Bottom Boundary Layer Research Articles

The contribution of wind-wave energy at sea bottom to the modelling of rhodolith beds distribution in an off-shore continental shelf

The study aims to investigate the relationship between the presence of rhodolith beds and the effect on the shelf bottom boundary layer due to the action of surface wind waves. The study area is situated off-shore and north-west of Elba Island in the Western Mediterranean Sea, an area known to be characterized by rhodolith beds. A binomial logistic regression model is used in order to analyse the relationship between wind-wave energy at sea bottom, bathymetry and rhodolith bed occurrence. The results indicate a positive correlation between rhodolith bed occurrence and wave energy, while the relation with bathymetry is weaker in all the trials. The wave energy confidence interval associated to the rhodolith bed probability is also estimated, thereby informing on wind wave energy values required for the modelling of this particular benthic habitat in off-shore shelf areas.

Wave flume experiments on dynamics of the bottom boundary layer in silty seabed

The objectives of this study are carried out a series of controlled large wave flume experiments using fine-grained sediment from the Huanghe River Delta, exploring the complete sequence of sediment behavior in the bottom boundary layer (BBL) during wave-induced liquefaction. The results show that: (1) The BBL in silty seabed is exposed to a progressive wave, goes through a number of different stages including compaction before liquefaction, sediment liquefaction, and compaction after liquefaction, which determines the range and thickness of BBL. (2) With the introduction of waves, first, the sediment surface has settled by an amount S (S=1–2 cm) in the course of wave loadings with an insufficient accumulation of pore water pressure. And a thin high concentration layer formed the near-bed bottom. (3) Once the liquefaction sets in, the liquefied sediment with an ‘orbital motion’ and the sub-liquefied sediment form a two-layer-sediment region. The range of BBL extends downwards and stopped at a certain depth, subsequently, develops upwards with the compaction process. Meanwhile, re-suspended sediments diffuse to the upper water column. (4) During the dynamics process of the BBL beneath progressive waves, the re-suspended sediment increment ranked as sediment liquefaction > erosion before liquefaction > compaction after liquefaction.

Calculation of Winds Induced Bottom Wave Orbital Velocity Using the Empirical Mode Decomposition Method

AbstractAccurate bottom wave orbital velocity (BWOV) calculation is important for understanding critical dynamic processes (e.g., turbulent mixing, sediment transport) at the bottom boundary layer of the oceans. Here we first use the empirical mode decomposition (EMD) method to calculate BWOV and evaluate its performance by comparing with two conventional methods (spectra and velocity methods) using field measurements collected from an acoustic Doppler velocimeter (ADV). The results suggest that BWOVs calculated by the EMD method were well correlated (R2 ≥ 0.97) to the results by the other two methods but with a few percent discrepancies. Under strong wavy conditions, BWOVs from the EMD method were 8% and 6% smaller than those from the spectra and velocity methods, respectively. Under weak wavy conditions, BWOVs from the EMD method were 14% and 11% smaller than those from the spectra and velocity methods, respectively. Statistical distributions of BWOV suggest that the EMD method calculated instantaneous BWOVs and BWOV amplitudes closely matched the Gaussian and Rayleigh distributions, respectively. Uncertainty analysis suggests that the EMD method was capable of calculating the most accurate BWOVs among the three methods. While the spectra and velocity methods can provide robust BWOV estimation, they cannot completely avoid the errors caused by wave-unrelated motions and instrumental noise.

Suspended sand concentration models under breaking waves: Evaluation of new and existing formulations

A total of 7 reference concentration (C0) models (6 existing and 1 newly proposed) were validated against 119 test cases from 4 recently published datasets collected under the LIP, CROSSTEX, SandT-Pro and SINBAD experimental studies. These models were evaluated for performance in different cross-shore regions: the shoaling zone, breaking (outer surf) zone and inner surf zone, under regular and irregular breaking wave conditions. In almost all existing C0 models, substantial under-prediction was found particularly around the wave plunging point (point at which breaking wave plunges and surface generated turbulent kinetic energy, TKE, is injected into the water column) where strong localised increases in C0 were observed. This strong increase in concentration was attributed to the large-scale breaking-generated turbulent vortices invading the wave bottom boundary layer (WBBL) and entraining dense clouds of sediment near the plunging point. Models that were directly or indirectly driven by local wave climate such as the local wave height (H), breaker height (Hb) or local water depth (d), were found to perform quite poorly in the breaking region under regular and irregular plunging breaker waves. Formulations that related C0 to the sand pickup rate (i.e. depending on exerted bed shear exceeding critical bed shear for entrainment) were adept in regions unaffected by external breaking-induced TKE (e.g. the shoaling zone) but could not account for the high levels of concentration observed at the plunging point. This is because these formulations were based on the implicit assumption that sediment entrainment is only induced by the local TKE generated by bed shear; not taking surface-generated breaking-induced TKE into account. This assumption was addressed in more recent studies, by including breaking-induced TKE into sediment pickup rate or reference concentration formulations. Though latest studies have shown promising relationships between near-bed TKE (kb) and reference concentration/sediment pickup, such formulations also face inherent limitations. These formulations are highly dependent on the accuracy of measured or modelled kb and are also sensitive to the magnitude of kb. For example, the magnitude of measured kb was found to vary by a factor of 1.1–1.3 between regular and irregular wave conditions, with kb being smaller under irregular wave conditions. This resulted in varied performance between datasets in kb-driven reference concentration formulations. The Froude-scaled TKE produced smaller deviations in magnitude of TKE between datasets, suggesting that it may be a more suitable driving parameter for reference concentration models than kb. A new reference concentration model, L19, was empirically derived from an inverse relationship observed between d and C0, and from the roller energy dissipation rate. The newly proposed L19 model shows good agreement with measured C0 (with RMSE ranging between 0.36 and 1.79 kg/m3 over the different datasets) in regular and irregular wave conditions, even at the plunging point where concentration is highest. The modified concentration profile [C(z)] equation also performs well, generally capturing the vertical concentration profile accurately throughout the whole water column.

Sensitivities of Bottom Stress Estimation to Sediment Stratification in a Tidal Coastal Bottom Boundary Layer

The bottom friction velocity (U*), which controls seabed erosion and deposition, plays a critical role in sediment transport in tidal coastal bottom boundary layers. Approaches have been proposed to calculate U*, including the log profile (LP) estimation, the direct covariance (COV) measurement, and the turbulent kinetic energy (TKE) method. However, the LP method assumes homogeneous flow and the effects of stratification need to be taken into account. Here, field investigations of hydrodynamics and sediment dynamics were carried out on the Jiangsu Coast, China. Two acoustic Doppler velocimeters (ADV) velocity measurements at 0.2 and 1 m above the seabed have been used to estimate U*, based on the aforementioned three methods. The COV and TKE methods provided reasonable estimations of U*, while a pronounced overestimation was identified when using the LP method. This overestimation can be attributed to the stratification effects associated with the vertical suspended sediment concentration (SSC) gradient near the bottom. Then, three models were utilized to correct the overestimation, in which the gradient/flux Richardson number was modified with empirical constants α, β, and A to parameterize the stratification effects in the logarithmic velocity distribution. The values of α, β, and A derived from the observation are smaller than the results from previous investigations. These modified logarithmic velocity distribution models can be applied in numerical simulations when sediment stratification is important.

Vertical variations in the bio-optical properties of seawater in the northern South China Sea during summer 2008

Vertical variability in the bio-optical properties of seawater in the northern South China Sea (NSCS) including inherent optical properties (IOPs) and chlorophyll a concentration (Chl) were studied on the basis of in situ data collected in summer 2008 using an absorption/attenuation spectrophotometer. An empirical model was developed to estimate Chl profiles based on the absorption line height at long wavelengths, with a relative root mean square error of 37.03%. Bio-optical properties exhibited large horizontal and vertical spatial variability. As influenced by coastal upwelling and the Zhujiang River (Pearl River) discharge, both IOPs and Chl exhibited high values in the surface waters of the inner shelf, which tended to decrease with distance offshore. Subsurface maximum layers of IOPs and Chl were observed in the middle and outer shelf regions, along with significantly higher values of attenuation coefficients beneath this layer that rapidly increased towards the bottom. In the open ocean, both IOPs and Chl exhibited consistent variability, with the subsurface maximum layer typically located at 34–84 m. Phytoplankton were found to be one of the major components in determining the vertical variability of bio-optical properties, with their vertical dynamics influenced by both physical forcing and light attenuation effects. The depth of the subsurface maximum layer was found to be closely related to the fluctuation of the oceanic thermocline and the depth of the euphotic zone, which also affected the total integrated biomass of the upper ocean. Typically high values of attenuation coefficients observed in the bottom waters of the continental shelf reflected the transport of particulate matter over the bottom boundary layer. Our results reveal large spatial differences in bio-optical profiles in response to complex marine ecodynamics in the NSCS. From the perspective of marine research, high-resolution optical measurements are clearly advantageous over conventional bottle sampling.

Gravity, Topography, and Melt Generation Rates From Simple 3‐D Models of Mantle Convection

AbstractConvection in fluid layers at high Rayleigh number (Ra ∼106) have a spoke pattern planform. Instabilities in the bottom thermal boundary layer develop into hot rising sheets of fluid, with a component of radial flow toward a central upwelling plume. The sheets form the “spokes” of the pattern, and the plumes the “hubs.” Such a pattern of flow is expected to occur beneath plate interiors on Earth, but it remains a challenge to use observations to place constraints on the convective planform of the mantle. Here we present predictions of key surface observables (gravity, topography, and rates of melt generation) from simple 3‐D numerical models of convection in a fluid layer. These models demonstrate that gravity and topography have only limited sensitivity to the spokes and mostly reflect the hubs (the rising and sinking plumes). By contrast, patterns of melt generation are more sensitive to short‐wavelength features in the flow. There is the potential to have melt generation along the spokes but at a rate which is relatively small compared with that at the hubs. Such melting of spokes can only occur when the lithosphere is sufficiently thin ( km) and mantle water contents are sufficiently high ( ppm). The distribution of volcanism across the Middle East, Arabia, and Africa north of the equator suggests that it results from such spoke pattern convection.

Influence of Coriolis Force Upon Bottom Boundary Layers in a Large‐Scale Gravity Current Experiment: Implications for Evolution of Sinuous Deep‐Water Channel Systems

AbstractOceanic density currents in many deep‐water channels are strongly influenced by the Coriolis force. The dynamics of the bottom boundary layer in large geostrophic flows and low Rossby number turbidity currents are very important for determining the erosion and deposition of sediment in channelized contourite currents and many large‐scale turbidity currents. However, these bottom boundary layers are notoriously difficult to resolve with oceanic field measurements or in previous small‐scale rotating laboratory experiments. We present results from a large, 13‐m diameter, rotating laboratory platform that is able to achieve both stratified and highly turbulent flows in regimes where the rotation is sufficiently rapid that the Coriolis force can potentially dominate. By resolving the dynamics of the turbulent bottom boundary in straight and sinuous channel sections, we find that the Coriolis force can overcome centrifugal force to switch the direction of near‐bed flows in channel bends. This occurs for positive Rossby numbers less than +0.8, defined as RoR = /Rf, where is the depth and time‐averaged velocity, R is the radius of channel curvature, and f is the Coriolis parameter. Density and velocity fields decoupled in channel bends, with the densest fluid of the gravity current being deflected to the outer bend of the channel by the centrifugal force, while the location of velocity maximum shifted with the Coriolis force, leading to asymmetries between left‐ and right‐turning bends. These observations of Coriolis effects on gravity currents are synthesized into a model of how sedimentary structures might evolve in sinuous turbidity current channels at various latitudes.

Dissipation of Turbulent Kinetic Energy in the Oscillating Bottom Boundary Layer of a Large Shallow Lake

AbstractMixing rates and biogeochemical fluxes are commonly estimated from the rate of dissipation of turbulent kinetic energy ε as measured with a single instrument and processing method. However, differences in measurements of ε between instruments/methods often vary by one order of magnitude. In an effort to identify error in computing ε, we have applied four common methods to data from the bottom boundary layer of Lake Erie. We applied the second-order structure function method (SFM) to velocity measurements from an acoustic Doppler current profiler, using both canonical and anisotropy-adjusted Kolmogorov constants, and compared the results with those computed from the law of the wall, Batchelor fitting to temperature gradient microstructure, and inertial subrange fitting to acoustic Doppler velocimeter data. The ε from anisotropy-adjusted constants in SFM increased by a factor of 6 or more at 0.2 m above the bed and showed a better agreement with microstructure and inertial method estimations. The maximum difference between SFM ε, computed using adjusted and canonical constants, and microstructure values was 25% and 50%, respectively. This difference was 30% and 55%, respectively, for those from inertial subrange fitting at times of high-intensity turbulence (Reynolds number at 1 m above the bed of more than 2 × 104). Comparison of the SFM ε to those from law of the wall was often poor, with errors as large as one order of magnitude. From the considerable improvement in ε estimates near the bed, anisotropy-adjusted Kolmogorov constants should be applied to compute dissipation in geophysical boundary layers.

Steady Streaming Induced by Asymmetric Oscillatory Flows over a Rippled Bed

The flow induced by progressive water waves propagating over a rippled bed is reproduced by means of the numerical solution of momentum and continuity equations to gain insights on the steady streaming induced in the bottom boundary layer. When the pressure gradient that drives the flow is given by the sum of two harmonic components an offshore steady streaming is generated within the boundary layer which persists in the irrotational region. This steady streaming depends on the Reynolds number and on the geometrical characteristics of the ripples. Nothwithstanding the presence of a steady velocity component, the time-average of the force on the ripples vanishes.

Study of the effects of Ekman dynamics in the bottom boundary layer on the Black Sea continental slope

Study of the effects of Ekman dynamics in the bottom boundary layer on the Black Sea continental slope

Cross-shelf exchanges in the northern Bay of Biscay

Abstract The spatio-temporal distribution of cross-shelf exchanges in the northern Bay of Biscay from 2007 to 2010 were investigated using a high-resolution three-dimensional model as well as sea-surface temperature and chlorophyll-a concentration satellite observations. Our results show that the net yearly mean transport was upslope each year, with 2010 showing the highest value (0.93 Sv upslope). Bottom fluxes showed peak values near Chapel Bank, with mean values of 0.1 m3 s−1 m−2 and maximum values of 0.2 m3 s−1 m−2. Our model demonstrated that cross-shelf exchanges can be divided into three vertical layers. At the surface, cross-shelf transports are driven by wind forcing (Ekman transport accounts for about 60% of the total cross-shelf transport) and mesoscale activity (eddy advection accounts for about 30% of transport). In the absence of mesoscale activity, Ekman transport at the surface is typically balanced out by a downslope flux at the bottom boundary layer. Exchanges at mid-depths are regulated by mesoscale activity and tides. A ubiquitous feature appeared at the bottom boundary with a cross-shelf flow in the downslope direction. Numerical simulations suggest bottom fluxes of 0.1 m3 s−1 m−2, in agreement with previous in situ observations. We discuss the impact of winds, tides and mesoscale eddies on cross-shelf exchanges using different examples. The eddy census was obtained using an eddy-tracking algorithm. The shelf break was shown to be an important area of eddy presence due to slope current instabilities. The impact of eddies on surface and mid-depth transport is illustrated with a shelf-break eddy as an example, from its generation to dissipation and its contribution to cross-shelf exchanges. Results suggest that the largest magnitudes of downslope transports occur in the presence of both strong winds and intense eddy activity. Our detailed and quantitative exploration of cross-shelf transports in the northern Bay of Biscay highlights the relative contribution of intermittent processes (e.g. wind-driven events, eddies, frictional bottom layer) and confirms the complex links between the coastal and open ocean over shelf breaks.

Bottom Boundary Layer and Particle Dynamics in an Upwelling Affected Continental Margin (NW Iberia)

AbstractThe dynamics of the bottom boundary layer and their effects on the development of nepheloid layers (NLs) on a high‐energy and upwelling‐affected margin, the NW Iberian continental shelf, were studied by means of ADP currents, wave time series, and across‐shelf hydrographic surveys covering an entire annual cycle. The bottom boundary layer hydrodynamics showed that high levels of bottom shear stress over the inner shelf occurred mainly during downwelling seasons, when there was a coupling between storm waves and intense currents. This wave‐current coupling promotes strong resuspension events that favored the generation of bottom NLs (BNLs). BNLs were well developed on the inner continental shelf inshore of a thermohaline front generated by the interaction of the Iberian Poleward Current and the West Iberian Buoyant Plume. The existence of this front limited the extent of offshore export of resuspended particles during the downwelling season. In contrast, during the upwelling season, only thin BNLs were developed, and surface NLs, principally composed by biogenic particulate material, were advected offshore in the Ekman layer, often as part of the development of upwelling filaments. Our study confirmed that wave orbital velocity under stormy conditions, in combination with the hydrography, could explain the shoreward boundary of the Galician mud depocenter.

Stratification and mixing processes associated with hypoxia in a shallow lake (Lake Kasumigaura, Japan)

Physical processes associated with hypoxia events in a shallow lake, Lake Kasumigaura, Japan, are investigated with long-term mooring observations at the middle of the lake basin. Results show that strong stratification during the summer suppresses vertical mixing resulting in hypoxia in the bottom boundary layer. The dissolved oxygen decreases due to the limited oxygen supply under strongly stratified conditions. The intensity of the stratification is controlled by the ratio between surface wind stress and the surface incoming buoyancy flux. The Monin–Obukhov length scale explains the stratification and shows good agreement with the Froude number and Wedderburn number. The stratification is also enhanced by the heat flux towards the sediments, reaching O (100) W m−2, during the summer season. In addition to field observations, this study conducted numerical simulations to further investigate mixing structure. Results from numerical simulations indicate that the bottom sediment heat flux significantly contributes to mixing and stratification in the lake. This study suggests that mixing processes associated with winds and heat flux at surface and bottom sediments are the key to understanding hypoxia events in Lake Kasumigaura.

ПРЕДВАРИТЕЛЬНЫЕ РЕЗУЛЬТАТЫ ЭКСПЕРИМЕНТАЛЬНОГО ИССЛЕДОВАНИЯ ДИНАМИКИ ПРИДОННОГО СЛОЯ НА ШЕЛЬФЕ–КОНТИНЕНТАЛЬНОМ СКЛОНЕ В ЧЕРНОМ МОРЕ

The preliminary results of an experimental study of a mechanism of the Black Sea anoxic layer ventilation related with the descent of oxygen-containing water down the bottom slope in Ekman boundary layer are presented. To study this mechanism, several automatic measuring stations were installed at the bottom of the shelf-continental slope zone in the depth range from 80 to 243 m, on the cross-section abeam of the Gelendzhik Bay. The observations that lasted for 1.5–2 months were fulfilled during two periods in the beginning and the end of 2018. The stations were registering hydrophysical (temperature, salinity, pressure and current velocity) and hydrochemical (dissolved oxygen concentration) parameters at a 0.5–2.5 m distance from the bottom. The acquired data are suitable for estimation of spatio-temporal parameters of water transfer in the bottom layer up and down the slope, depending on direction and intensity of alongshore current. Preliminary analysis of the first installation data confirmed the presence of bottom water transfer along the slope perpendicular to the shore. Also, in case of intense alongshore north-western current, a descent of the bottom layer water was observed. Such manner of water transfer conforms to both geostrophic adjustment and dynamics of the bottom Ekman boundary layer. However, variability of water density in the bottom layer, caused by vertical water transport, had about the same range as density fluctuations in the water column on the same depths. This fact disputes the effectiveness of Ekman transfer in the bottom layer as a water ventilation mechanism for the upper part of continental slope of the Black Sea (Zatsepin et al., 2007; Elkin, Zatsepin, 2017).

Study on boundary layer development and bottom shear stress beneath a tsunami

ABSTRACTThis paper presents a detailed investigation on unsteady effects in bottom boundary layer beneath a tsunami. In numerical simulation of tsunami, bottom shear stress has commonly been evaluated by using steady flow friction laws such as Manning equation, simply assuming that long-period wave motion satisfies quasi-steady flow conditions. However, the present study found that the behavior of tsunami-induced bottom boundary layer has an unsteady characteristics and acts similar to that induced by wind-generated waves even under long-period wave motion. As a result, bottom shear stress under tsunami is much larger than the estimation using steady friction coefficient due to a steep velocity gradient in the bottom boundary layer. Surprisingly, the steady flow friction law is not valid in almost the entire computational domain, from the source area to shallow region. This result highly coincides with the field measurement data obtained by Lacy group during the 2010 Chilean Tsunami at the Monterey Bay mouth in U.S. A correction coefficient is proposed to take into account unsteady effects in the conventional estimation method using a steady friction factor; this approach is validated by comparing with numerical simulation results using k-ω turbulence model.

Research progress of turbulence characteristic parameters in bottom boundary layer

The bottom boundary layer in estuaries and coastal waters is an area where water flows interact with the bottom bed, and its flow structure is very complex. The influence on the bed morphology, energy and material exchange, circulation and other processes can not be ignored. With the rapid development of technology, the measurement methods of the parameters of the bottom boundary layer are more mature, and the measurement and analysis of turbulence are more in-depth. In addition, the study of the bottom boundary layer has a great role in promoting the development of other similar disciplines, and its application value is increasingly obvious. This paper presents the latest progress in the study of turbulent characteristic parameters of the bottom boundary layer in estuaries and coastal waters, including measurement methods, frictional velocity, bottom roughness length and bottom drag coefficient. These achievements are summarized and analyzed, and the shortcomings of the current research are discussed, and the future research is prospected.

Particulate organic matter controls benthic microbial N retention and N removal in contrasting estuaries of the Baltic Sea

Abstract. Estuaries worldwide act as “filters” of land-derived nitrogen (N) loads, yet differences in coastal environmental settings can affect the N filter function. We investigated microbial N retention (nitrification, ammonium assimilation) and N removal (denitrification, anammox) processes in the aphotic benthic system (bottom boundary layer (BBL) and sediment) of two Baltic Sea estuaries differing in riverine N loads, trophic state, geomorphology, and sediment type. In the BBL, rates of nitrification (5–227 nmol N L−1 d−1) and ammonium assimilation (9–704 nmol N L−1 d−1) were not enhanced in the eutrophied Vistula Estuary compared to the oligotrophic Öre Estuary. No anammox was detected in the sediment of either estuary, while denitrification rates were twice as high in the eutrophied (352±123 µmol N m−2 d−1) as in the oligotrophic estuary. Particulate organic matter (POM) was mainly of phytoplankton origin in the benthic systems of both estuaries. It seemed to control heterotrophic denitrification and ammonium assimilation as well as autotrophic nitrification by functioning as a substrate source of N and organic carbon. Our data suggest that in stratified estuaries, POM is an essential link between riverine N loads and benthic N turnover and may furthermore function as a temporary N reservoir. During long particle residence times or alongshore transport pathways, increased time is available for the recycling of N until its eventual removal, allowing effective coastal filtering even at low process rates. Understanding the key controls and microbial N processes in the coastal N filter therefore requires to also consider the effects of geomorphological and hydrological features.

Hydrography of a shelf ecosystem inshore of a major Western Boundary Current

The Agulhas Current, the largest Western Boundary Current in the Southern Hemisphere, strongly influences the oceanographic conditions of its adjacent shelf system. As a result of limited societal drivers, such as large-scale fisheries and marine mining, the South African southeast shelf, between Port Elizabeth and Port Shepstone, is one of the least studied and under-sampled shelf systems in Southern Africa. This study describes hydrographic conditions from the first two shelf-wide high-resolution surveys (January/February and July/August 2017) of this region. These data provide a baseline of environmental conditions and hydrographic processes across the shelf. The Agulhas Current, as well as cyclonic mesoscale eddies along the inshore edge of the Current, were observed to strongly drive conditions on the shelf. Substantial differences in the impacts of these cyclonic eddies were evident, with the smaller less intense January/February eddy inducing much stronger upwelling than the larger more intense July/August eddy. Indications of seasonality in temperature, salinity and chlorophyll-a were observed in the surface layers. Surface waters were 4 °C warmer in January/February than in July/August. Considerably lower salinities (33.77 g kg−1) were observed in January/February as a result of high rainfall and subsequent increased river outflow. Upwelling mechanisms such as divergence-induced upwelling, Ekman veering in the bottom boundary layer, Ekman pumping associated with cyclonic eddies, were observed to uplift colder, nutrient-rich water from deeper layers onto the mesotrophic shelf. These colder waters ultimately reached the surface as a result of wind-driven Ekman transport and vertical mixing. Tropical Surface Water, Subtropical Surface Water and South Indian Central Water were observed on the shelf. Red Sea Water as well as Antarctic Intermediate Water generally occupied the slope regions except where cyclonic eddies uplifted intermediate waters onto the shelf. The shelf-wide dissolved oxygen levels (>3 ml l−1) suggested that oxygen availability adequately facilitated the survival of the existing shelf biology.

Winter storms induced high suspended sediment concentration along the north offshore seabed of the Changjiang estuary

Fine-grained sediments suspended in coastal waters play an important role in submarine topography evolution and associated environment changes. The convergence of suspended sediments concentrated near the seabed results in high sediment concentration, and contributes significantly to sediment transport. In order to investigate the mechanism triggering high suspended sediment concentration (SSC), we deployed a tripod to the seabed to obtain in situ bottom boundary layer measurements of sediment dynamics and a buoy to the sea surface to collect meteorological and wave data at the northern Changjiang River mouth from December 20, 2015 to January 20, 2016. The high SSC (e.g. >3 g/L) events were observed together with fluid mud (thicknesses of 4–16 cm) near the seabed during neap tides when cold air intrusion generated winter storms and strong waves. Further, we found that the high SSC event was mainly resulted from wind waves and sediment resuspension supported by local benthic fluid mud, which was associated with three stages. At the setting up stage, the winter storm brought long duration of strong-waves (e.g. significant wave height >1.5 m) more than 15 h, resulting in a maximum wave-current combined bottom shear stress of 3.2 Pa and the increase of SSC to >1 g/L. At the reinforcement stage, the strong waves and bottom shear stress lasted for several hours, and further increased the SSC to >3 g/L. At the final decay stage, wind waves and the maximum wave-current shear stresses decreased significantly, with the disappearance of high SSC. Thus, there was no high SSC event observed even with strong wind waves during spring tides because strong wave duration was too short for the reinforcement process. The sediment source for the high SSC events was mainly from bottom fluid mud resuspension, as well as the advection transport from the adjacent subaqueous Changjiang River delta.