Near-bed Sediment Research Articles

Turbulence and sediment deposition in a channel with floating vegetation

Floating canopies of vegetation alter the vertical profile of flow velocity within and below canopies, which in turn impacts sediment deposition below canopies. This study explored the impacts of channel-average velocity (U0), vegetation density (a), and relative flow depth (hg/H, where hg is the height of the free flow region below the floating canopy and H is the entire flow depth) on the longitudinal distributions of near-bed turbulent kinetic energy (TKE) and sediment deposition in a channel with a floating vegetation canopy. Below a floating canopy, sediment deposition was correlated with the near-bed TKE. For the low-density and low-velocity cases, sediment deposition was spatially uniform across the entire channel because the near-bed TKE was lower than the critical value for sediment resuspension. As the vegetation density and channel-average velocity increased, the near-bed TKE increased and surpassed the critical TKE, which caused resuspension and reduced deposition below the canopy relative to the upstream reference. For the same vegetation density and channel-average velocity, a lower relative flow depth (hg/H) produced a higher below-canopy depth-averaged velocity and near-bed TKE, resulting in less deposition below the canopy. A model was proposed for predicting the longitudinal evolution of near-bed TKE based on the predicted flow velocity below the canopy throughout the channel. The longitudinal evolution of deposition below a floating canopy was predicted by combining the predicted near-bed TKE with the deposition probability. The predicted near-bed TKE and deposition matched laboratory measurements. Finally, the proposed model can be used to predict the longitudinal profile of deposition below a real Eichhornia crassipes canopy.

Mechanism of rapid accretion-erosion transition in a complex hydrodynamic environment based on refined in-situ data

Morphological changes in estuaries are typically discussed on decadal time scales, with reported changes often exceed several decimeters. In our study, we utilized refined in-situ data to investigate bed-level changes and rapid accretion-erosion transitions during tidal cycles off the Changjiang Estuary. A field study employing a high-resolution bottom tripod system was conducted, measuring various parameters such as water level, waves, velocity profiles, near-bottom suspended sediment concentration (SSC), and bed level changes over ten tidal cycles. Our findings reveal significant and intense bed level changes occurring at the scale of tidal cycles. The data suggests that downward sediment transport plays a dominant role in the overall accretion tendency throughout the study period. Additionally, our research introduces a dynamic critical bed shear stress that increases during tidal cycles, deviating from fixed values. We also present a new quadrant analysis method to elucidate the primary processes controlling near-bed sediment transport. The results highlight the importance of horizontal advective sediment transport in frequent accretion-erosion transitions at tidal-cycle timescales. This implies a frequent exchange of sediment between adjacent shoals and channels in the seaward region of the North Channel in the Changjiang Estuary, which holds significance for future navigation and construction management.

Improved XBeach model and its application in coastal beach evolution under wave action

ABSTRACT Beach evolution is one of the key issues in coastal engineering. High computational efficiency model of beach evolution is limited by strong wave nonlinearity, flow acceleration, and sediment phase lag. The XBeach model is improved to cover the three limitations in the study. First, the near-bed velocity of nonlinear wave is reconstructed by using nonlinear parameters. Second, flow acceleration and sediment phase lag are taken into account for the near-bed sediment conditions to calculate the equilibrium concentration of suspended sediment and the transport rate of bed load. The response of sediment is not necessarily instantaneous to near-bed velocity in practical problems, but separated into a phase (time) shift between sediment concentration and velocity and a phase residual denoting sediment amount in moving at flow reversal. The improved XBeach model performs better results than the original XBeach model. Numerical experiments are carried out using the improved XBeach model to investigate the effects of wave height and sediment size. The numerical experiments show that the volume and range of erosion/deposition increase with the increase of wave height and decrease of sediment size, and deposition center moves to offshore. The perspective of wave energy and phase lag can explain the erosion/deposition variation.

Effects of currents on nonlinear wave transformation in a reef-lagoon-channel system

Wave nonlinearity can drive the near-bed sediment transport and nutrient delivery on coral reefs. Previous studies have primarily concentrated on the wave nonlinearity evolution for a specific cross-shore profile and have scarcely taken the effects of currents into consideration. To better understand the nonlinear wave transformation on reef coasts, the wave height distribution and wave nonlinearity evolution are analyzed based on 3D reef-lagoon-channel experiments in a wave basin. By taking the current velocity into consideration, the Glukhovskiy distribution function is improved in calculating the probability of large wave height in the sequence. Wave skewness and asymmetry on the reef and in the channel are parameterized with the Ursell number considering the Doppler effect induced by opposing or following currents. The accuracies for estimating wave skewness and asymmetry are improved by 39% and 66%, respectively. The influence of Doppler effect on wave shapes on the reef flat and in the channel varies significantly with the incident wave heights and reef submergences. Furthermore, it is also noticed that the reduction of local water depth caused by the propagation of infragravity wave troughs increases short-wave skewness at individual wave scale on the middle of reef flat, while its effect on short-wave asymmetry is not significant.

Development, calibration and validation of a phase-averaged model for cross-shore sediment transport and morphodynamics on a barred beach

Simulating cross-shore sediment transport and associated sandbar migration is still a challenging task for phase-averaged coastal morphological models. Numerical studies have mostly relied on beach morphology prediction for calibration and validation, without examining in much detail the underlying hydrodynamics, sediment concentrations and transport rates. This paper reports on a new three-dimensional coastal morphodynamic model based on the hydrodynamic model of Zheng et al. (2017), combined with an advection-diffusion type suspended sediment transport model and the extended SANTOSS near-bed sediment transport formula of Van der A et al. (2013), to represent the key cross-shore transport mechanisms. The model is are calibrated based on comprehensive measurements from a large-scale laboratory experiment involving regular plunging breaking waves over an evolving sandbar, covering detailed comparisons on hydrodynamics, sediment suspension, transport rates, and bed level evolution. Separate validation using large scale wave flume experiments were also conducted to confirm the model's performance on different conditions. Good agreements are obtained between measurements and model results, which demonstrates the model's ability to reproduce cross-shore sediment transport processes under breaking waves correctly, given that the appropriate parameterizations for intra-wave processes are included. Model results also reveal the onshore near-bed transport is related to wave-induced near-bed streaming, wave skewness and asymmetry, and bed slope effects at different locations across the beach surface. Wave breaking-induced turbulence enhances the near-bed transport within the bed boundary layer which needs to be taken into account in order to achieve good model prediction skills.

A new method for measurement and calculation of unit width suspended sediment transport rate

Abstract The unit width suspended sediment transport rate is the basis for calculating river sediment flux, which provides an important measure of river hydrology and morpho-dynamics. By integrating the vertical distribution of the suspended sediment concentration and logarithmic flow velocity, the formula for unit width suspended sediment transport rate is presented. Based on this formula, the calculation of the near-bed relative transport rate missing in field measurement is given. Moreover, a new stratified water sampling device is invented for suspended sediment transport rate measurement by the six-point method at Mengsheng hydrological station in the Lancang River basin. Results of 24 sets of measurements show that the near-bed relative sediment transport rate ranges from 3.4 to 10.7% and is significantly positively correlated with the suspension index. The unit width suspended sediment transport rate measured by the traditional depth integrating method is on average 15.0% smaller than that calculated by the theoretical formula, which can be a reference for correcting the sediment transport rate based on the traditional method. This work provides a new device and a new method for the measurement and calculation of the unit width transport rate of suspended sediment, which has theoretical and practical significance.

The impacts of near-bed flow characteristics on river bed sediment transport under ice-covered conditions in 2016–2021

Global climate change has been projected to affect hydrology, the ice-covered flow period and river morphology (including changed sediment transport conditions) in northern high-latitude regions. To understand the impact of the expected shortening of the ice-covered period on bedload transport, one needs to understand the present sediment transport in these high-latitude rivers with annually occurring ice cover. Thus, the aims are (1) to define the impacts of ice cover on near-bed flow characteristics during hydrologically varying years, and (2) to analyse the impacts of these mid-winter flow characteristics on the bed sediment transport potential. The analyses are based on Acoustic Doppler Current Profiler (ADCP: 2016–21) and Acoustic Doppler Velocimeter (ADV: 2020–21) measurements performed in mid-winter ice-covered conditions of the sandy and small (circa 20 m wide) Pulmanki River in northern Finnish Lapland.Despite the ice-covered river conditions in winter, sediment transport occurs even during these harshest mid-winter conditions. The critical velocities and shear velocities of mid-winter conditions were exceeded in winters 2016–2021, and bedload transport occurred according to bedload measurements. Three different situations occurred regarding the bed sediment transport and near-bed velocity conditions: (1) high measured mid-winter discharges indicate high velocities throughout the meander bend; (2) low measured mid-winter discharges cause low near-bed velocities throughout the meander bend; (3) winters having intermediate discharges indicate near-bed velocities and sediment transport potential being higher at the upstream inlet and apex sections of the meander bend but clearly lower downstream of the apex. The confinement by the river ice cover, i.e. bottom-fast ice, explains the velocity variation. The near-bed velocities were the highest at the upstream inlet section of a symmetrical meander bend, where the measurement cross-sections were narrower and shallower. The velocities were the lowest downstream of the apex, where the channel changed from relatively narrow to wider and deeper.

Numerical investigation of sheet flow driven by a near-breaking transient wave using SedFoam

Sheet flow driven by a near-breaking transient wave is numerically investigated using SedFoam, a two-phase Eulerian sediment transport model in the OpenFOAM framework. SedFoam resolves the full profile of sediment transport processes within the bottom boundary layer based on closures for intergranular stresses and fluid–particle interactions. Compared with large-scale wave flume data, good agreements are obtained for streamwise flow velocity profiles, sediment concentration profiles, and the sheet flow layer thickness. Model results show near-bed velocity and sediment profile evolution within the sheet flow layer. Intense sediment suspension trailing the wave crest generates stable density stratification that dampens near-bed turbulent kinetic energy and contributes to decreasing bed shear stress. Results suggest that the buoyant flux dominates turbulent kinetic energy dissipation after the passing of the wave crest, coinciding with the reduction of bed shear stress. The instantaneous upper bound of the sheet flow layer exists in different log-law regimes under the transient wave, giving rise to a near-bed velocity profile that is highly dependent on the variable sheet flow layer wall unit. The effect of the profile shape parameter on bedload sediment transport is studied where the bedload predicted using the time-varying optimal profile shape parameter yields good agreement compared to the directly modeled bedload. Modeled sediment transport rates demonstrate that reduced bed shear stress caused by density stratification limits bedload and results in a suspended load-dominant mode.

Influence of artificial sandbar on nonlinear wave transformation: Experimental investigation and parameterizations

Wave nonlinearity is a main driving force for near-bed sediment transport, and thus affects the beach evolution. Accurate prediction of wave nonlinearity is an essential need for the design of beach nourishment and restoration projects. So far, wave nonlinearity evolution patterns on the nourished beaches with abrupt changes of bottom slopes and water depth are not fully understood. This paper presents experimental results of nonlinear wave transformation across the submerged artificial sandbar. It can be found that the artificial sandbar intensifies wave shoaling processes and increases the magnitudes of the wave nonlinearity. Wave skewness and asymmetry are parameterized and both dependent on Ursell number and bottom slopes. By taking bottom slopes into consideration, the prediction accuracy for wave skewness is improved 17% with dataset measured in this study and 21%–65% with an independent dataset, compared with a common-used formula also calibrated on a steep low-crest breakwater without considering the bottom slope. Wave skewness decreases with increasing Ursell number in the inner surf zone, indicating the artificial sandbar influences its shoreward wave nonlinearities through the wave history. Finally, it is found that wave triads interactions can be enhanced by the steep seaward slope and small water depth of the artificial sandbar.

Transport and deposition processes of the sediment depocenter off the Shandong Peninsula: An observational study

The long-distance transport of Yellow River sediments has formed an omega-shaped (‘Ω’) distal muddy deposit in the Yellow Sea. Numerous studies have investigated the sediment dynamics and sedimentary evolution of the deposit, but several key questions remain unanswered due to insufficient observations. To reveal the mechanisms of sediment dynamics and quantify sediment transport and deposition processes, this study collected field datasets in the depocenter of the muddy deposit off the Shandong Peninsula. Bottom mounted tripods were deployed from August 18 to 27, 2017, and from February 23 to March 2, 2018, during which two strong wind events with speeds of over 10.8 m/s occurred. The results showed that near-bed suspended sediment concentration responded directly to varying hydrodynamic conditions, and significant differences in suspended sediment concentration between two wind events could be ascribed to a number of factors (e.g., tidal current, wave periods, wind stress, turbulence). The harmonic analysis demonstrated that suspended sediment variations during the spring tide were directed mainly by resuspension, while resuspension and advection contributed equally at neap tide. The variations in background suspended sediment concentration can be ascribed to the residual current. Furthermore, the sediment transport pattern is similar to the pattern of water circulation. Generally, the along-shore suspended sediment flux is about one order of magnitude higher than the cross-shore flux. In summer, the suspended sediment was transported mainly downcoast. In winter, sediment was transported downcoast during wavy periods and upcoast during calm periods. Advection dominated sediment flux in most cases, except during wavy periods when local resuspension increased significantly, and the contribution of resuspension term to along-shore flux was up to 87%. Meanwhile, cross-shore flux also increased significantly under strong winds, which was simultaneously modulated by downcoast and offshore winds. After statistical analysis, we found that offshore winds controlled the cross-shore sediment transport in winter. Using parameterized calculation and statistical analysis, the average accumulation rate of the study site was estimated to be 0.6 cm/yr. During calm periods, lower background sediment concentration in summer led to lower deposition rates than in winter. In addition, the frequent strong wind events in winter cause extremely high erosion rates. This study provides new perspectives on quantitative variations in suspended sediment transport and regional erosion/deposition from synoptic to seasonal variations in the distal mud deposit.

Dynamic evolution of a secondary turbidity maximum under various forcing conditions in a microtidal estuary

Axial survey and in situ mooring in microtidal Masan Bay were conducted to reveal the mechanism for movement of secondary turbidity maximum (STM) and sediment resuspension. The Bay is characterized by sluggish water circulation with a vertically well-mixed water column. The strength of the residual estuarine current (Δu¯) was enhanced by the combined effects of decreased tidal currents, increased precipitation, and down-estuary (northerly) winds, whereas Δu¯ was weakened by increased tidal currents and up-estuary (southerly) winds. The variability of current asymmetry originated from Δu¯ contributed to the entrapment of suspended sediments, creating a mobile sediment pool in central regions of the Bay. The sediments resuspended from the mobile sediment pool were more influenced by residual currents than the tidal currents, and they remained in suspension near bed. When the down-estuary winds were applied to the mobile sediment pool, the near-bed sediments were readily resuspended to form an STM > 40 mg l−1. The STM moved up-estuary by persistent high Δu¯ and down-estuary by an intermittent low Δu¯. Over the entire measurement periods, the STM moved by at least 450 m toward the up-estuary. As the consolidated sediment layer was exposed by the movement of the STM, near-bed currents were ineffective in resuspending sediments from the consolidated layer even though the winds provided sufficient current strength. Due to the depletion of available sediments for resuspension, the STM was not fully generated. This study highlights that the STM in a microtidal estuary can be determined by the combined function of various forcings such as tide, wind, and precipitation.

Seagrass Meadows Reduce Wind-Wave Driven Sediment Resuspension in a Sheltered Environment

Seagrass meadows are prominent in many coastal zones worldwide and significant contributors to global primary production. The large bottom roughness (or canopy) created by seagrass meadows substantially alters near-bed hydrodynamics and sediment transport. In this study, we investigate how a seagrass meadow in a low-energy environment (forced by local winds) modifies near-bed mean and wave-driven flows and assess how this relates to suspended sediment concentration (SSC). A two-week field study was conducted at Garden Island in southwestern Australia, a shallow and sheltered coastal region subjected to large diurnal sea-breeze cycles, typical of many low-energy environments where seagrasses are found. The mean and turbulent flow structure, along with optical estimates of SSC, were measured within both a seagrass canopy and over an adjacent bare bed. Near-bed mean current velocities within the seagrass canopy were on average 35% of the velocity above the canopy. Oscillatory wave velocities were less attenuated than mean current velocities, with near-bed values on average being 83% of those above the canopy. Mean and maximum shear velocities inferred from currents and waves above the canopy frequently exceeded the threshold for sediment resuspension, but no significant variation was observed in the SSC. However, a significant correlation was observed between SSC and bed shear stress estimated using near-bed velocities inside the canopy. When sediment was resuspended, there were substantial differences between the SSCs within and above the canopy layer, with higher levels confined within the canopy. This study demonstrates the importance of measuring near-bed hydrodynamic processes directly within seagrass canopies for predicting the role seagrass meadows play in regulating local rates of sediment resuspension.

Sediment suspension affected by submerged rigid vegetation under waves, currents and combined wave–current flows

Laboratory experiments and mathematical analyses were carried out to investigate and quantify the impacts of submerged rigid vegetation on sediment suspension under waves, currents and combined wave–current flows. Mimic canopies constructed from wooden cylinders were used in the experiments with three configurations (sparse, dense and vertically varying density). It was found that more sediment was suspended within the vegetation canopies, where smaller velocities and higher turbulence were observed, indicating that vegetation-induced turbulence rather than mean flow was the main driver of sediment suspension over vegetated beds. Meanwhile, higher sediment concentration near the bed was observed within a denser canopy, especially in the case of vertically varying vegetation density. The near-bed suspended sediment concentration could be described by an exponential formulation using an effective bed shear velocity considering different turbulence components (i.e. vegetation wake turbulence, bed shear turbulence and near-bed coherent structures). The formula was applicable in both vegetated and bare beds with different hydrodynamic conditions. To predict the vertical distributions of sediment suspension, two theoretical models of turbulent diffusion were improved by incorporating the vertically varying turbulent intensity in the water column.

Fluid mud dynamics in a tide-dominated estuary: A case study from the Yangtze River

Fluid mud plays an important role in the maintenance of the estuarine turbidity maximum (ETM) and makes a significant contribution to estuarine sediment transport. A seabed tripod system and a shipboard vertical profile were deployed to explore the spatial-temporal variation and mechanism of fluid mud in the ETM of the Yangtze Estuary. The near-bed suspended sediment concentration (SSC) measured by OBS and ABS exceeded 50 kg/m3, confirming the existence of fluid mud. The fluid mud layer varied frequently in thickness during spring and intermediate tides, with a maximum of up to 0.6 m, but maintained stability during neap tides. The thickness rose rapidly during the larger ebb tide in semidiurnal tidal cycles of intermediate tides when the gradient Richardson number (Ri) increased to exceed the critical value of 0.25, and the velocity at 0.2 mab was in the range of 0.5–0.8 m/s. The fluid mud layer transported more than twice as much sediment as the low-concentration upper layer did. Waves and intense currents at the flood phase of tide enhanced vertical mixing within the water column, with Ri being reduced to lower than 0.25 and turbulent kinetic energy (TKE) exceeding 0.01 m2/s2, leading to the breakdown of the fluid mud layer. Sediment stratification damps turbulence close to the seabed and leads to gradual solidification of the bottom fluid mud. Approximately 0.2 m of fluid mud deposit was formed on the seabed within 148 h of the observation period.

Surf Zone Turbulence and Suspended Sediment Dynamics—A Review

The existence of sandy beaches relies on the onshore transport of sand by waves during post-storm conditions. Most operational sediment transport models employ wave-averaged terms, and/or the instantaneous cross-shore velocity signal, but the models often fail in predictions of the onshore-directed transport rates. An important reason is that they rarely consider the phase relationships between wave orbital velocity and the suspended sediment concentration. This relationship depends on the intra-wave structure of the bed shear stress and hence on the timing and magnitude of turbulence production in the water column. This paper provides an up-to-date review of recent experimental advances on intra-wave turbulence characteristics, sediment mobilization, and suspended sediment transport in laboratory and natural surf zones. Experimental results generally show that peaks in the suspended sediment concentration are shifted forward on the wave phase with increasing turbulence levels and instantaneous near-bed sediment concentration scales with instantaneous turbulent kinetic energy. The magnitude and intra-wave phase of turbulence production and sediment concentration are shown to depend on wave (breaker) type, seabed configuration, and relative wave height, which opens up the possibility of more robust predictions of transport rates for different wave and beach conditions.

Effect of surface wave skewness on near-bed sediment transport velocity

The paper presents a two-dimensional model, based on the Lagrangian description of sand grain dynamics, for the estimation of mean velocities of sediment particles driven by the oscillatory motion of water generated by non-linear gravitational surface waves of the Stokes type. The model was verified against experimental results obtained in a wave flume, by comparing its predictions with the sediment velocities measured by applying a PIV (particle image velocimetry) technique. The model was used to calculate near-bed sediment transport velocities generated by surface waves of the parameters encountered in real sea conditions, with the purpose to investigate the effect of the wave velocity skewness on the near-bed sediment net transport rates. The results of numerical simulations, carried out for a range of wave parameters and sand grain sizes, showed that normalized sediment transport velocities vary approximately linearly with an index defining the wave skewness, and are practically independent of sediment grain diameters. Based on the laboratory measurements, also approximate formulae relating a bed ripple asymmetry index with the surface wave skewness index were derived.

Acoustic scattering characteristics and inversions for suspended concentration and particle size above mixed sand and mud beds

The majority of reported field studies, using acoustic backscattering, for the measurement of nearbed suspended sediment processes, have been focussed on field sites with sand size fractions and unimodal size distributions. However, in many sedimentary environments, and particularly for estuaries and rivers, sands and muds coexist in the bed sediment substrate, forming a size regime that is often bimodal in nature. To examine the interaction of sound in these more complex sedimentary environments a numerical study is presented based on observations of sediment size distributions measured in the Dee estuary, UK. The work explores the interpretation of the backscatter signal from a mixed sediment composition in suspension, with mud-sand fractions varying with height above the bed. Consideration is given to the acoustical scattering properties and the inversion of the backscatter signal to extract information on the suspension. In common with most field deployments, the scenarios presented here use local bed sediments for the acoustic inversion of the backscattered signal. The results indicate that in general it is expected that particle size and concentration will diverge from what is actually in suspension, with the former being overestimated and the latter underestimated.

Field methods of a near-bed suspended sediment experiment in the Yangtze River, China

Using experimental data of near-bed suspended sediment concentrations at five typical hydrometric stations of the Three Gorges Reservoir at the early reserving stage, the differences were investigated between the common method and improved method during flood seasons and non-flood seasons. The impact of taking measurements below 0.2 times the water depth on the results was discussed. The results show that the average discharges and velocities at each station calculated by the common method were slightly larger than those calculated by the improved method. Regarding the suspended sediment concentration at each station, the errors in the reservoir and downstream channels in dynamic equilibrium state were small, and the largest errors occurred where the river bed was strongly scoured in the downstream reach below the large dam. There was no significant relationship between water discharge and flow velocity, and the missed measurement phenomenon also occurred. The sediment discharge error was affected by the suspended sediment concentration, implying that errors usually occurred in channels with serious erosion during flood seasons. The correction coefficients (R2) of sediment discharge at each station were given during the experiment, which showed that the sediment discharges at the hydrometric stations where a large amount of sediment transport occurred near the river bottom, needed to be modified. Furthermore, the test methods proposed in this study were applied to calculate the sediment discharges of three rivers, and the results indicate that this method can narrow the gap between bathymetric comparisons and sediment load measurements.

Estimation of near-bed sediment concentrations in turbulent flow beyond normality

Abstract Recent experiments have established that sediment particle motion, especially for particles near the bed, may not follow a normal (Fickian) diffusion behavior. To modify the diffusion equation when the fluctuation velocity is based on a normal distribution is a key research goal. This study represents an attempt to acquire some insight into the particle diffusion behavior by considering the turbulent fluctuation velocity based on bivariate probability distributions and by introducing the stochastic particle alignment into the particle–bed collision process in an open channel. The fluctuation velocity distribution is obtained using the Gram–Charlier expansion, which considers the first four statistical moments of turbulent fluctuation velocity. Fluctuation velocities in both streamwise and vertical directions are generated by conducting correlative Monte Carlo simulations. Moreover, in this study, the stochastic particle alignment composed of random particle sizes on the bed, as well as the particle collision process with random angles of incidence of particles is introduced to the particle collision mechanism. The associated probabilities of particle rebounding then would depend on the conditional probability distribution of the collision height. Particle trajectories simulated based on the saltation model and the random particle-bed collision process can be used to estimate the ensemble mean sediment concentration profile and the ensemble variance of sediment concentrations. In this study, the collision process, sediment concentration, and variance of particle motion are verified using available experimental data. Anomalous diffusion of particles can be observed in this study. In the proposed model, particle diffusion in the streamwise direction is found to be superdiffusive and that in the vertical direction is subdiffusive.

Direct evidence of a high-concentration basal layer in a submarine turbidity current

Submarine turbidity currents are one of the most important sediment transfer processes on earth. Yet the fundamental nature of turbidity currents is still debated; especially whether they are entirely dilute and turbulent, or a thin and dense basal layer drives the flow. This major knowledge gap is mainly due to a near-complete lack of direct measurements of sediment concentration within active submarine flows. Here we present the most detailed near-bed sediment concentrations measurements from a powerful turbidity current in Monterey Canyon, offshore California. We employ a novel approach using correlations between conductivity and sediment concentration, which unlike previous methods can measure very high concentrations and not sensitive to grain size. We find that sediment concentrations close to the canyon floor gradually increased after the arrival of the turbidity current, until reaching a maximum value of 12%, the highest concentration ever inferred from direct measurements in turbidity currents. We also show a two-layer flow head, with a fast (up to 4 m/s), thin and dense basal layer overlain by a thicker (~50 m) dilute flow. At the interface of these two layers, there seems to be a sharp steep concentration gradient. Such quantitative measurements of sediment concentration can produce a key step forward in understanding the basic character and dynamics of these powerful submarine flows.