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1.
The mouth of the Seine River estuary (France) has undergone marked morphological evolution over several decades mainly due to engineering works aimed at improving access to Rouen and Le Havre harbours. The intertidal areas are decreasing in size and the lower estuary is accumulating sediment and prograding. In order to understand and better describe the major morphological behaviours of the estuary, a morphodynamic numerical model was developed within the Seine-Aval program. At the end of the 1st part of the research program, a validated fine sediment transport model (3D) was available (Le Hir et al., 2001b). As the present morphological study addresses medium-term issues (a few decades), and because of the need to investigate impacts of local structures or events, we chose to use the so-called “process-based approach” starting from the existing model. First, the existing model was upgraded to account for (suspended) sand transport, and to achieve coupling between morphological changes and sediment transport. Erodability of the sediment accounts for the respective proportions of mud and sand. Simulations starting from an arbitrary surficial sediment cover show that the model is able to reproduce realistic sediment patterns. For example, it is able to change the sediment nature on the intertidal flat near Le Havre from sand to mud. Observed structures of suspended sediment are also reproduced: fine particles mainly follow the turbidity maximum whereas significant concentrations of sand grains in suspension are found where the hydrodynamic stresses are intense. Concerning morphodynamics, simulations with real forcing over one year are discussed. The effect of waves on the bathymetric evolution of the mouth is shown and the sensitivity of morphodynamics to the coupling procedure is tested.  相似文献   

2.
The Scheldt river drains a densely populated and industrialized area in northern France, western Belgium and the south-west Netherlands. Mineralization of the high organic load carried by the river leads to oxygen depletion in the water column and high concentrations of dissolved nitrogen and phosphorus compounds. Upon estuarine mixing, dissolved oxygen concentrations are gradually restored due to reaeration and dilution with sea water. The longitudinal redox gradient present in the Scheldt estuary strongly affects the geochemistry of nutrients. Dissolved nutrients in the water column and dissolved nitrogen species in sediment porewaters were determined for a typical summer and winter situation. Water column concentration-salinity plots showed conservative behaviour of dissolved Si during winter. During summer (and spring) dissolved Si may be completely removed from solution due to uptake by diatoms. The geochemistry of phosphorus was governed by inorganic and biological processes. The behaviour of nitrogen was controlled by denitrification in the anoxic fluvial estuary, followed by nitrification in the upper estuary (prior to oxygen regeneration). In addition, nitrogen was taken up during phytoplankton blooms in the lower estuary. Dissolved inorganic nitrogen species in porewaters from the upper 20 cm of sediments were obtained from a subtidal site in the middle of the lower estuary. Dissolved nutrient concentrations were low in the upper 10–15 cm of the sandy and organic poor (<1% POC) sediments mainly as a result of strong sediment mixing. The porewater profiles of ammonium and nitrate were evaluated quantitatively, using a one-dimensional steady-state diagenetic model. This coupled ammonium-nitrate model showed ammonification of organic matter to be restricted to the upper 4 to 7 cm of the sediments. Total nitrification ranged from 3.7–18.1 mmol m?2 d?1, converting all ammonium produced by ammonification. The net balance between nitrification and denitrification depended on the season. Nitrate was released from the sediments during winter but is taken up from the water column during summer. These results are in good agreement with data obtained from the independently calibrated water column model for the Scheldt Estuary (VAN GILSet al., 1993).  相似文献   

3.
A numerical model to simulate the transport of suspended sediment in tidal estuaries is presented. The model is applied to the two large European estuaries the Tagus (Portugal) and the Scheldt (Belgium-The Netherlands). Calculated suspended sediment concentrations compare favourably with observations in the Tagus (r=0.84) and in the Scheldt (r=0.73). The parametrization scheme indicates that the bottom content of fine sediment is correlated with depth in the Tagus; but a different relationship is used in the Scheldt. Because of tidal range differences, average suspended sediment concentrations are lower in the Tagus (80 mg l−1) than in the Scheldt (130 mg l−1), but a larger relative variation between spring and neap tide concentrations may occur in the Tagus.  相似文献   

4.
The behaviour of some organic micropollutants in the abiotic compartments water, particulate matter and bottom sediments of the Scheldt estuary, S.W. Netherlands, was studied between 1986 and 1989. Special attention was given to two individual PolyChlorinated Biphenyl congeners (PCB 52 and PCB 153), two Polycyclic Aromatic Hydrocarbons (PAHs) fluoranthene and benzo(a)pyrene and γ-HCH. Dissolved PCBs and γ-HCH behave conservatively during estuarine mixing. The concentration of γ-HCH in the fresh-water is extremely high in spring compared to the other seasons. Dissolved fluoranthene does not behave conservatively. The concentration of dissolved fluoranthene in the freshwater endmember shows a seasonal dependence with highest concentrations in winter and lowest in summer. Benzo(a)pyrene could hardly be detected in the dissolved phase with the methods used. PCBs and PAHs in particulate matter and sediments behave essentially conservatively as a result of the mixing of riverine particulates with high, and marine particulates with low organic micropollutant contents. Particulate fluoranthene is removed at low salinities under (nearly) anoxic conditions probably by microbial degradation. The rivers Scheldt and Rupel appear to be the major sources for the compounds studied. However, in particular the organic micropollutant contents of the sediments indicate that important emissions along the estuary were or are still present. Measurements of individual PCBs and PAHs in the <63 μm fraction of a dated sediment core from a salt marsh in the eastern part of the Scheldt estuary show that the recent input of PCBs and PAHs into the estuaryis probably 2–3 times lower than the maximum input in the mid-1960s. The PCB profile shows postdepositional congener-selective mobilization caused by advective transport. The PAH assemblage is remarkably uniform in the sediment core. The PAH ratios indicate that combustion of coal is the main source of PAHs in the Scheldt estuary.  相似文献   

5.
Suspended matter in the Scheldt estuary   总被引:1,自引:0,他引:1  
The Scheldt estuary is characterised by a specific energy pattern resulting from the interaction of wave energy, tidal energy and river energy. It divides the estuary into three parts and governs suspended matter transport and distribution pattern. Observation of suspended matter transport shows the existence of three estuarine turbidity maxima (ETM), a marine-dominated ETM in the lower estuary at the river mouth, a river-dominated ETM in the upper estuary with suspended matter concentration reaching up to 300 mg/l, and the most important tide-dominated ETM in the middle estuary with suspended matter concentrations from several hundred milligrams per litre up to a few grams per litre. Resuspension is the dominant phenomenon in this last ETM due to the tidal related bottom scour, which is initiated when a critical erosion velocity of 0.56 m/s is exceeded. An assessment of residual current along the axis of the estuary shows distinctive pattern between the surface water flow and the near bottom water flow. Also the local morphology of the river, natural or man-made, has a prominent effect on the orientation and strength of the residual currents flowing along either side of the river or river bend. Evaluation of suspended matter concentration in relation to the current flow shows no systematic correlation either because of phenomena as scour lag and settling lag mainly in the middle estuary, or because of the current independency character of uniform-suspension mainly in the upper and lower estuary. Quantification of suspended matter load exhibits a net downstream transport from the upper estuary, a near-equilibrium sustainable status in the middle estuary and a net upstream transport of suspended matter from the lower estuary. The characteristic of suspended matter is induced by and is a function of e.g. tidal phase, spring-neap tide, longitudinal and vertical distribution mechanisms, seasons, short and long terms of anthropogenic influence and/or estuarine maintenance. Suspended matter is dominated by complex and cohesive organo-mineral aggregates. It consists of a variable amount of an inorganic fraction (average of 89%) and an organic fraction and occurs largely as flocs, the size of which is remarkably larger in the upper estuary and smallest within the ETM in the middle estuary. Independent time series measurements (1990–2000) of suspended matter property show an increasing sand fraction, a decreasing organic matter content, a rise in 13C as well as a decrease in water transparency. These independent measurements exhibit coherent consequences of estuarine maintenance operations. Maintenance dredging of the shipping channel and harbours and dumping operation in the Scheldt strengthen marine influence further landward, resulting in a sustained tidal range increment and upstream flow and transport of suspended matter.  相似文献   

6.
The mollusc fauna of 64 sites in 31 tidal marshes was surveyed along a salinity gradient from freshwater to marine conditions in the river Scheldt (Belgium–The Netherlands). A total of 10649 specimens involving 31 taxa were identified. Salinity turned out to be a major factor in mollusc assemblages in the Scheldt estuary, but other factors can not be excluded. In the marine part five species were common, compared to the brackish part where only Assiminea grayana was abundant. In the freshwater zone species richness was highest (24). There was a significant correlation between flooding frequency and species richness in the tidal freshwater marsh `Durmemonding'. Finally, the survey confirmed the distribution of the amphibious hygromiid snail Pseudotrichia rubiginosa, a species which in Belgium only occurs in the marshes of the tidal freshwater part of the Scheldt and its tributaries.  相似文献   

7.
The existence of a negative relationship between fine sediment infiltration during the incubation period and salmonid embryo survival has often been discussed in the literature. However, few studies have specifically addressed this relationship in the field. We conducted a field experiment to examine the relationship between survival of Atlantic salmon (Salmo salar) embryos contained in incubation baskets and the patterns of fine sediment infiltration into the baskets during the incubation period. The results indicate that survival to pre-eyed (STPE), eyed (STE) and hatched (STH) stages of development were all negatively correlated with the percentage of fine sediment entering the baskets. STPE and STE were most strongly affected by silts and clays (<0.063 mm) although this size class represented only a small fraction of the grain size distribution inside the incubation baskets (0.03–0.41%). STH was most strongly correlated with the infiltration of medium sand (0.25–0.50 mm) material. On average, 66% of the implanted embryos survived to the pre-eyed stage of development compared to 63% for the eyed and 48% for the hatched stages of development.  相似文献   

8.
The temporal dynamics and spatial distribution of microphytobenthic chlorophyll-a in the layer 0–1 cm were determined in the Western Scheldt estuary over the period 1991–1992. Connections between the annually averaged benthic chlorophyll-a and station elevation and sediment composition (as a measure of the hydrodynamic energy caused by currents and waves) were also examined.Microphytobenthic chlorophyll-a showed one main peak in early summer and a smaller peak in autumn. The mean chlorophyll-a concentration of 113 mg Chl-a m–2 in the upper centimeter is of the same order of magnitude as in other estuarine areas. The average annual primary production of the microphytobenthos has been estimated at 136 g C m–2 y–1 The primary production of sediment inhabiting microalgae is at least 17% of the total primary production in the estuary.Considerable differences in annually averaged chlorophyll-a emerges between the stations. These differences are related mainly to the interaction between station elevation and clay content of the sediment.  相似文献   

9.
A cohesive sediment transport model considering the effects of flocculation, deposition and erosion is used in an attempt to simulate the suspended sediment distribution in a mesotidal estuary. The numerical model solves the three-dimensional (3D) advection-diffusion equation using a two-time level scheme, and a semi-implicit finite difference approach. The transport model is coupled to a 3D-barotropic hydrodynamic model for the simulation of the major tidal components reproducing the non-linear effects. An application of these models in the Western Scheldt estuary is described. The results of the different tests show that the adopted approach provides a useful basis for a good understanding of the physical processes involved in sediment transport and for the study of practical problems. The sensitivity of the model to key parameters controlling the simulation of bed sediment/water exchanges, shows the importance of a good definition of bottom sediment characteristics and the importance of further development of a consolidation algorithm.  相似文献   

10.
Information on movement patterns of marine fishes between estuarine populations and stocks at sea is fundamental to understanding their population dynamics, life history tactics and behavior. Furthermore, understanding estuarine habitat use by marine fishes is crucial for their effective conservation and integrated estuarine management. Although large numbers of young marine fish make use of temperate estuaries in highly predictable abundance patterns, very little is known about how estuarine populations interact with the populations at sea. Recruitment of sand goby Pomatoschistus minutus (Pallas, 1770) into the low salinity zone of the Scheldt estuary (Belgium) was reconstructed over an entire year by means of an isotopic clock. These results were combined with a growth model to yield age and length at immigration. Sand gobies entered the upper Scheldt estuary almost continuously from May onwards, except in July when they appeared to avoid the estuary due to warm summer temperatures. About 70% of the fish caught in the upper estuary resided there for less than 1 month, which indicates a strong temporal overlap of immigration and emigration. This complex migration pattern suggests that estuarine residence is caused by trade-offs made at the individual level, whereby migration is probably triggered by temperature. The high turnover of individuals in the estuarine population leads us to question the functional role of the estuary for marine fishes. Sand gobies entering the upper estuary had a wide range of ages and body sizes, although they were at least 2 months old and had a minimum standard length of approximately 20 mm. This study shows that the use of an isotopic clock strongly complements catch data and is useful to describe the connectivity between populations.  相似文献   

11.
12.
Biofilms in marine and fluvial environments can comprise strong bacterial and diatom mats covering large areas of the bed and act to bind sediments. In this case the bed material becomes highly resistant to shear stresses applied by the overlying fluid motion and detachment, when it does occur, is manifest in patches of biofilm of the order cm(2) being entrained into the flow. This article is the first to report tensile test data specific to the centimeter scale using moist biofilm/sediment composite materials; the strain (ε)-stress (σ) relationships permit quantification of the elasticity (Young's modulus, E) and cohesive strength of each specimen. Specifically, we compare the mechanical strength of cyanobacterial biofilm-only samples to that of biofilm cultured over sediment samples (glass beads or natural sands of d ~ 1 mm) for up to 8 weeks. The range of tensile strength (1,288-3,283 Pa) for composite materials was up to three times higher than previous tensile tests conducted at smaller scale on mixed culture biofilm [Ohashi et al. (1999) Water Sci Technol 39:261-268], yet of similar range to cohesive strength values recorded on return activated sludge flocs [RAS; Poppele and Hozalski (2003) J Microbiol Methods 55:607-615]. Composite materials were 3-6 times weaker than biofilm-only samples, indicating that adhesion to sediment grains is weaker than cohesion within the biofilm. Furthermore, in order to relate the tensile test results to the more common in-situ failure of bio-mats due to shear flow, controlled erosion experiments were conducted in a hydraulic flume with live fluid flow. Here, the fluid shear stress causing erosion was 3 orders of magnitude lower than tensile stress; this highlights both the problem of interpreting material properties measured ex-situ and the need for a better mechanistic model of bio-mat detachment.  相似文献   

13.
The geochemistry of dissolved and particulate trace metals has been studied in the water column and the sediments of the Scheldt estuary between 1987 and 1990. A strong seasonal influence on the behaviour of dissolved Cd, Cu and Zn is observed, related to the redox conditions in the upper estuary and phytoplankton activity in the lower estuary (which are both seasonally dependent variables). The dissolved trace metal concentrations in the fresh water end-member are remarkably low during spring and summer, due to metal sulphide precipitation in the anoxic Scheldt river. However, the dissolved concentrations increase rapidly with increasing salinity, due to oxidation of metal sulphides that are present in the suspended matter, accompanied by (e.g. chloro-)complexation of the released metals. Readsorption of Cd and Zn occurs in the lower estuary during the spring phytoplankton bloom. During winter, when the Scheldt river is not completely anoxic, much higher dissolved trace metal concentrations are observed in the fresh water end-member since metal sulphide precipitation in the water column is precluded. Rapid trace metal removal is observed in the low salinity, high turbidity zone, due to adsorption onto suspended matter and freshly precipitated iron and manganese oxyhydroxides. Upon further mixing, desorption is apparent, due to a similar oxidation-complexation mechanism as observed during spring and summer. Pore water infusion may also contribute to the enrichment of dissolved Cd, Cu and Zn in the mid-estuarine region. The trace metal contents of the suspended matter and the sediments show a continuous decrease with increasing salinity. This behaviour is to a very large extent due to physical mixing of contaminated fluvial particulates and relatively unpolluted marine particulates. Desorption of Cd, Cu and Zn can be identified but is of minor importance compared to the conservative mixing process. The distribution of dissolved Cd, Cu and Zn in the pore waters of the mid-estuarine region reflects the impact of early diagenetic processes. Trace metal peaks are observed near the sediment-water interface, and at greater depth in the manganese and iron reduction zones. These peaks are attributed to oxidation of reduced trace metal compounds (e.g. sulphides) and reduction of the (iron and manganese) oxide carrier phases, respectively. At greater depth, the dissolved trace metal concentrations are much lower due to metal sulphide precipitation in the sulphate reduction zone. Analysis of a large sediment dataset indicates severe trace metal pollution of the Scheldt estuary at the end of the fifties. A major reduction of the pollution by As, Cr, Hg, Pb, and Zn has occurred in the seventies, and of Cd and Cu in the eighties. The Ni pollution has increased over the time period considered. In spite of this improvement, the present-day pollution status of the Scheldt estuary is still reason for concern.  相似文献   

14.
Sly  P. G. 《Hydrobiologia》1989,(1):111-124
In part 2 of this contribution, examples are drawn from the River Mersey and Liverpool Bay illustrating the use of simple statistical parameters to describe dispersion of sands and muddy sediments. The River Mersey and Liverpool Bay, eastern Irish Sea, were sites of intensive studies on the dispersal of dumped harbour mud and sewage sludge during the mid 1960's–70's. The combined effects of strong tidal scour, wave action and shoreward near-bed residual drift result in shoreward transport of large volumes of sand in the bay. Large amounts of mud (silt/clay mixtures) oscillate in the river estuary, and naturally derived and dumped muds also move shoreward in the bay. Unpublished historic geochemical data have been combined with reprocessed particle size data and both have been used to reassess sedimentological techniques for defining transport and dispersal pathways. River and bay muds have similar size compositions, but river muds have excess Cd > V > U > As = Zn relative to bay muds. The lower relative concentrations of heavy metals in the bay are thought to reflect desorption and degradation of organic matter from the river. Trends in sediment distribution data based on the means of the sand size fraction, alone, provide sensitivities comparable to those of higher order moment measures and are usually easier to interpret than full size spectrum analyses.  相似文献   

15.
Fine sediment forms tidal flats along most of the Severn Estuary coastline but subtidal deposits are localized, mainly confined to Newport Deep and Bridgwater Bay. The estuary is experiencing a period of sea level rise which has led to coastal mud erosion in recent centuries. Sediment exchanges with the sea and river inputs are negligible compared to the large-scale exchanges within the estuary itself. Measurements of tidal flat level changes, archaeological finds and anthropogenic chemical concentrations confirm that mud flat erosion is long term and typical of the entire estuary. Sea level rise and coast erosion would normally lead to much of this sediment being redeposited inshore at the landward limit of the alluvium. Other than in narrow salt marsh strips, such deposition is prevented in the Severn by the river walls. In cores of subtidal muddy sediment, the number and thickness of sand layers decreases upwards. The top metre or so is entirely mud and exhibits thousands of layers of varying thickness and origin. Radiochemical analysis confirms that mud is accumulating in the subtidal zone of Newport Deep and the seaward periphery of the Bridgwater Bay mud patch and is increasing in proportion to sand. Circumstantial evidence strongly suggests that for at least the last 600 years the dominant pattern has been one of erosion of fine sediment from the coastal margin and its accumulation in subtidal sinks.  相似文献   

16.
The dynamics of carbon and nitrogen in carbonate mud were examined in the lagoons of Arlington and Sudbury Reefs, Great Barrier Reef. Most (89–93%) of the organic carbon and total nitrogen depositing to the carbonate mud zones was mineralized over a sediment depth of 1 m, with ∼50% of CO2 produced during microbial decomposition involved in carbonate precipitation/dissolution reactions. There was proportionally little burial of organic carbon (10–11%) or nitrogen (7–10%). Nitrogen budgets suggest rapid turnover of porewater inorganic N pools on the order of hours to a few days. Incubation experiments indicate carbonate dissolution in surface deposits (≤20 cm depth) and carbonate precipitation in deeper sediments. Depth-integrated reaction rates indicate net carbonate precipitation of 7–10 mol CaCO3 m2 year−1 over a depth of 1 m. Budget calculations at the whole-reef scale imply that deposition of CaCO3 in the mud zones of both lagoons may equate to 50–90% of total reef carbonate production, with organic carbon fluxes equating to nearly all net primary production on each reef. These biogeochemical estimates point to the functional importance of carbonate mud zones in the lagoons of the shelf reefs of the Great Barrier Reef.  相似文献   

17.
Catsadorakis  G. 《Hydrobiologia》1997,366(1-3):143-155
The main processes that determine the behaviour ofheavy metals in the Scheldt estuary are tidalhydrodynamics, sediment transport, and sorption ofheavy metals on suspended matter. The water qualitymodel WASP is applied to simulate the spatialdistribution of five heavy metals in the estuary,under average hydrodynamic and suspended sedimenttransport regimes. First, the hydrodynamical part ofthe model is constructed and the results are verifiedby comparison with measured water levels and flowvelocities. Secondly, a salt transport model is set upin order to evaluate the hydrodynamical dispersivemixing characteristics. Thirdly, a suspended sedimenttransport model is constructed and finally a transportmodel for heavy metals. The simulated distributions of the sorbedamounts of heavy metals, suspended sediment andsalinity in the estuary agree well with observations.The calculated profiles of dissolved and sorbedconcentrations of heavy metals in the water columnindicate an accumulation of heavy metals in the zoneof the turbidity maximum, while closer to the sea theconcentrations diminish due to mixing of the pollutedfluvial sediments with unpolluted marine sediments andbecause of sediment deposition in the estuary. It canbe concluded that only a small part of the heavymetals reaches the sea. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

18.
Modelling of water and sediment quality in the Scheldt estuary   总被引:2,自引:0,他引:2  
In 1987 the Tidal Waters Division (Ministry of Transport, Public Works and Water Management, The Netherlands) initiated the SAWES project (Systems Analysis WEstern Scheldt). In the framework of this project, a water quality model for the Scheldt estuary was made. The model aims to establish the relation between pollution inputs and the concentration of pollutants in the abiotic compartments and to establish the fate of wasted substances. The model includes oxygen, nitrogen and trace metals. Due to the low oxygen content of the upper Scheldt estuary, a new model approach for trace metals had to be developed, taking into account precipitated metal sulphides. The calibration, verification and sensitivity analysis of the model provided a good understanding of the chemistry of the estuary. Afterwards the model was used to support policy making by computing how the water and sediment quality in the estuary respond to reduced inputs of waste.  相似文献   

19.
The application of correspondence factorial analysis to four types of sediment in the upper reaches of the Loire estuary enabled the significance of environmental factors in the spatial distribution, abundance and seasonal dynamics of the biotic communities to be determined. In coarse sand, the communities are stable but species are few in number and population densities are low. In muddy sediments (from muddy sand to fluid mud) population densities are high but undergo seasonal fluctuations due to changes in the hydrological regime (especially the dissolved oxygen regime). This is true for the composition of the biotic communities. The oligohaline phase, in which the fluvial element is predominant, is characterized by abundance maxima due to freshwater species. The mesohaline phase, in which the marine element is predominant, exerts a restrictive action on population densities; certain species are supplanted by others, their development being governed by the quality of the environment.  相似文献   

20.
Most aquatic vegetation restorations involve the transplantation of submerged macrophytes. Sediment type and the clonal size are of great significance as they determine the fate of submerged macrophytes. In order to ensure successful restoration, a simulation experiment was conducted using aquarium mesocosms to investigate the response of stolon propagation capacity, the morphological features and productivity of Vallisneria natans for four types of sediment (lake mud [L], lake mud + sand [L + S, 50:50, v/v mixture], sand [S], clay [C]), and three types of clonal sizes. Results showed that sediment types significantly affected V. natans biomass accumulation, stolon propagation ability, ramet morphological characteristics, and productivity, where the asexual reproduction ability and productivity ranked as L > L + S > S > C in four sediment types. Total biomass, maximum net production, number of ramets, root diameter, number of stolons, and stolon propagation rate were all highest in L. In L and L + S, the plant chlorophyll content was higher than in S and C. The root diameter and the ratio of aboveground/underground biomass in S were the smallest among the four sediments. Moreover, when more V. natans seedlings were linked, more ramets and biomass were produced. The stolon propagation rate was ranked as the stolon with single seedling greater than the stolon with two‐linked seedlings greater than the stolon with three‐linked seedlings in L and L + S. The concentration of total nitrogen, total phosphorus, and NO3?‐N in water was remarkably reduced in four aquariums. Findings provide a scientific basis for restoring submerged macrophytes in different sediment settings.  相似文献   

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