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1.
A three-dimensional hydrodynamic model is used to investigate intra-tidal and spring–neap variations of turbulent mixing, stratification and residual circulation in the Chesapeake Bay estuary. Vertical profiles of salinity, velocity and eddy diffusivity show a marked asymmetry between the flood and ebb tides. Tidal mixing in the bottom boundary layer is stronger and penetrates higher on flood than on ebb. This flood–ebb asymmetry results in a north–south asymmetry in turbulent mixing because tidal currents vary out of phase between the lower and upper regions of Chesapeake Bay. The asymmetric tidal mixing causes significant variation of salinity distribution over the flood–ebb tidal cycle but insignificant changes in the residual circulation. Due to the modulation of tidal currents over the spring–neap cycle, turbulent mixing and vertical stratification show large fortnightly and monthly fluctuations. The stratification is not a linear function of the tidal-current amplitude. Strong stratification is only established during those neap tides when low turbulence intensity persists for several days. Residual circulation also shows large variations over the spring–neap cycle. The tidally averaged residual currents are about 50% stronger during the neap tides than during the spring tides.  相似文献   

2.
An observational study in the middle reach of Delaware Bay shows that vertical stratification is often enhanced during flood tide relative to ebb tide, contrary to the tidal variability predicted by the tidal straining mechanism. This tidal period variability was more pronounced during times of high river discharge when the tidally mean stratification was higher. This tidal variability in stratification is caused by two reinforcing processes. In the along-channel direction, the upstream advection of a salinity front at mid-depth causes an increase of the vertical stratification during the flood tide and a decrease during the ebb tide. In the cross-channel direction, the tilting of isohalines during the ebb reduces vertical stratification, and the subsequent readjustment of the salinity field during the flood enhances the water column stability. A diagnosis of the cross-channel momentum balance reveals that the lateral flows are driven by the interplay of Coriolis forcing and the cross-channel pressure gradient. During the flood tide, these two forces mostly reinforce each other, while the opposite occurs during the ebb tide. This sets up a lateral circulation that is clockwise (looking landward) during the first half of the flood and then reverses and remains counterclockwise during most of the ebb tide. Past maximum ebb, the cross-channel baroclinic term, overcomes Coriolis and reverses the lateral flows.  相似文献   

3.
A numerical modeling study of the influence of the lateral flow on the estuarine exchange flow was conducted in the north passage of the Changjiang estuary. The lateral flows show substantial variabilities within a flood-ebb tidal cycle. The strong lateral flow occurring during flood tide is caused primarily by the unique cross-shoal flow that induces a strong northward (looking upstream) barotropic force near the surface and advects saltier water toward the northern part of the channel, resulting in a southward baroclinic force caused by the lateral density gradient. Thus, a two-layer structure of lateral flows is produced during the flood tide. The lateral flows are vigorous near the flood slack and the magnitude can exceed that of the along-channel tidal flow during that period. The strong vertical shear of the lateral flows and the salinity gradient in lateral direction generate lateral tidal straining, which are out of phase with the along-channel tidal straining. Consequently, stratification is enhanced at the early stage of the ebb tide. In contrast, strong along-channel straining is apparent during the late ebb tide. The vertical mixing disrupts the vertical density gradient, thus suppressing stratification. The impact of lateral straining on stratification during spring tide is more pronounced than that of along-channel straining during late flood and early ebb tides. The momentum balance along the estuary suggests that lateral flow can augment the residual exchange flow. The advection of lateral flows brings low-energy water from the shoal to the deep channel during the flood tide, whereas the energetic water is moved to the shoal via lateral advection during the ebb tide. The impact of lateral flow on estuarine circulation of this multiple-channel estuary is different from single-channel estuary. A model simulation by blocking the cross-shoal flow shows that the magnitudes of lateral flows and tidal straining are reduced. Moreover, the reduced lateral tidal straining results in a decrease in vertical stratification from the late flood to early ebb tides during the spring tide. By contrast, the along-channel tidal straining becomes dominant. The model results illustrate the important dynamic linkage between lateral flows and estuarine dynamics in the Changjiang estuary.  相似文献   

4.
Tidal straining effect on sediment transport dynamics in the Huanghe (Yellow River) estuary was studied by field observations and numerical simulations. The measurement of salinity, suspended sediment concentration, and current velocity was conducted during a flood season in 1995 at the Huanghe river mouth with six fishing boats moored at six stations for 25-h hourly time series observations. Based on the measurements, the intra-tidal variations of sediment transport in the highly turbid river mouth was observed and the tidal straining effect occurred. Our study showed that tidal straining of longitudinal sediment concentration gradients can contribute to intra-tidal variability in sediment stratification and to asymmetries in sediment distribution within a tidal cycle. In particular, the tidal straining effect in the Huanghe River estuary strengthened the sediment-induced stratification at the flood tide, thus producing a higher bottom sediment concentration than that during the ebb. A sediment transport model that is capable of simulating sediment-induced stratification effect on the hydrodynamics in the bottom boundary layers and associated density currents was applied to an idealized estuary to demonstrate the processes and to discuss the mechanism. The model-predicted sediment processes resembled the observed characteristics in the Huanghe River estuary. We concluded that tidal straining effect is an important but poorly understood mechanism in the transport dynamics of cohesive sediments in turbid estuaries and coastal seas.  相似文献   

5.
Using in situ, continuous, high frequency (8–16 Hz) measurements of velocity, suspended sediment concentration (SSC), and salinity, we investigate the factors affecting near-bed sediment flux during and after a meteorological event (cold front) on an intertidal flat in central San Francisco Bay. Hydrodynamic forcing occurs over many frequency bands including wind wave, ocean swell, seiching (500–1000 s), tidal, and infra-tidal frequencies, and varies greatly over the time scale of hours and days. Sediment fluxes occur primarily due to variations in flow and SSC at three different scales: residual (tidally averaged), tidal, and seiching. During the meteorological event, sediment fluxes are dominated by increases in tidally averaged SSC and flow. Runoff and wind-induced circulation contribute to an order of magnitude increase in tidally averaged offshore flow, while waves and seiching motions from wind forcing cause an order of magnitude increase in tidally averaged SSC. Sediment fluxes during calm periods are dominated by asymmetries in SSC over a tidal cycle. Freshwater forcing produces sharp salinity fronts which trap sediment and sweep by the sensors over short (∼30 min) time scales, and occur primarily during the flood. The resulting flood dominance in SSC is magnified or reversed by variations in wind forcing between the flood and ebb. Long-term records show that more than half of wind events (sustained speeds of greater than 5 m/s) occur for 3 h or less, suggesting that asymmetric wind forcing over a tidal cycle commonly occurs. Seiching associated with wind and its variation produces onshore sediment transport. Overall, the changing hydrodynamic and meteorological forcing influence sediment flux at both short (minutes) and long (days) time scales.  相似文献   

6.
In situ observations and numerical simulations of turbulence are essential to understanding vertical mixing processes and their dynamical controls on both physical and biogeochemical processes in coastal embayments. Using in situ data collected by bottom-mounted acoustic Doppler current profilers(ADCPs) and a free-falling microstructure profiler, as well as numerical simulations with a second-moment turbulence closure model, we studied turbulence and mixing in the Xiamen Bay, a freshwater-influenced tidal bay located at the west coast of the Taiwan Strait. Dynamically, the bay is driven predominantly by the M2 tide, and it is under a significant influence of the freshwater discharged from the Jiulong River. It is found that turbulence quantities such as the production and dissipation rates of the turbulent kinetic energy(TKE) were all subject to significant tidal variations, with a pronounced ebb-flood asymmetry. Turbulence was stronger during flood than ebb. During the flooding period, the whole water column was nearly well mixed with the depth-averaged TKE production rate and vertical eddy viscosity being up to 5?10?6 W kg?1 and 2?10?2 m2 s?1, respectively. In contrast, during the ebb strong turbulence was confined only to a 5?8 m thick bottom boundary layer, where turbulence intensity generally decreases with distance from the seafloor. Diagnosis of the potential energy anomaly showed that the ebb-flood asymmetry in turbulent dissipation and mixing was due mainly to tidal straining process as a result of the interaction between vertically shared tidal currents and horizontal density gradients. The role of vertical mixing in generating the asymmetry was secondary. A direct comparison of the modeled and observed turbulence quantities confirmed the applicability of the second-moment turbulence closure scheme in modeling turbulent processes in this weakly stratified tidally energetic environment, but also pointed out the necessity of further refinements of the model.  相似文献   

7.
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   

8.
Sediment flux in marsh tidal creeks is commonly used to gauge sediment supply to marshes. We conducted a field investigation of temporal variability in sediment flux in tidal creeks in the accreting tidal marsh at China Camp State Park adjacent to northern San Francisco Bay. Suspended‐sediment concentration (SSC), velocity and depth were measured near the mouths of two tidal creeks during three 6‐ to 10‐week deployments: two in winter and one in summer. Currents, wave properties and SSC were measured in the adjacent shallows. All deployments spanned the largest spring tides of the season. Results show that tidally averaged suspended‐sediment flux (SSF) in the tidal creeks varied from slightly landward to strongly bayward with increasing tidal energy. SSF was negative (bayward) for tidal cycles with maximum water surface elevation above the marsh plain. Export during the largest spring tides dominated the cumulative SSF for each deployment. During ebb tides following the highest tides, velocities exceeded 1 m s?1 in the narrow tidal creeks, resulting in negative tidally averaged water flux, and mobilizing sediment from the creek banks or bed. Storm surge also produced negative SSF. Tidally averaged SSF was positive in wavy conditions with moderate tides. Spring tide sediment export at the creek mouth was about twice that at a station 130 m further up the tidal creek. The negative tidally averaged water flux near the creek mouth during spring tides indicates that in the lower marsh some of the water flooding directly across the bay–marsh interface drains through the tidal creeks, and suggests that this interface may be a pathway for sediment supply to the lower marsh as well. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

9.
The morphodynamics of shallow, vertically well-mixed estuaries, characterised by tidal flats and deeper channels, have been investigated. This paper examines what contributes to flood/ebb-dominant sediment transport in localised regions through a 2D model study (using the TELEMAC modelling system). The Dyfi Estuary in Wales, UK has been used as a case study and, together with idealised estuary shapes, shows that shallow water depths lead to flood dominance in the inner estuary whilst tidal flats and deep channels cause ebb dominance in the outer estuary. For medium sands and with an artificially ‘flattened’ bathymetry (i.e. no tidal flats), the net sediment transport switches from ebb-dominant to flood-dominant where the parameter a/h (local tidal amplitude ÷ local tidally averaged water depth) exceeds 1.2. Sea level rise will reduce this critical value of a/h and also reduce the ebb-directed sediment transport significantly, leading to a flood-dominated estuarine system. A similar pattern, albeit with greater transport, was simulated with tidal flats included and also with a reduced grain size. This suggests that analogous classifications for flood/ebb asymmetry of the tide in estuaries as a whole may not represent the local sediment transport in sufficient detail. Through the Dyfi simulations, the above criterion involving a/h is shown to be complicated further by augmented flow past a spit at the estuary mouth which gives rise to a self-maintaining scour hole. Simulations of one year of bed evolution in an idealised flat-bottomed estuary, including tidal flow past a spit, recreate the flood/ebb dominance on either side of the spit and the formation of a scour hole in between. The erosion rate at the centre of the hole is reduced as the hole deepens, suggesting the establishment of a self-maintaining equilibrium state.  相似文献   

10.
The geomorphology of the southern Yellow Sea（SYS） is characterized by offshore radial sand ridges（RSR）.An offshore tidal channel（KSY Channel） is located perpendicular to the coast,comprised of a main and a tributary channel separated by a submarine sand ridge（KSY Sand Ridge） extending seaward.In order to investigate the interactions among water flow,sediment transport,and topography,current velocity and suspended sediment concentration（SSC） were observed at 11 anchor stations along KSY Channel in RSR during a spring tide cycle.High resolution bottom topography was also surveyed.Residual currents and tidally averaged suspended sediment fluxes were calculated and analyzed by using the decomposition method.Results suggested that the water currents became stronger landward but with asymmetrical current speed and temporal duration of flood and ebb tides.Residual currents showed landward water transport in the nearshore channel and a clockwise circulation around the KSY Sand Ridge.Tidally-averaged SSC also increased landward along the channel.The main mechanisms controlling SSC variations were resuspension and horizontal advection,with spatial and temporal variations in the channel,which also contributed to sediment redistribution between channels and sand ridges.Residual flow transport and the tidal pumping effect dominated the suspended sediment flux in the KSY Channel.The KSY Sand Ridge had a potential southward migration due to the interaction between water flow,sediment transport,and topography.  相似文献   

11.
Bastos  A.  Collins  M.  Kenyon  N. 《Ocean Dynamics》2003,53(3):309-321
Numerical simulations of tidal flow and sand transport around a coastal headland (Portland Bill, southern UK) were undertaken to investigate patterns of sand transport during the development of tidally induced transient eddies. Results obtained from a 2-D finite-element hydrodynamic model (TELEMAC-2D) were combined with a sediment transport model (SEDTRANS), to simulate the sand transport processes around the headland. Simulation of the tidal flow around Portland Bill has shown the formation and evolution of tidally induced transient eddies, around the headland. During the evolution of these transient eddies, no current-induced bedload (transport) eddy is formed for either side of the headland. Net bedload sand transport direction, around a coastal headland, is the result of instantaneous gradients in bedload transport rates, during flood and ebb flows, rather than the average (residual) flow. Thus, the use of residual (water) circulation to describe patterns of sediment movement as bedload is not an appropriatedapproach. In the case study presented here, the distinct characteristics of the coastal and seabed morphology around the Isle of Portland (i.e. headland shape and the bathymetry) indicate that these parameters can be influencing tidal (flow) and sediment dispersion around the headland. Such an interpretation has broader implications and applications to headland-associated sandbanks elsewhere.Responsible Editor: Hans Burchard  相似文献   

12.
Despite vigorous tidal and wind mixing, observations in an estuarine tidal inlet in the Wadden Sea show that during part of the tidal cycle, vertical stratification and internal waves may still develop. Acoustic Doppler current profiler (ADCP) and conductivity, temperature, depth observations, collected over the past 6 years at 13 h anchor stations (ASs), reveal that these occur especially during slack tide, when there is little wind and large freshwater discharge from nearby Lake IJssel. Measurements with a moored ADCP show that in the same tidal phase, strong cross-channel circulation develops, which may suddenly reverse circulation sense due to passing density fronts. In the vertically stratified phase that follows after the front passage, propagating mode-one solitary internal waves are observed. These are resonantly generated during decelerating tidal ebb currents when the (shear) flow passes a transcritical regime (Froude number equal to 1). A combination of photographs (including one from the International Space Station), bathymetric data, and ASs data leads to the discovery of yet another source of internal waves in this area, produced during slackening tide by propagating lee waves that develop over a deep trench. We suggest that both the cross-channel circulation as well as the (solitary) internal waves may locally be of importance for the (re)distribution and transport of sediments and nutrients and may influence tidally averaged transports.  相似文献   

13.
Data from time series of transects made over a tidal period across a section of the upper Chesapeake Bay, USA, reveal the influence of lateral dynamics on sediment transport in an area with a deep channel and broad extents of shallower flanks. Contributions to lateral momentum by rotation (Coriolis plus channel curvature), cross channel density gradients and cross channel surface slope were estimated, and the friction and acceleration terms needed to complete the balance were compared to patterns of observed lateral circulation. During ebb, net rotation effects were larger because of river velocity and reinforcement of Coriolis by curvature. During flood, stratification was greater because of landward advection of strong vertical density gradients. Together, the ebb intensified lateral circulation and flood intensified stratification focused sediment and sediment transport along the left side of the estuary (looking seaward). The tendency for greater stratification on flood and net sediment flux toward the left-hand shoal are contrary to more common models which, in the northern hemisphere, predict greater resuspension on flood and move sediment toward the right-hand shoal. These tidal asymmetries interact with the lateral circulation to focus net sediment flux on the left side of the estuary, and to produce net ebb directed sediment transport at the surface of the same order of magnitude as net flood directed sediment transport at the bottom.  相似文献   

14.
In this paper, we analyse the behaviour of fine sediments in the hyper-turbid Lower Ems River, with focus on the river’s upper reaches, a stretch of about 25 km up-estuary of Terborg. Our analysis is based on long records of suspended particulate matter (SPM) from optical backscatter (OBS) measurements close to the bed at seven stations along the river, records of salinity and water level measurements at these stations, acoustic measurements on the vertical mud structure just up-estuary of Terborg and oxygen profiles in the lower 3 m of the water column close to Leerort and Terborg. Further, we use cross-sectionally averaged velocities computed with a calibrated numerical model. Distinction is made between four timescales, i.e. the semi-diurnal tidal timescale, the spring–neap tidal timescale, a timescale around an isolated peak in river flow (i.e. about 3 weeks) and a seasonal timescale. The data suggest that a pool of fluid/soft mud is present in these upper reaches, from up-estuary of Papenburg to a bit down-estuary of Terborg. Between Terborg and Gandersum, SPM values drop rapidly but remain high at a few gram per litre. The pool of fluid/soft mud is entrained/mobilized at the onset of flood, yielding SPM values of many tens gram per litre. This suspension is transported up-estuary with the flood. Around high water slack, part of the suspension settles, being remixed during ebb, while migrating down-estuary, but likely not much further than Terborg. Around low water slack, a large fraction of the sediment settles, reforming the pool of fluid mud. The rapid entrainment from the fluid mud layer after low water slack is only possible when the peak flood velocity exceeds a critical value of around 1 m/s, i.e. when the stratified water column seems to become internally supercritical. If the peak flood velocity does not reach this critical value, f.i. during neap tide, fluid mud is not entrained up to the OBS sensors. Thus, it is not classical tidal asymmetry, but the peak flood velocity itself which governs the hyper-turbid state in the Lower Ems River. The crucial role of river flow and river floods is in reducing these peak flood velocities. During elongated periods of high river flow, in e.g. wintertime, SPM concentrations reduce, and the soft mud deposits consolidate and possibly become locally armoured as well by sand washed in from the river. We have no observations that sediments are washed out of the hyper-turbid zone. Down-estuary of Terborg, where SPM values do not reach hyper-turbid conditions, the SPM dynamics are governed by classical tidal asymmetry and estuarine circulation. Hence, nowhere in the river, sediments are flushed from the upper reaches of the river into the Ems-Dollard estuary during high river flow events. However, exchange of sediment between river and estuary should occur because of tide-induced dispersion.  相似文献   

15.
The MARS-3D model in conjunction with the particle tracking module Ichthyop is used to study circulation and tracer dynamics under a variety of forcing conditions in the eastern English Channel, and in the Boulogne-sur-Mer harbour (referred to hereafter as BLH). Results of hydrodynamic modelling are validated against the tidal gauge data, VHF radar surface velocities and ADCP measurements. Lagrangian tracking experiments are performed with passive particles to study tracer dispersal along the northern French coast, with special emphasis on the BLH. Simulations revealed an anticyclonic eddy generated in the harbour at rising tide. Tracers, released during flood tide at the Liane river mouth, move northward with powerful clockwise rotating current. After the high water, the current direction changes to westward, and tracers leave the harbour through the open boundary. During ebb tide, currents convergence along the western open boundary but no eddy is formed, surface currents inside the harbour are much weaker and the tracer excursion length is small. After the current reversal at low water, particles are advected shoreward resulting in a significant increase of the residence time of tracers released during ebb tide. The effect of wind on particle dispersion was found to be particularly strong. Under strong SW wind, the residence time of particles released during flood tide increases from 1.5 to 6 days. For release during ebb tide, SW wind weakens the southward tidally induced drift and thus the residence time decreases. Similar effects are observed when the freshwater inflow to the harbour is increased from 2 to 10 m3/s during the ebb tide flow. For flood tide conditions, the effect of freshwater inflow is less significant. We also demonstrate an example of innovative coastal management targeted at the reduction of the residence time of the pathogenic material accidentally released in the harbour.  相似文献   

16.
Jouanneau  Nicolas  Sentchev  Alexei  Dumas  Franck 《Ocean Dynamics》2013,63(11):1321-1340

The MARS-3D model in conjunction with the particle tracking module Ichthyop is used to study circulation and tracer dynamics under a variety of forcing conditions in the eastern English Channel, and in the Boulogne-sur-Mer harbour (referred to hereafter as BLH). Results of hydrodynamic modelling are validated against the tidal gauge data, VHF radar surface velocities and ADCP measurements. Lagrangian tracking experiments are performed with passive particles to study tracer dispersal along the northern French coast, with special emphasis on the BLH. Simulations revealed an anticyclonic eddy generated in the harbour at rising tide. Tracers, released during flood tide at the Liane river mouth, move northward with powerful clockwise rotating current. After the high water, the current direction changes to westward, and tracers leave the harbour through the open boundary. During ebb tide, currents convergence along the western open boundary but no eddy is formed, surface currents inside the harbour are much weaker and the tracer excursion length is small. After the current reversal at low water, particles are advected shoreward resulting in a significant increase of the residence time of tracers released during ebb tide. The effect of wind on particle dispersion was found to be particularly strong. Under strong SW wind, the residence time of particles released during flood tide increases from 1.5 to 6 days. For release during ebb tide, SW wind weakens the southward tidally induced drift and thus the residence time decreases. Similar effects are observed when the freshwater inflow to the harbour is increased from 2 to 10 m3/s during the ebb tide flow. For flood tide conditions, the effect of freshwater inflow is less significant. We also demonstrate an example of innovative coastal management targeted at the reduction of the residence time of the pathogenic material accidentally released in the harbour.

  相似文献   

17.
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   

18.
A new depth-averaged exploratory model has been developed to investigate the hydrodynamics and the tidally averaged sediment transport in a semi-enclosed tidal basin. This model comprises the two-dimensional (2DH) dynamics in a tidal basin that consists of a channel of arbitrary length, flanked by tidal flats, in which the water motion is being driven by an asymmetric tidal forcing at the seaward side. The equations are discretized in space by means of the finite element method and solved in the frequency domain. In this study, the lateral variations of the tidal asymmetry and the tidally averaged sediment transport are analyzed, as well as their sensitivity to changes in basin geometry and external overtides. The Coriolis force is taken into account. It is found that the length of the tidal basin and, to a lesser extent, the tidal flat area and the convergence length determine the behaviour of the tidally averaged velocity and the overtides and consequently control the strength and the direction of the tidally averaged sediment transport. Furthermore, the externally prescribed overtides can have a major influence on tidal asymmetry in the basin, depending on their amplitude and phase. Finally, for sufficiently wide tidal basins, the Coriolis force generates significant lateral dynamics.  相似文献   

19.
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   

20.
This paper describes an analysis of the observed up-river transport of fine sediments in the Ems River, Germany/Netherlands, using a 1DV POINT MODEL, accounting for turbulence-induced flocculation and sediment-induced buoyancy destruction. From this analysis, it is inferred that the net up-river transport is mainly due to an asymmetry in vertical mixing, often referred to as internal tidal asymmetry. It is argued that the large stratification observed during ebb should be attributed to a profound interaction between turbulence-induced flocculation and sediment-induced buoyancy destruction, as a result of which the river became an efficient trap for fine suspended sediment. Moreover, an asymmetry in flocculation processes was found, such that during flood relative large flocs are transported at relative large flow velocity high in the water column, whereas during ebb, the larger flocs are transported at smaller velocities close to the bed??this asymmetry contributes to the large trapping mentioned above. The internal tidal asymmetry and asymmetry in flocculation processes are both driven by the pronounced asymmetry in flow velocities, with flood velocities almost twice the ebb values. It is further argued that this efficient trapping is the result of a continuous deepening of the river, and occurs when concentrations in the river become typically a few hundred mg/l; this was the case during the 1990 survey analyzed in this paper. We also speculate that a second regime shift did occur in the river when fluid mud layers become so thick that net transport rates are directly related to the asymmetry in flow velocity itself, probably still in conjunction with internal asymmetry as well. This would yield an efficient mechanism to transport large amounts of fine sediment far up-river, as currently observed.  相似文献   

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