. <br/
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