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1.
Measurements are presented of the properties of suspended particulate matter (SPM) in the estuarine turbidity maximum (ETM) of the upper Humber and Ouse estuaries during transient, relatively low freshwater inflow conditions of September 1995. Very high concentrations of near-bed SPM (more than 100 g l−1) were observed in the low-salinity (less than 1), upper reaches. SPM within the ETM consisted largely of fine sediment (silt and clay) that existed as microfloc and macrofloc aggregates and individual particles. Primary sediment particles were very fine grained, and typically, about 20–30% was clay-sized at high water. The clay mineralogy was dominated by chlorite and illite. There was a pronounced increase in particle size in the tidal river, up-estuary of the ETM. The mean specific surface area (SSA) of near-bed SPM within the ETM was 22 m2 g−1 on a spring tide and 24 m2 g−1 on a neap tide. A tidal cycle of measurements within a near-bed, high concentration SPM layer during a very small neap tide gave a mean SSA of 26 m2 g−1. The percentage of silt and clay in surficial bed sediments along the main channel of the estuary varied strongly. The relatively low silt and clay percentage of surficial bed sediments (about 10–35%) within the ETM’s region of highest near-bed SPM concentrations and their low SSA values were in marked contrast to the overlying SPM. The loss on ignition (LOI) of near-bed SPM in the turbid reaches of the estuary was about 10%, compared with about 12% for surface SPM and more than 40% in the very low turbidity waters up-estuary of the ETM. Settling velocities of Humber–Ouse SPM, sampled in situ and measured using a settling column, maximized at 1.5 mm s−1 and exhibited hindered settling at higher SPM concentrations.  相似文献   

2.
Few hyperpycnal flows have ever been observed in marine environments although they are believed to play a critical role in sediment dispersal within estuarine and deltaic depositional systems. The paper describes hyperpycnal flows observed in situ off the Huanghe (Yellow River) mouth, their relationship to tidal cycles, and the mechanisms that drive them. Simultaneous observations at six mooring stations during a cruise off the Huanghe mouth in the flood season of 1995 suggest that hyperpycnal flows observed at the river mouth are initiated by high concentrations of sediment input from river and modulated by tides. Hyperpycnal flows started near the end of ebb tides, when near‐bottom suspended sediment concentration (SSC) increased rapidly and salinity decreased drastically (an inverse salt wedge). The median grain size of suspended particles within the hyperpycnal layer increased, causing strong stratification of the suspended sediments in the water column. Towards the end of flood tides, the hyperpycnal flow attenuated due to frictions at the upper and lower boundaries of the flow and tidal mixing, which collapsed the stratification of the water column. Both sediment concentration and median grain size of suspended particles within the bottom layer significantly decreased. The coarser sediment particles were deposited and the hyperpycnal flows stopped. The intra‐tidal behaviors of hyperpycnal flows are closely associated with the variations of SSC, salinity, and stratification of the water column. As nearly 90% of riverine sediment is delivered to the sea during the flood seasons when hyperpycnal flows are active, hyperpycnal flows at the Huanghe mouth and the river's high sediment loads have caused rapid accretion of the Huanghe delta. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

3.
Muddy sediments with their potential for containing contaminants are commonly deposited and remobilized by tidal currents in estuarine environments. We examined the mobilization and subsequent redeposition of mud in a coastal plain estuary located in the southeastern United States. Time-series data for salinity, suspended sediment concentrations and quality (percent organic matter and pigment concentrations) were obtained over a 13-hour tidal cycle. We found that fast-settling mud particles are found during the highest tidal current speeds. Particle quality analyses suggest that all the material is of similar origin, and that phaeopigment can be used as a tracer of particles in this system. These particles settle onto the bed when current speeds approach slack conditions. We speculate that the quantity of mud mobilized during neap tide is less than during spring tide resulting in an opportunity for the mud to partially consolidate on the bottom and be removed from resuspension. We further speculate that the muddy sediments are mainly derived from fringing marshes in this estuary.  相似文献   

4.
The erodibility of intertidal sediments is an important factor affecting coastal erosion.In July and October 2008,in situ measurement of erodibility of the surficial sediment were conducted using a recirculating flume at 20 tidal flat experiment sites along the seashore of the Yellow River delta.At the same time,the characteristics of sand ripples and biogenic features on the tidal flat were observed and the physical-mechanical sediment properties such as bulk density,water content,grain size distribution,plasticity,penetration resistance,shear strength and compressibility,were measured.By field measurement,it is obtained that the critical erosion shear stress of the surficial sediment on the tidal flat varies between 0.088 Pa and 0.254 Pa.The factors influencing sediment erodibility are complicated because of physical and biological reworking after the sediment deposited.There’s a positive correlation between shear strength and critical erosion shear stress.The burrowing crabs’ activities changed the sediment microtopography and made the sediment have greater roughness,and that is one possible reason for the higher erodibility.The formation of scour pits on the tidal flat correlates with the heterogeneous erodibility of the surficial sediment.  相似文献   

5.
In tidal environments, the response of suspended sediment concentration (SSC) to the current velocity is not instantaneous, the SSC lagging behind the velocity (phase lag), and the amplitude of SSC variation decreasing with height above the bed (amplitude attenuation). In order to quantitatively describe this phenomenon, a one-dimensional vertical advection–diffusion equation of SSC is derived analytically for uniform unsteady tidal flow by defining a concentration boundary condition using a constant vertical eddy diffusivity and sediment settling velocity. The solution, in simple and straightforward terms, shows that the vertical phase lag increases linearly with the height above the bed, while the amplitude of the SSC variation decreases exponentially with the height. The relationship between the SSC and the normalized current velocity can be represented by an ellipse or a line, depending on the phase lag. The lag of sediment movement or “diffusion/settling lag” is the mechanism generating the phase lag effect. Field observations used for validation show that the theoretically predicted and the observed curves of the vertical SSC phase lag and amplitude attenuation show reasonable agreement. The procedure proposed in this paper substantially simplifies the modeling of suspended matter transport in tidal flows.  相似文献   

6.
A tidal bore is a water discontinuity at the leading edge of a ood tide wave in estuaries with a large tidal range and funneling topography. New measurements were done in the Garonne River tidal bore on 14 15 November 2016, at a site previously investigated between 2010 and 2015. The data focused on long, continuous, high-frequency records of instantaneous velocity and suspended sediment con- centration (SSC) estimate for several hours during the late ebb, tidal bore passage and ood tide. The bore passage drastically modi ed the ow eld, with very intense turbulent and sediment mixing. This was evidenced with large and rapid uctuations of both velocity and Reynolds stress, as well as large SSCs during the ood tide. Granulometry data indicated larger grain sizes of suspended sediment in water samples compared to sediment bed material, with a broader distribution, shortly after the tidal bore. The tidal bore induced a sudden suspended sediment ux reversal and a large increase in suspended sedi- ment ux magnitude. The time-variations of turbulent velocity and suspended sediment properties indicated large uctuations throughout the entire data set. The ratio of integral time scales of SSC to velocity in the x-direction was on average TE,SSC/TE,x 0.16 during the late ebb tide, compared to TE,SSC/ TE,x 0.09 during the late ood tide. The results imply different time scales between turbulent velocities and suspended sediment concentrations.  相似文献   

7.
The formation and evolution of tidal platforms are controlled by the feedbacks between hydrodynamics, geomorphology, vegetation, and sediment transport. Previous work mainly addresses dynamics at the scale of individual marsh platforms. Here, we develop a process-based model to investigate salt marsh depositional/erosional dynamics and resilience to environmental change at the scale of tidal basins. We evaluate how inputs of water and sediment from river and ocean sources interact, how losses of sediment to the ocean depend on this interaction, and how erosional/depositional dynamics are coupled to these exchanges. Model experiments consider a wide range of watershed, basin, and oceanic characteristics, represented by river discharge and suspended sediment concentration, basin dimensions, tidal range, and ocean sediment concentration. In some scenarios, the vertical accretion of a tidal flat can be greater than the rate of sea level rise. Under these conditions, vertical depositional dynamics can lead to transitions between tidal flat and salt marsh equilibrium states. This type of transition occurs much more rapidly than transitions occurring through horizontal marsh expansion or retreat. In addition, our analyses reveal that river inputs can affect the existence and extent of marsh/tidal flat equilibria by both directly providing suspended sediment (favoring marshes) and by modulating water exchanges with the ocean, thereby indirectly affecting the ocean sediment input to the system (favoring either marshes or tidal flats depending on the ratio of the river and ocean water inputs and their sediment concentrations). The model proposed has the goal of clarifying the roles of the main dynamic processes at play, rather than of predicting the evolution of a particular tidal system. Our model results most directly reflect micro- and meso-tidal environments but also have implications for macro-tidal settings. The model-based analyses presented extend our theoretical understanding of marsh dynamics to a greater range of intertidal environments. © 2020 John Wiley & Sons, Ltd.  相似文献   

8.
Numerical models of fine sediment transport depend on different approaches to parameterize the erosion properties of surficial sediment strata. These properties, namely the critical shear stress for erosion and the erosion rate coefficient, are crucial for reproducing the short-term and long-term sediment dynamics of the system. Methods to parameterize these properties involve either specialized laboratory measurements on sediment samples or optimization by model calibration. Based on observations of regular patterns in the variation of suspended sediment concentrations (SSC) over the tidal cycle in a small, narrow estuary, an alternate approach, referred to as the entrainment flux method, for quantifying the erosion properties of surficial bed strata is formulated and applied. The results of this method are shown to be analogous to the erosion data used to formulate the standard linear erosion formulation developed by various authors. The erosion properties inferred from the entrainment flux method are also compared to direct measurements of erodibility on sediment samples from the same site using the Gust microcosm apparatus. The favorable comparison of the two approaches suggests that the entrainment flux method can be used to infer and quantify the erodibility of surficial sediment strata in similar small and narrow estuaries. This method has certain advantages, chiefly its ease of implementation and the fact that it uses SSC time series which would typically be expected to be available for the study of or for model application at a given site. Guidelines for selecting the appropriate dataset for the application of the method are also presented.  相似文献   

9.
Liu  Guangliang  Liu  Zhe  Gao  Huiwang  Gao  Zengxiang  Feng  Shizuo 《Ocean Dynamics》2012,62(10):1443-1456

The Eulerian residual transport velocity and the first-order Lagrangian residual velocity for weakly nonlinear systems have been used extensively in the past to depict inter-tidal mass transport. However, these could not explain the observed net surface sediment transport pattern in Jiaozhou Bay (JZB), located on the western Yellow Sea. JZB is characterized by strong tidal motion, complex topography and an irregular coastline, which are features of typical nonlinear systems. The Lagrangian residual velocity, which is applicable to general nonlinear systems, was simulated with the water parcel tracking method. The results indicate that the composition of the Lagrangian residual velocity at different tidal phases coincides well with the observed net surface sediment transport pattern. The strong dependence of water flushing time on the initial tidal phase can also be explained by the significant intra-tidal variation of the Lagrangian residual velocity. To investigate the hydrodynamic mechanism governing the nonlinearity of the M 2 tidal system, a set of nonlinearity indexes were defined and analysed. In the surface layer, horizontal advection is the main contributor to the strong nonlinearity near the bay mouth, while in the bottom layer, the strong nonlinearity near the bay mouth may result from the vertical viscosity and horizontal advection.

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10.
The Eulerian residual transport velocity and the first-order Lagrangian residual velocity for weakly nonlinear systems have been used extensively in the past to depict inter-tidal mass transport. However, these could not explain the observed net surface sediment transport pattern in Jiaozhou Bay (JZB), located on the western Yellow Sea. JZB is characterized by strong tidal motion, complex topography and an irregular coastline, which are features of typical nonlinear systems. The Lagrangian residual velocity, which is applicable to general nonlinear systems, was simulated with the water parcel tracking method. The results indicate that the composition of the Lagrangian residual velocity at different tidal phases coincides well with the observed net surface sediment transport pattern. The strong dependence of water flushing time on the initial tidal phase can also be explained by the significant intra-tidal variation of the Lagrangian residual velocity. To investigate the hydrodynamic mechanism governing the nonlinearity of the M 2 tidal system, a set of nonlinearity indexes were defined and analysed. In the surface layer, horizontal advection is the main contributor to the strong nonlinearity near the bay mouth, while in the bottom layer, the strong nonlinearity near the bay mouth may result from the vertical viscosity and horizontal advection.  相似文献   

11.
Hyperconcentrated flows as influenced by coupled wind-water processes   总被引：4，自引：0，他引：4  
Hyperconcentrated flow is a natural phenomenon, which is widely observed on the Loess Plateau of China[1,2]. So far, much research has been done with hyperconcentrated flows in China[1―7], although hy-perconcentrated flows are also observed in many riv-ers in other countries[8―10]. In the monograph edited by Chien[11], hyperconcnetrated flows were studied in depth, involving the physical properties, resistance, sediment-carry behavior and channel-forming pro- cesses. Wang and Chien el al.[…  相似文献   

12.
Seasonal observations on the nature and concentration of suspended particulate matter (SPM) are presented for a cross-section of the English Channel, between the Isle of Wight (UK) and Cotentin peninsula (France) i.e. the western boundary of the eastern English Channel. The highest concentrations of suspended material are found adjacent to the English coastline, whereas the offshore waters are associated with low concentrations. Seasonal variations in the concentration and nature of suspended material are identified, with highest concentrations in winter. At this time, the suspended particles are characterised generally by peaked grain size spectra and an enrichment in coarse silt particles; in summer, the distributions are generally flat. The diatom communities found within the suspended matter indicate that material resuspended in the coastal zone and the estuarine environments is transported offshore. SPM fluxes (based upon the observed SPM concentrations and the output from a 2-D hydrodynamic model) from the western Channel through the Wight–Cotentin Section, ranged between 2 and 71×106 t a−1 with a mean of around 20×106 t a−1 over the period of the observations (1994–1995). These fluxes are comparable to the order of magnitude and mean value reported as output through the Dover Strait. Therefore, it is possible that the eastern English Channel may be characterised as an area of fine-grained sediment ‘bypass'. This interpretation is corroborated by: (a) the absence of fine-grained sediment deposits over the area; and (b) correlation between the potential resuspension time of the fine particles and the seabed sediment distribution.  相似文献   

13.
The macro-tidal Gulf of Kachchh, covering nearly 7000 km(2), is located about 150 km south of the Indus River mouth. In spite of semi-arid climate and lack of major rivers flowing into it, the Gulf is highly turbid with suspended sediment concentrations (SSC) during October-November 2002 ranging between 0.5 and 674 mgl(-1). Highly turbid waters are observed towards the northern portion of the mouth of the Gulf, at the head of the Gulf and adjacent to the numerous shoals present within the Gulf. Perennial high SSC in the Gulf is due to resuspension of sediments by strong tidal currents, shallow bathymetry and presence of fine-grained sediments on the sea floor. Numerical model studies show that there is a dynamic barrier in the central Gulf, which prevents the exchange of water and suspended sediments between the outer and inner Gulf. This dynamic barrier associated with strong east-west tidal currents restricts the turbid waters mainly to the northern Gulf, resulting in relatively clear waters (SSC<10 mgl(-1)) in the southern and central portions of the Gulf. Laser particle size distribution, clay mineralogy and geochemistry of the suspended matter show that the main source of sediments to the Gulf of Kachchh is the Indus River. Although the Indus discharge has been severely curtailed in the recent decades due to construction of numerous dams and barrages, the Gulf of Kachchh continues to receive resuspended sediments from the numerous meso and macro-tidal creeks of the Indus delta. The sediments at the head of the Gulf appear to be a mixture of sediments derived from the Indus as well as the numerous seasonal rivers draining the Rann of Kachchh.  相似文献   

14.
Shallow tidal environments (e.g. bays, estuaries, lagoons) represent one of the most productive ecosystems in the world, and they are threatened by current climate change and increasing human pressure. Monitoring the bio-morphodynamic evolution of these environments is therefore a crucial task that requires a detailed and holistic scrutiny. The present study aims to investigate the temperature of the water–sediment continuum, its effect on the related microphytobenthos (MPB) growth and the related bio-stabilization of the bed sediment surface under different water depth and water turbidity conditions. We investigated the vertical energy transfer and the temperature dynamics by applying a 1-D model to a shallow coastal lagoon. Our results show that the water temperature does not substantially change under different turbidity conditions, whereas the sediment temperature exhibits important changes. Two major factors driving the MPB photosynthetic growth are the sediment surface temperature and the light availability at the sediment bed, which can both be computed using the vertical energy transfer model. We observed that, in general, clear water conditions better promote MPB growth over the entire year. The limiting factor for the photosynthetic process is usually the light availability at the bottom, which increases under clear water conditions. As MPB provides a bio-stabilizing effect on the bed sediments by producing a biofilm on the sediment surface that reduces sediment resuspension, our results suggest a positive feedback between MPB growth and water column turbidity. Furthermore, MPB growth and the related sediment bio-stabilization are clearly affected by the seasonal variation of surface sediment temperature and light availability. This seasonal variation of MPB growth rate and surface sediment bio-stabilization must be considered when studying the long-term morphodynamic evolution of tidal environments. © 2018 John Wiley & Sons Ltd.  相似文献   

15.
In many tidal embayments, bottom patterns, such as the channel-shoal systems of the Wadden Sea, are observed. To gain understanding of the mechanisms that result in these bottom patterns, an idealized model is developed and analyzed for short tidal embayments. In this model, the water motion is described by the depth- and width-averaged shallow water equations and forced by a prescribed sea surface elevation at the entrance of the embayment. The bed evolves due to the divergence and convergence of suspended sediment fluxes. To model this suspended-load sediment transport, the three-dimensional advection–diffusion equation is integrated over depth and averaged over the width. One of the sediment fluxes in the resulting one-dimensional advection–diffusion equation is proportional to the gradient of the local water depth. In most models, this topographically induced flux is not present. Using standard continuation techniques, morphodynamic equilibria are obtained for different parameter values and forcing conditions. The bathymetry of the resulting equilibrium bed profiles and their dependency on parameters, such as the phase difference between the externally prescribed M2 and M4 tide and the sediment fall velocity, are explained physically. With this model, it is then shown that for embayments that are dominated by a net import of sediment, morphodynamic equilibria only exist up to a maximum embayment length. Furthermore, the sensitivity of the model to different morphological boundary conditions at the entrance of the embayment is investigated and it is demonstrated how this strongly influences the shape and number of possible equilibrium bottom profiles. This paper ends with a comparison between the developed model and field data for the Wadden Sea’s Ameland and Frisian inlets. When the model is forced with the observed M2 and M4 tidal constituents, morphodynamic equilibria can be found with embayment lengths similar to those observed in these inlets. However, this is only possible when the topographically induced suspended sediment flux is included. Without this flux, the maximum embayment length for which morphodynamic equilibria can be found is approximately a third of the observed length. The sensitivity of the model to the topographically induced sediment flux is discussed in detail.  相似文献   

16.
The isotopic composition and concentration of Pb was measured in suspended particulate matter of the Irish Sea. Aerosol, surficial and pre-industrial sediment was also analysed to provide information on sources terms. Concentrations of Pb in suspended sediments were lower than previously reported which presumably reflects the international effort to reduce Pb inputs to the environment. Lead concentrations were highest in Liverpool Bay and lowest in the western Irish Sea. A significant negative relationship between Pb and salinity suggests that present inputs and the resuspension of relict lead associated with particles in areas significantly affected by freshwater discharges are the predominant sources of Pb to the Irish Sea. The isotopic composition of Pb in the stratified region of the western Irish Sea demonstrates that atmospheric sources are also significant to this region, which is consistent with current knowledge on the hydrography. Pb isotopic ratios show that water entering the Irish Sea through St George's Channel is significantly influenced by anthropogenic inputs prior to additional contamination by direct inputs to the Irish Sea.  相似文献   

17.
The German Wadden Sea (southern North Sea) sediments are composed of both cohesive and non-cohesive deposits. The spatial distribution patterns are mainly driven by wind-induced waves and tidal currents. Transport intensity and duration depend on the hydrodynamic conditions, which vary over time. In this paper, the transport of suspended sediment was investigated on seasonal, tidal and hourly time scales in the back-barrier system of Spiekeroog Island. Long- and short-term data of fair weather periods and two storm events were investigated based on stationary and mobile measurements of currents and waves by Acoustic Doppler Current Profiler (ADCP), in situ particle size and suspended sediment concentration (SSC) measurements by laser in situ scattering and transmissometry (LISST) as well as wind records. The ADCP backscatter intensities were calibrated by means of LISST volume concentration data in order to quantify longer term SSCs and fluxes in the back-barrier system. Values up to 120 mg l−1 were recorded, but concentrations more commonly were below 60 mg l−1. The long-term results confirm former observations of a balanced budget during low-energy (fair weather) conditions in the study area. In general, SSCs were higher during spring tides than during neap tides. The data also clearly show the remobilisation of sediment by tidal current entrainment. The records include two severe storm events, “Britta” (1st November 2006) and “Kyrill” (18th January 2007). The data reveal very complex temporal flow and transport patterns. During both storm events, the export of material was mainly controlled by the interaction of wind, waves and tidal phase. The typical ebb-dominance occurring during fair-weather conditions was temporarily neutralised and even reversed to a flood-dominated situation. During “Kyrill”, the wind and high-waves setup in conjunction with the tidal phase was even able to compress the duration of two successive ebb cycles by over 70%. Although SSCs increased during both storms and higher turbulence lifted particle clouds upwards, an export of suspended matter towards the North Sea was only observed under the conditions taking place during “Britta”. Such fluxes, however, are currently still difficult to quantify because the backscatter intensity during high energy events includes a substantial amount of noise produced by the high turbulence, especially near the water surface.  相似文献   

18.
The Rouse formula and its variants have been widely used to calculate the steady-state vertical concentration distribution for suspended sediment in steady sediment-laden flows, where the diffusive flux is assumed to be Fickian. Turbulent flow, however, exhibits fractal properties, leading to non-Fickian diffusive flux for sediment particles. To characterize non-Fickian dynamics of suspended sediment, the current study proposes a Hausdorff fractal derivative based advection-dispersion equation(H...  相似文献   

19.
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20.
The paper analyses the concentrations of total phosphorus and its forms in sediments from the Gulf of Gdańsk on the basis of studies conducted at 25 sampling sites in 2001–05. The phosphorus speciation analysis was performed by sequential extraction. The extensive spatial variability of Ptot concentrations and speciation was found to be dependent on the physicochemical properties of the sediments, the oxygen conditions in the water and sediments, and the depth of the water column above the sediment surface. In the coastal zone, the sedimentation of riverine suspended matter and the sorption and chemisorption processes exert a considerable influence on P speciation. Over 70% of variation of total phosphorus concentration in sediments in the Gulf of Gdańsk could be explained by changes of proportion of fine fraction of sediments (grain size <0.0625 mm). Maximum Ptot concentrations were recorded in clays and silts in the deep water, stratified part of the Gulf of Gdańsk. In the coastal zone, where sandy sediments are dominant, phosphorus concentrations were much lower; this was due to the considerable dynamics of the bottom water and intensive sea floor transport. Ptot concentrations in the Gulf of Gdańsk sediments ranged from 1.75 to 957.17 μmol g−1 d.w. Of all the forms of phosphorus, the highest concentrations were found for organic phosphorus (Org-P). Of its inorganic forms, the highest concentrations were of phosphorus bound to clay minerals and aluminium oxides (NaOH-P), the lowest ones were of loosely bound phosphorus (NaCl-P). On the basis of determinations of total phosphorus concentrations in sediments of a given type and the available data on the seabed areas covered by particular sediment types in the Gulf of Gdańsk, the mass of total phosphorus in the surficial sediment layer (0–2 cm) was estimated at ca. 15.6×103 tonnes.  相似文献   

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