Three-dimensional numerical modeling of cohesive sediment transport and wind wave impact in a shallow oxbow lake

It was observed that in some closed inland lakes sediment transport was dominated by wind-induced currents, and the sediment resuspension was primarily driven by wind-induced waves. This paper presents the development and application of a three-dimensional numerical model for simulating cohesive sediment transport in water bodies where wind-induced currents and waves are important. In the model, the bottom shear stresses induced by currents and waves were calculated, and the processes of resuspension (erosion), deposition, settling, etc. were considered. This model was first verified by a simple test case consisting of the movement of a non-conservative tracer in a prismatic channel with uniform flow, and the model output agreed well with the analytical solution. Then it was applied to Deep Hollow Lake, a small oxbow lake in Mississippi. Simulated sediment concentrations were compared with available field observations, with generally good agreement. The transport and resuspension processes of cohesive sediment due to wind-induced current and wave in Deep Hollow Lake were also discussed.     

Sediment characteristics and wind-induced sediment dynamics in shallow Lake Markermeer, the Netherlands

In 2007/08, a study was undertaken on the sediment dynamics in shallow Lake Markermeer (the Netherlands). Firstly, sediment characteristics were determined at 49 sites in the lake. Parameters such as median grain size and loss on ignition showed a spatial as well as water depth related pattern, indicating wind-induced sediment transport. Highly significant correlations were found between all sediment parameters. Lake Markermeer sediment dynamics were investigated in a sediment trap field survey at two permanent stations in the lake. Sediment yields, virtually all coming from sediment resuspension, were significantly correlated with average wind speeds, though periods of extreme winds also played a role. Sediment resuspension rates for Lake Markermeer were high, viz. on average ca. 1,000 g m⁻² day⁻¹. The highly dynamic nature of Lake Markermeer sediments must be due to the overall shallowness of the lake, together with its large surface area (dynamic ratio = [√(area)]/[average depth] = 7.5); wind-induced waves and currents will impact most of the lake’s sediment bed. Indeed, near-bed currents can easily reach values >10 cm/s. Measurements of the thickness of the settled “mud” layer, as well as ¹³⁷Cs dating, showed that long-term deposition only takes place in the deeper SE area of the lake. Finally, lake sediment dynamics were investigated in preliminary laboratory experiments in a small “micro-flume”, applying increasing water currents onto five Lake Markermeer sediments. Sediment resuspension started off at 0.5–0.7 cm/s and showed a strongly exponential behaviour with respect to these currents.     

Sediment mobility due to currents and waves in the Torres Strait–Gulf of Papua region

The MECO hydrodynamic model (MECO Technical Report No. OMR-118/120, CSIRO Marine Research, 1998) was adapted for the Torres Strait–Gulf of Papua region at 0.05° resolution. Validation of the hydrodynamic model was carried out against observed current meter data and calculated tidal sea levels. Dispersal pathways of sediments derived from the Fly River, and from a resuspension event along the northern Great Barrier Reef were investigated using an Eulerian approach. Sediment input into Torres Strait is found to be greater during the Trade season by approximately 10%. Wave data were also obtained, and together with hydrodynamic model output, sediment mobility due to currents, waves and wave–current interactions was considered for both the Trade and Monsoon seasons. Sediment mobility in the Gulf of Papua is dominated by wave motion, whereas Torres Strait is a mixed environment of waves and tidal currents.     

Modeling of turbidity dynamics caused by wind-induced waves and current in the Taihu Lake

A simple turbidity model was developed with a sound physical basis based on in situ high-frequency observations of short-term, strong wind-induced sediment suspension in Taihu Lake, China. The validation results show that the model could successfully simulate turbidity caused by strong wind events, despite the relatively poor simulation accuracy for high values of turbidity caused by the entrainment of cyanobacteria by turbulence. The in situ observations and model simulation results indicate that the wind waves were within a narrow spectral band, with spectral energy mainly distributed within the 0.28–0.75 Hz band on opposite sides of the peak frequency. These high-frequency and low-energy wind waves are sensitive to depth filtering. However, the average depth of the lake is only 1.9 m, and wind waves still represent the main force of sediment suspension at the sediment-water interface. Moreover, lake currents were of significance to the maintenance of background turbidity in calm waves or ripples and in the determination of critical shear stress. By considering the spatial distribution of hydrodynamics and sediment, the model can be used to simulate the turbidity of the entire lake as well as boundary conditions for three-dimensional numerical models.     

Suspended sediment transport in the German Wadden Sea—seasonal variations and extreme events

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.     

Numerical simulation of tidal current and erosion and sedimentation in the Yangshan deep-water harbor of Shanghai

Yangshan near-shore sea area is the multi-island and multi-channel area with strong flow velocity and high suspended sediment concentration. Based on the characteristics of tidal currents, waves, and sediment in the Yangshan area, a two-dimensional numerical model of tidal currents, sediment transport, and sea bed deformation is developed. In the model, the effects of tidal currents and wind waves on sediment transport are considered. According to characteristics of the study area, unstructured grids are applied to fit the boundaries of the near-shore sea area. The results show that the calculated values are in good agreement with the measured data. The field of tidal currents, suspended sediment concentrations, and the deformation of the seabed can be successfully simulated.     

Modeling the impact of wind and waves on suspended particulate matter fluxes in the East Frisian Wadden Sea (southern North Sea)

Suspended particulate matter (SPM) fluxes and dynamics are investigated in the East Frisian Wadden Sea using a coupled modeling system based on a hydrodynamical model [the General Estuarine Transport Model (GETM)], a third-generation wave model [Simulating Waves Nearshore (SWAN)], and a SPM module attached to GETM. Sedimentological observations document that, over longer time periods, finer sediment fractions disappear from the Wadden Sea Region. In order to understand this phenomenon, a series of numerical scenarios were formulated to discriminate possible influences such as tidal currents, wind-enhanced currents, and wind-generated surface waves. Starting with a simple tidal forcing, the considered scenarios are designed to increase the realism step by step to include moderate and strong winds and waves and, finally, to encompass the full effects of one of the strongest storm surges affecting the region in the last hundred years (Storm Britta in November 2006). The results presented here indicate that moderate weather conditions with wind speeds up to 7.5 m/s and small waves lead to a net import of SPM into the East Frisian Wadden Sea. Waves play only a negligible role during these conditions. However, for stronger wind conditions with speeds above 13 m/s, wind-generated surface waves have a significant impact on SPM dynamics. Under storm conditions, the numerical results demonstrate that sediments are eroded in front of the barrier islands by enhanced wave action and are transported into the back-barrier basins by the currents. Furthermore, sediment erosion due to waves is significantly enhanced on the tidal flats. Finally, fine sediments are flushed out of the tidal basins due to the combined effect of strong erosion by wind-generated waves and a longer residence time in the water column because of their smaller settling velocities compared to coarser sediments.

Wave- and current-induced bottom shear stress distribution in the Gulf of Lions

Simulations of both currents and waves were performed throughout the year 2001 to assess the relative contribution of each to their overall erosive potential on the Gulf of Lions shelf. Statistical analysis of bottom shear stress (BSS) was compared to sediment grain-size distribution on the bottom. The hydrodynamic features of the bottom layer coincide with the distribution of surficial sediments, and three areas with different hydro-sedimentary characteristics were revealed. (i) The sandy inner shelf (<30 m) area is a high-energy-wave dominated area but may be subjected to intense current-induced BSS during on-shore winds along the coast and during continental winds mainly in the up-welling cells. (ii) The middle shelf (30–100 m) is a low-energy environment characterised by deposition of cohesive sediments, where the wave effect decreases with depth and current-induced BSS cannot reach the critical value for erosion of fine-grained sediments. (iii) The outer shelf, which has a higher bottom sand fraction than the middle shelf, may be affected by strong south-westward currents generated by on-shore winds, which can have an erosive effect on the fine-grained sediments.     

3D Modeling of sediment movement by ships-generated wakes in confined shipping channel

Ship-generated waves and return currents are capable of re-suspending significant quantities of bottom and bank sediments.However,most of the previous studies done on the subject do not show how and where sediment is re-suspended by the wakes and the directions of net transport.In this paper,a 3D numerical model based on hydro-sedimentary coupling is presented to search the relationship between the sediment movement,and the pattern of ship-generated waves around and far away from the vessel and the return currents around the ships.The hydrodynamic model is based on 3D Navier-Stokes equations including the standard k-ε model for turbulence processes,and the sediment transport model is based on a 3D equation for the re-suspended sediment transport.The computation results show that the areas of sediment concentration and transport(whether by resuspension or by the bedload) depend mainly on the position,the speed of the ship in the waterways,the kinematics of ship-generated waves and on the return flows.Thus,a map of sediment distribution and the modes of sediment transport generated by the passage of the ship are presented.     

Sediment transport measurements with tracers in very low‐energy beaches

This study investigates sediment transport at a very low‐energy backbarrier beach in southern Portugal, from a spring‐to‐neap tide period, during fair‐weather conditions. Rates and directions of transport were determined based on the application of fluorescent tracer techniques. Wind and currents were collected locally, whereas the dominant small and short‐period wind waves were characterized using a morphodynamic modelling system coupling a circulation model, a spectral wave model, and a bottom evolution model, well validated over the study area. For the recorded conditions sediment transport was small and ebb oriented, with daily transport rates below 0.02 m³ day^‐1. Tidal currents (mainly ebb velocities) were found to be the main causative forcing controlling sediment displacements. Transport rates were higher during spring tides, tending towards very small values at neap tides. Results herein reported points towards the distinction between tracer advection and tracer dispersion in this type of environment. Transport by advection was low as a consequence of the prevailing hydrodynamic conditions (H_s < 0.1 m, and max. current velocity of 0.5 m s^‐1) and the tracer adjustment to the transport layer, whereas dispersion was relatively high (few metres per day). Tracer techniques allowed distinguishing the broad picture of transport, but revealed the need for refinement in this type of environments (bi‐directional forcing by ebb and flood cycles). Copyright © 2012 John Wiley & Sons, Ltd.     

Observations of asymmetry in contrasting wave‐ and tidally‐dominated environments within a mesotidal basin: implications for estuarine morphological evolution

Tides are often considered to be the dominant hydrodynamic process within mesotidal estuaries although waves can also have a large influence on intertidal erosion rates. Here, we use a combination of hydrodynamic measurements and sediment deposition records to determine the conditions under which observed waves are ‘morphologically significant’, in which case they influence tidal and suspended sediment flux asymmetry and subsequently infilling over geomorphological timescales. Morphological significant conditions were evaluated using data from contrasting arms in a dendritic mesotidal estuary, in which the orientation of the arms relative to the prevailing wind results in a marked difference in wave conditions, deposition rates and morphology. By defining the morphological significance of waves as a product of the magnitude of bed shear stress and frequency of occurrence, even small (but frequently occurring) winds are shown to be capable of generating waves that are morphologically significant given sufficient fetch. In the arm in which fetch length is restricted, only stronger but rare storm events can influence sediment flux and therefore tides are more morphologically significant over longer timescales. Water depth within this mesotidal estuary is shown to be a critical parameter in controlling morphological significance; the rapid attenuation of short period waves with depth results in contrasting patterns of erosion occurring during neaps and accretion during springs. Copyright © 2016 John Wiley & Sons, Ltd.     

Contemporary hydrodynamics and morphological change of a microtidal estuary: a numerical modelling study

Contemporary hydrodynamics and morphological change are examined in a shallow microtidal estuary, located on a wave-dominated coast (Port Stephens, NSW, Australia). Process-based numerical modelling is undertaken by combining modules for hydrodynamics, waves, sediment transport and bathymetry updates. Model results suggest that the complex estuarine bathymetry and geometry give rise to spatial variations in the tidal currents and a marked asymmetry between ebb and flood flows. Sediment transport paths correspond with tidal asymmetry patterns. The SE storms significantly enhance the quantities of sediment transport, while locally generated waves by the westerly strong winds also are capable of causing sediment entrainment and contribute to the delta morphological change. The wave/wind-induced currents are not uniform with flow over shoals driven in the same direction as waves/winds while a reverse flow occurring in the adjacent channel. The conceptual sediment transport model developed in this study shows flood-directed transport occurs on the flood ramp while ebb-directed net transport occurs in the tidal channels and at the estuary entrance. Accretion of the intertidal sand shoals and deepening of tidal channels, as revealed by the model, suggest that sediment-infilling becomes advanced, which may lead to an ebb-dominated estuary. It is likely that a switch from flood- to ebb-dominance occurs during the estuary evolution, and the present-day estuary acts as a sediment source rather than sediment sink to the coastal system. This is conflictive to the expectation drawn from the estuarine morphology; however, it is consistent with previous research suggesting that, in an infilling estuary, an increase in build-up of intertidal flats/shoals can eventually shift an estuary towards ebb dominance. Thus, field data are needed to validate the result presented here, and further study is required to investigate a variety of estuaries in the Australian area.     

Currents induced by vertical varied radiation stress in standing waves and evolution of the bed composed of fine sediments

This paper extends the conventional concept of radiation stress （Longuet-Higgins and Stewart, 1964） in progressive water waves to standing waves, so that its vertical profile could be defined and calculated in a new technical way. The hydrodynamic numerical model being coupled with the vertically varying radiation stress in standing waves is used to simulate the currents being induced by standing waves in the vertical section. Numerical modeling of suspended sediment transport is then carried out to simulate the evolution of the bed composed of fine sediments by the currents. The scour and deposition patterns simulated are in qualitative agreement with prior laboratory and field observations.     

Sediment dynamics in the Black Sea: numerical modelling and remote sensing observations

Here, we address the sediment dynamics in the Black Sea based on analysis of remote sensing data from the Medium Resolution Imaging Spectrometer and numerical simulations with Nucleus for European Modelling of the Ocean model. Boundary conditions consist of realistic meteorological forcing, including significant wave height generated by wave prediction model. A number of sensitivity runs was analysed with the aim to find the most suitable parameters governing sediment fluxes. The comparison between numerical simulations and remote sensing data gives credibility to the quality of simulations. The combined effect of wind waves and currents in the bed layer controls the sediment resuspension that appears to be the major basin-wide source of sediment. Sensitivity experiments included or excluded different forcing terms, e.g. sediment flux from rivers enable to determine the spatial extensions of different point sources. It is concluded that wind-wave forcing is manifested in the sediment dynamics through episodic high energy events contributing to the increase of horizontal sediment fluxes over the northwestern shelf. Both satellite images and numerical model simulations demonstrated that the penetration of suspended sediment into the basin interior was governed by the dynamics of coastal and open-ocean eddies. While fine sediment at sea surface could cross the continental slope propagating into the open ocean, coarser fractions follow the bottom and their penetration into the open ocean is limited. The conclusion is thus that the deposition patterns correlate with the specific shape of Black Sea topography, and the largest depositions are observed in the area of continental slope.     

Mixing processes in lakes: mechanisms and ecological relevance

In Lake Baldegg, Switzerland (surface area 5.3 km², maximum depth 66 m) the analysis of data from moored instrument systems (atmospheric boundary layer, lake temperature distribution, bottom currents) was correlated to the long-term development of vertical mixing as seen from profiles of natural isotopes (radon-222, tritium and helium-3) and chemical species. The investigation shows: 1. Vertical mixing coefficients below 25 m are small. Consequently the vertical concentration distribution of sediment emanating species in the deep hypolimnion is controlled by the bottom topography. 2. Renewal of deep hypolimnic water is significant even during stratification. 3. Weakly damped internal waves characterize the internal dynamics during stratification. 4. Horizontal bottom currents play an important role in the hypolimnion mixing and can be correlated to internal waves during stratification.     

Development of a modelling methodology for simulation of long-term morphological evolution of the southern Baltic coast

The Darss–Zingst peninsula at the southern Baltic Sea is a typical wave-dominated barrier island system which includes an outer barrier island and an inner lagoon. The formation of the Darss–Zingst peninsula dates back to the Littorina Transgression onset about 8,000 cal BP. It originated from several discrete islands, has been reshaped by littoral currents, wind-induced waves during the last 8,000 years and evolved into a complex barrier island system as today; thus, it may serve as an example to study the coastal evolution under long-term climate change. A methodology for developing a long-term (decadal-to-centennial) process-based morphodynamic model for the southern Baltic coastal environment is presented here. The methodology consists of two main components: (1) a preliminary analysis of the key processes driving the morphological evolution of the study area based on statistical analysis of meteorological data and sensitivity studies; (2) a multi-scale high-resolution process-based model. The process-based model is structured into eight main modules. The two-dimensional vertically integrated circulation module, the wave module, the bottom boundary layer module, the sediment transport module, the cliff erosion module and the nearshore storm module are real-time calculation modules which aim at solving the short-term processes. A bathymetry update module and a long-term control function set, in which the ‘reduction’ concepts and technique for morphological update acceleration are implemented, are integrated to up-scale the effects of short-term processes to a decadal-to-centennial scale. A series of multi-scale modelling strategies are implemented in the application of the model to the research area. Successful hindcast of the coastline change of the Darss–Zingst peninsula for the last 300 years validates the modelling methodology. Model results indicate that the coastline change of the Darss–Zingst peninsula is dominated by mechanisms acting on different time scales. The coastlines of Darss and the island of Hiddensee are mainly reshaped by long-term effects of waves and longshore currents, while the coastline change of the Zingst peninsula is due to a combination of long-term effects of waves and short-term effects caused by wind storms.     

Modelling larval dispersal of the king scallop (Pecten maximus) in the English Channel: examples from the bay of Saint-Brieuc and the bay of Seine

The king scallop (Pecten maximus) is one of the most important benthic species of the English Channel as it constitutes the first fishery in terms of landings in this area. To support strategies of spatial fishery management, we develop a high-resolution biophysical model to study scallop dispersal in two bays along the French coasts of the English Channel (i.e. the bay of Saint-Brieuc and the bay of Seine) and to quantify the relative roles of local hydrodynamic processes, temperature-dependent planktonic larval duration (PLD) and active swimming behaviour (SB). The two bays are chosen for three reasons: (1) the distribution of the scallop stocks in these areas is well known from annual scallop stock surveys, (2) these two bays harbour important fisheries and (3) scallops in these two areas present some differences in terms of reproductive cycle and spawning duration. The English Channel currents and temperature are simulated for 10 years (2000–2010) with the MARS-3D code and then used by the Lagrangian module of MARS-3D to model the transport. Results were analysed in terms of larval distribution at settlement and connectivity rates. While larval transport in the two bays depended both on the tidal residual circulation and the wind-induced currents, the relative role of these two hydrodynamic processes varied among bays. In the bay of Saint-Brieuc, the main patterns of larval dispersal were due to tides, the wind being only a source of variability in the extent of larval patch and the local retention rate. Conversely, in the bay of Seine, wind-induced currents altered both the direction and the extent of larval transport. The main effect of a variable PLD in relation to the thermal history of each larva was to reduce the spread of dispersal and consequently increase the local retention by about 10 % on average. Although swimming behaviour could influence larval dispersal during the first days of the PLD when larvae are mainly located in surface waters, it has a minor role on larval distribution at settlement and retention rates. The analysis of the connectivity between subpopulations within each bay allows identifying the main sources of larvae which depend on both the characteristics of local hydrodynamics and the spatial heterogeneity in the reproductive outputs.     

The shoreface-connected ridges along the central Dutch coast — part 2: morphological modelling

The hydrodynamic and sediment transport processes on the Dutch inner-shelf were studied on the basis of short- and medium-term (days, month) field experiments during fair-weather and storm conditions. The primary goal of this study was to identify the mechanisms that may be responsible for the formation and maintenance of the shoreface-connected ridges in the study area. The hydrodynamic field observations do not consistently support existing theoretical models explaining the formation of such ridges. Instead, the water motion appears to be dominated by wind- and density-driven effects. A review of literature indicates that other field studies also fail to observe the flow response expected from theory. Sediment transport during fair-weather is very episodic, with bed load transport slightly dominant over suspended load transport. The sediment transport processes during storms are dominated by the mean fluxes, with waves acting as a stirring mechanism. The contribution of wave-oscillatory fluxes cannot be neglected and may be directed with and against the waves. Long-wave fluxes are present, but very small.     

Numerical modelling of the suspended sediment transport in Torres Strait

One-dimensional vertical and three-dimensional fine-resolution numerical models of sediment transport have been developed and applied to the Torres Strait region of northern Australia. The one-dimensional model, driven by measured waves and currents, was calibrated against measured suspended sediment concentrations using a sequential data assimilation algorithm. The algorithm produced a good match between model and data, but this was achieved only by allowing some temporal variability in parameter values, suggesting that there were underlying uncertainties in the model structure and forcing data. Implications of the assimilation results to the accuracy of the numerical modelling are discussed and the need for observational programmes having an extensive spatial and temporal coverage is highlighted. The three-dimensional sediment model, driven by modelled waves and currents, simulates sediment transport over the shelf during the monsoon and trade-wind seasons covering 1997–2000. The model predicts strong seasonal variability of the sediment transport on the shelf attributed to seasonally varying hydrodynamics, and illustrates significant inter-annual variability of the sediment fluxes driven by extreme events. The developed model provides a platform for testing scientific hypothesis. With additional calibration, including uncertainty analysis, it can also be used in a management context.     

Post-glacial sediment dynamics in the Irish Sea and sediment wave morphology: Data–model comparisons

The irregular seafloor of the narrow Irish Sea on the NW European Shelf has been documented over several decades. From recently collected swath bathymetry data, very large trochoidal, nearly symmetrical sediment waves are observed in many parts of the Irish Sea and appear similar to those described from other continental shelf seas in North America that were covered by glacigenic sediments during the Last Glacial Maximum. Swath multibeam and single beam bathymetry data, backscatter intensity, shallow seismic imagery, video footage and sediment cores from the Irish Sea high sediment waves have been integrated to identify their genesis with reference to present and past hydrodynamic variability. From cross-sectional profiles over asymmetrical sediment waves in the Irish Sea the direction of asymmetry is used to map residual bed stress directions and associated bedload transport paths. Irish Sea peak bed stress vectors were generated using a two-dimensional palaeo-tidal model for the NW European shelf seas and compare well with the observations. Tidally induced bed stresses are modelled to have increased between 7–10 ka BP, to be nearly symmetrical in magnitude and to have reversed in dominant direction on a millennial scale. These environmental conditions during the post-glacial marine transgression are suggested here to help comprehend the construction of the very large sediment waves, with local variations due to differences in sediment grain size, sediment supply, water depth and intensified currents due to seafloor slopes. Model parameterisation using an open ocean boundary with time-dependent tidal changes and the implementation of high-resolution bathymetric information will improve future models of small-scale bed shear stress patterns and improve the predictive value of such modelling efforts.