Currently, the estimation of suspended sediment concentration (SSC) fluxes in rivers relies on river discharge and an average SSC, the latter is commonly determined through optical turbidity measurements at a single point in the river cross-section. This approach has limitations, such as the SSC data being extrapolated from a one-point measurement and indirectly determined depending on regular sampling and laboratory analysis, which is cost-intensive. Hydro-acoustic echosounders are an alternative to derive SSC across an entire profile, for accurate conversion from backscatter intensity to SSC knowledge of particle size is a requirement. In this approach, we present a method utilizing multi-frequency hydro-acoustic echosounding in addition to velocity measurements via an ADCP. Operating on various acoustic frequencies allows for the direct estimation of mean particle size from backscatter data at different frequencies over a water profile. River in-situ measurements as well as laboratory experiments have been conducted in different concentration as well as particle size distribution regimes.

Accurate modelling and prediction of sediment transport in aquatic environments is essential for sustainable coastal and riverine management. Current capabilities rely on physical process-based numerical models and fine-scale sediment flux measurements. High-resolution hydroacoustic instrumentation has emerged as a promising tool for such measurements. However, challenges arise due to the inherent complexity of ultrasound scattering processes. This study introduces a numerical modelling using a point-particle approach to simulate the echoes backscattered by such instrumentation in sediment-laden flow conditions. The model considers geometric, statistical, particle cloud, and flow-induced effects on sediment velocity, concentration, and flux estimates using an acoustic concentration and velocity profiler as a reference. The model performance is assessed here under unidirectional constant flow conditions in terms of velocity, concentration, and time-resolved sediment flux estimates for a large range of the particles' advection speed and sampled volume sizes. Application to the estimation of the measurement accuracy of sediment flux in these flows is also considered, with a final error on the flux seen to be partially controlled by the residence time of particles within the sampled volumes. The proposed model provides insights into scattering processes and offers a tool for investigating robust sediment flux estimation techniques in various flow conditions.

Large-scale wave flume experiments are conducted in the ripple vortex regime to study near bed coarse sand transport processes below asymmetric surface waves typical of the coastal nearshore region. For this purpose, a set of complementary acoustic instruments were deployed under regular nearshore wave conditions. Time-resolved velocity, sand concentration and sand flux profiles are measured across both the dense bedload and dilute suspension layers with an Acoustic Concentration and Velocity Profiler. The equilibrium 2D suborbital ripples are in good agreement in terms of dimensions, shape and onshore migration rate with Wang and Yuan (2018, , 2020, ). Stoss ripple vortex entrainment around the trough-to-crest flow reversal (FR+) is found to be more energetic in terms of sand pick-up into suspension compared to the counter rotating lee side vortex around the FR- flow reversal, as a consequence of the onshore skewed wave acceleration. Ripple vortex driven nearbed velocity phase leads around both flow reversals exceed typical bed friction induced values found in turbulent Wave Boundary Layers. Intrawave sand erosion events can be distinguished locally at the two ripple vortex positions around the flow reversals and two events more uniformly distributed along the ripple profile at wave crest and trough. Spatial fields of sand flux reveal the origin of the net onshore directed suspended and bedload transport. Good agreement is found with the mechanism identified under asymmetric oscillatory flows in Wang and Yuan (2020, ). Differences with ripple vortex regime under skewed shoaling waves and symmetric oscillatory flows are highlighted. Ripples are common bedform features on sandy beaches. They are formed under specific wave conditions and influence sand transport under waves. Indeed, when the wave passes over the wave crest, a vortex is created on the ripple flank. This modifies sand transport on beaches and makes it harder to predict. Predictions of sand transport on beaches are nevertheless extremely important to study long-term coastal evolution, which is crucial to solve social and environment issues. In this paper, ripple formation and transport under specific wave conditions are studied. Inside a ripple cycle, two flow reversals are present (i.e., the moments where the direction of the flow changes), and in this particular case, we focus on the waves for which the second flow reversal (from negative to positive flow velocity) happens much quicker than the first flow reversal (from positive to negative flow velocity). This influences the creation of the vortices above ripple flanks and therefore sand transport: indeed, it turns out that the vortex created on the offshore side of the ripple during the second flow reversal is more energetic and therefore leads to more sand transport in the onshore direction above the ripple than the other vortex. Good agreement in terms of ripple shape, migration speed and net transport rate is found with U-tube studies realized under similar hydrodynamic and sediment conditions Pick up due to vortex entrainment on the stoss side is more important than on the lee side due to the acceleration skewed flow conditions Contrary to the velocity-skewed case, the net resulting ripple averaged transport is onshore directed

Bar and berm morphology characterize the seasonal beach evolution, and determine the protection against storm erosion as well as the touristic use of beaches. Thus, they are of particular interest for coastal management and engineering in the nearshore zone. This study used large-scale wave flume experiments to observe the transition from fully dissipative to fully reflective beach profile at a high level of detail. Starting from a post-storm profile generated under energetic waves, a very low energy wave condition caused dissipation of the outer and the inner bar, shoreline recovery, and berm accretion. Measurements revealed feedback between hydrodynamics and beach profile evolution with an onshore shift of the wave breaking location. As a result, the magnitude and cross-shore evolution of wave asymmetry-related bedload net onshore and suspended net offshore transport changed. The relative magnitudes of the two transport components and the way they shifted relative to each other caused the observed beach recovery. Additionally, a link between bar and berm morphology (surf-swash sand exchange) was observed. The shifting breakpoint enabled sustained, wave asymmetry-related onshore transport in the inner surf zone, feeding the berm accretion which occurred through advective swash zone processes including berm overwash.

New experiments in highly turbulent, steady, subcritical and uniform water open-channel flows have been carried out to measure the mean turbulent kinetic energy (TKE) budget of sediment-laden boundary layer flows with two sizes (dp = 3 mm and 1 mm) of Plexiglas particles $({\rm relative\ density}\ = 1.192)$ . The experiments covered energetic sediment transport conditions (Shields number of $0.35 < \theta < 1.2$ ) ranging from non-capacity to full-capacity flows in bedload-to-suspension-dominated transport modes (suspension number of $0.5 < {w_s}/{u_\ast } < 1.3$ where ${w_s}$ is the settling velocity and $u_*$ is the friction velocity) and for weakly to highly inertial, finite size turbulence-particle conditions (Stokes number of 0.1 < St < 3.5 and dp/η > 10 where $\eta$ is the Kolmogorov length scale). It was shown that the effects of sediments on the TKE budget are very pronounced in all large particle experiments for which a bedload layer of several grain diameter thickness is developed above the channel bed. When compared with the corresponding reference clear-water flows, the TKE shear-production rate for the 3 mm particle flows is strongly reduced in the wall region corresponding to the bedload layer. This turbulence damping is seen to increase with sediment load until full capacity for flows with constant Shields value, as well as with Shields number value. Inside this damped TKE shear-production zone, a distinct peak of maximal turbulence production appears to coincide with the upper edge of the bedload layer delimited by a sharp gradient in mean sediment concentration. This vertically upshifted peak of TKE production is accompanied by an enhanced net downward oriented TKE flux when compared with the reference clear-water flows. The downward diffused TKE is found to act in the bedload layer as a local energy source in reasonable balance with the sediment transport term. The mechanism behind this downward TKE transport was further analysed on the basis of coherent flow structure dynamics controlled by ejection- and sweep-type events. The agreement between the height of downward directed mean TKE flux and the height below which sweep-type events dominate the Reynolds shear-stress contribution over ejections, revealed the leading role played by sweeps in mean TKE transport. This agreement holds for all reference clear-water flows supporting the well-known wall-roughness-induced dominance of the sweep contribution in turbulent, rough clear-water boundary layer flows. Furthermore, for all 3 mm particle flows, the two referred to transition levels were significantly and similarly upshifted to the upper edge of the bedload layer. Only for these sediment-laden flows, the bedload layer thickness is seen to exceed the wall-roughness sublayer of the reference clear-water flows. This supports a strong analogy between wall-roughness effects in clear-water flows and bedload layer effects in sediment-laden flows, on the mean TKE budget induced by a similarly modified coherent flow structure dynamics. The bedload layer-controlled wall roughness is finally confirmed by the good prediction of the wall-roughness parameter ks of the logarithmic velocity distribution. An empirical formulation fitting the presented measurements is presented, valid over the range of Shields number values covered herein.

&lt;p&gt;At present, SSC fluxes in rivers are typically estimated by multiplying the river discharge with the&lt;br /&gt;average suspended sediment concentration (SSC). The latter is typically obtained from optical turbidity&lt;br /&gt;measurements in one single point of the river cross&amp;#8208;section. The optical turbidity is converted in&lt;br /&gt;average SSC based on a relation that is derived from the laboratory analysis of regular SSC samples.&lt;br /&gt;This method has the disadvantages that it is based on a one&amp;#8208;point measurement and that it is&lt;br /&gt;expensive.&lt;/p&gt; &lt;p&gt;The SSC distribution in an entire profile &amp;#8211; vertical or horizontal &amp;#8211; can also be derived from the&lt;br /&gt;backscatter of single&amp;#8208;frequency echosounders. The disadvantage of this method is that the particle size&lt;br /&gt;of the suspended sediment needs to be known in order to convert the profile of backscatter into a&lt;br /&gt;profile of SSC.&lt;/p&gt; &lt;p&gt;Here we present a hydro&amp;#8208;acoustic method based on multi&amp;#8208;frequency echosounding. Operating on&lt;br /&gt;multiple acoustic frequencies allows estimating the mean particle size directly from the backscatter at&lt;br /&gt;the different frequencies. The method based on multi&amp;#8208;frequency echosounding is illustrated with&lt;br /&gt;measurements on the Rh&amp;#244;ne River just upstream of Lake Geneva in Switzerland. The results are&lt;br /&gt;compared to measurements based on optical turbidity measurements and to measurements based on&lt;br /&gt;single&amp;#8208;frequency echosounding.&lt;/p&gt;

Onshore bar migration is a characteristic bar behavior during post-storm beach recovery. The present large-scale experiments, feature bichromatic wave groups over an initially steep (1:15), fully-evolving beach. The same accretive wave condition is applied on two different post-storm beach profiles featuring outer and inner bars. They are characterized by a larger (smaller) shoreline erosion and a larger (smaller) outer breaker bar located farther away from (closer to) the shoreline depending on the larger (smaller) energy of the storm condition. After a considerable post-storm recovery time, similar equilibrium profiles are obtained, stressing the link between wave condition and equilibrium beach configuration. However, the evolution toward the equilibrium is different and depends on the initial morphological condition (post-storm beach profile). After the larger storm, the morphological evolution is termed accretive merging (AM) and characterized by merging of the two bars (outer bar dissipation). After the smaller storm, the morphological evolution denoted as accretive non-merging (AN) is characterized by onshore migration of the two bars with constant distance between them (bar maintenance). This study focuses on processes around the outer bar. During AN it features wave breaking, causing large suspended net offshore transport. AM, in contrast, mainly features bedload related to short wave asymmetries and low decomposed net transport rate magnitudes. High suspended net offshore transport occurs solely onshore of the outer bar trough. This causes filling of the bar trough and bar dissipation during migration. Additionally, processes around the outer bars are linked to accretion onshore of the bars and at the shoreline.

Detailed information on nearshore sediment transport processes during onshore bar migration were obtained from large-scale laboratory experiments with bichromatic wave groups on a relatively steep initial beach slope (1:15). Detailed measurements of velocity and sand concentration near the bed from shoaling up to the outer breaking zone including suspended sediment and sheet flow transport are presented. The analysis focuses on onshore migration under an accretive wave condition but comparison to an erosive condition highlights important differences. Decomposition shows that total transport mainly results from a balance of short wave-related, bedload onshore transport and current-related, suspended offshore transport. When comparing the accretive to the more energetic erosive condition, the balance shifts toward onshore transport, and onshore migration, because the short wave-related transport does not decrease as much as the current-related transport. This is related to the effects of skewness and asymmetry combined with less sediment suspension in the water column and undertow magnitude under the accretive condition. Transports from streaming in the wave boundary layer and from infragravity waves become visible but only play a subordinate role. Identified priorities for numerical model development include parametrization of wave nonlinearity effects and better description of wave breaking and its influences on sediment suspension.

This study presents novel insights into hydrodynamics and sediment fluxes in large-scale laboratory experiments with bichromatic wave groups on a relatively steep initial beach slope (1:15). An Acoustic Concentration and Velocity Profiler provided detailed information of velocity and sand concentration near the bed from shoaling up to the outer breaking zone including suspended sediment and sheet flow transport. The morphological evolution was characterized by offshore migration of the outer breaker bar. Decomposition of the total net transport revealed a balance of onshore-directed, short wave-related and offshore-directed, current-related net transport. The short wave-related transport mainly occurred as bedload over small vertical extents. It was linked to characteristic intrawave sheet flow layer expansions during short wave crests. The current-related transport rate featured lower maximum flux magnitudes but occurred over larger vertical extents. As a result, it was larger than the short wave-related transport rate in all but one cross-shore position, driving the bar's offshore migration. Net flux magnitudes of the infragravity component were comparatively low but played a nonnegligible role for total net transport rate in certain cross-shore positions. Net infragravity flux profiles sometimes featured opposing directions over the vertical. The fluxes were linked to a standing infragravity wave pattern and to the correlation of the short wave envelope, controlling suspension, with the infragravity wave velocity.

A new set of open-channel flow experiments and turbulence resolved data are presented in heavy particle sheet flows (Shields number 0.35 <= theta <= 0.85) for which the proportions of bedload and suspended load are both important (ratio of settling velocity and friction velocity in the range 0.8 <= w(s)/u(*) <= 1.3). The effects of sediments and particularly the bedload on the turbulent suspension have been addressed by gradually increasing the concentration from clear water to capacity conditions. Distinction between the bedload and the suspension layer is discussed on the basis of the linearity of turbulent mixing length profiles. It is shown that the bedload layer has important impact on the vertical structure of the particle-laden flow. An upward shift of the logarithmic velocity layer is seen to be accompanied by a strong reduction of turbulent momentum mixing. The modification of the mixing length affects the theoretical formulation of both velocity and concentration profiles in the suspension layer. A modified analytical solution is derived for the suspended sediment concentration profile taking into account the presence of the bedload layer for improved predictions compared with the classical Rouse equation. Based on the present experiments, as well as literature data, an alternative parametrization for the beta-factor (ratio of sediment and momentum diffusivities) is proposed over an extended range of suspension number, 0 < w(s)/u(*) < 1.5. (C) 2022 American Society of Civil Engineers.

A new set of open-channel flow experiments and turbulence resolved data are presented in sediment-laden flows (Shields number 0.35 <=theta <= 1.2 and ratio of settling velocity and friction velocity (0.8 <= w(s)/u(*) <= 1.3). The effects of sediments and particularly the bed-load on the turbulent suspension have been addressed by gradually increasing the concentration from clear-water to capacity conditions. Distinction between the bed-load and the suspension layer is discussed based on the linearity of turbulent mixing length profiles. It is shown that the bed-load layer has important impact on the vertical structure of the particle-laden flow. An upward shift of the logarithmic velocity layer is seen to be accompanied by a strong reduction of turbulent momentum mixing. The modification of the mixing length affects the theoretical formulation of concentration profiles in the suspension layer. A modified analytical solution is derived for the suspended sediment concentration profile taking into account the presence of the bed-load layer for improved predictions compared with the classical Rouse equation. Based on the present experiments, as well as literature data, an alternative parametrization for the beta-factor (ratio of sediment and momentum diffusivities) is proposed over an extended range of suspension number, 0 < w(s)/u(*) < 1.5.

&lt;p&gt;Intense sediment transport regimes are important in river and coastal geomorphology as they are responsible for important morphological evolution occurring during extreme climatic events. Under such transport conditions, a complex interplay between suspended load, dominated by turbulence-particle interactions, and bed-load, dominated by particle-particle interactions, is taking place. Both turbulence and granular processes are interacting with each other in the so-called four-way coupling corresponding to a modification of fluid turbulence due to granular interactions and vice-versa.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;In order to better understand these fine-scale turbulent and granular processes in intense sediment transport regime we acquired new experimental data at the lab in a 10m long open-channel flume.&amp;#160; Two plastic sediment sizes have been used separately, 1 and 3mm in diameter, they are introduced at the upstream end of the flume using a sediment feeder system combining a hopper and a conveyor belt. A single profile is measured at 3m from the outlet in the centerline of the flume using the Acoustic Concentration and Velocity Profiler (ACVP) having a 1.5mm vertical resolution and 78Hz temporal resolution. The acoustic system allows to concurrently measure the mean velocity and the concentration profiles as well as second order statistics.&amp;#160; The experimental dataset contains 3 flow regimes for each particle size and 4 sediment load for each flow condition ranging from no sediments to almost saturated (or capacity) flow conditions. Ultimately, we acquired 86 runs with some redundancy to evaluate the repeatability of the experiments. In terms of dimensionless numbers, we cover the following &amp;#160;range of Shields number &amp;#120579; &amp;#8712;[0.3 ; 1.5] and suspension number w&lt;sub&gt;s&lt;/sub&gt;/u&lt;sub&gt;*&lt;/sub&gt; &amp;#8712;[0.4 ; 1.3] where w&lt;sub&gt;s &lt;/sub&gt;stands&lt;sub&gt;&lt;/sub&gt;for the settling velocity of the individual particles and u&lt;sub&gt;* &lt;/sub&gt;stands for the bed friction velocity. All flow conditions are in the subcritical fully turbulent hydraulically rough regime.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;This extensive dataset is further used to develop and validate a two-phase flow Eulerian-Eulerian model. The new experimental data combined with Eulerian-Lagrangian simulations (CFD-DEM) provide a strong guideline to establish constitutive relations for granular stress models as well as for turbulence models. We propose an empirical modification of the kinetic theory of granular flows to account for particle-particle friction essentially through a modification of the radial distribution function and by adding a dependency of the restitution coefficient to particle friction. A two-equation turbulence model, k-omega SST, is used for the boundary layer. The key terms that are the most uncertain in this problem are the fluctuating energy transfer terms between the fluid (Turbulent Kinetic Energy) and the particles (granular temperature). The objective of the present contribution is to try to elucidate this question using the combined experimental, theoretical and numerical approach presented above. Preliminary results are very encouraging and a synthesis of this work will be presented at the conference.&lt;/p&gt;

One of the most enigmatic science questions concerning inertial particle transport by a turbulent boundary layer flow is the value of the turbulent Schmidt number defined as the ratio of turbulent eddy viscosity to particle concentration diffusivity. Using direct acoustic measurement of turbulent particle flux profile, and two-phase flow turbulence-resolving numerical simulation, it is demonstrated that turbulent dispersion of particles is reduced rather than enhanced as predicted by many existing literature models. The explanation lies in the misleading assumption of settling velocity in quiescent water to estimate the turbulent particle diffusivity, while direct measurements and simulations of turbulent particle flux support the occurrence of settling retardation. The analysis presented herein suggests that the value of the turbulent Schmidt number is always larger than unity with values between 3 and 4 based on the directly measured turbulent particle flux. The observed settling reduction cannot be explained by the well-known hindrance effects related to particle concentration. This effect seems to be related to turbulence-particle interactions and correlates more with the Stokes number. Finally, our parameters, namely, the turbulent Schmidt number higher than unity, modified von Karman constant, and settling retardation, are successfully tested for the modeling of particle concentration profile using the well-known Rouse formulation. This result suggests that alternative parametrizations are possible to reduce the degree of empiricism to predict suspended particle transport by a boundary layer flow.

Dunes dominate the bed of sandy rivers and they respond to flow by changing shape and size, modifying flow, and sediment transport dynamics of rivers. Our understanding of and ability to predict dune adaptation, particularly dune growth and decay, remain incomplete. Here, we investigate dune growth from an initial flatbed in a laboratory setting by continuously mapping the 3D bed topography using a line laser scanner combined with a 3D camera. High‐resolution profiles of flow velocity and sediment concentration providing both bedload and suspended sediment fluxes were obtained by deploying Acoustic Concentration and Velocity Profiler technology. Our analysis reveals that the magnitude of the dune slipface angle, which determines flow separation and controls turbulence production, adjusts to the imposed flow at time scales similar to the evolution of dune height and length. The initiation of a flow separation zone intensifies through scour, and results in acceleration of the dune growth. Gradients in sediment transport and the rate of dune growth are inherently linked to spatial variations in slipface angles. During dune growth, the slipface angle evolves differently than the ratio of dune height to length, which immediately reaches its equilibrium value after dune initiation.

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.

A new dataset of co-located 2C velocity, sediment concentration and sediment flux profile measurements collected with an Acoustic Concentration and Velocity Profiler (ACVP), are presented in this paper. The experimental protocol developed herein is described in details. The first results are displayed in terms of mean velocity, concentration and sediment flux profiles. The validity of the logarithmic velocity profile and the Rouse concentration profile for the suspension load are analyzed. The effect of the Shields and suspension numbers on the solid transport mode is discussed. The flow quantities and turbulent transport processes to be analyzed with this new High-Resolution dataset and the corresponding questions to be addressed are raised in the conclusion.

In the state-of-the-art for suspended-load modeling it is commonly assumed that the concentration profile results from a balance between a settling flux, in which the settling velocity is considered as equal to its value for a single settling particle in quiescent water, and an upward turbulent flux modeled using a Fickian gradient diffusion approximation. While this model provides a general framework, comparison with experiments reveals that the concentration diffusivity is not equal to the eddy viscosity and a turbulent Schmidt number needs to be introduced. Based on Coleman (1970,1981) data, van Rijn (1984) proposed an empirical model which suggests that the Schmidt number is a decreasing function of Ws/u*. This result is intriguing as it suggests that the turbulent dispersion of sediment concentration is enhanced when the particle’s settling velocity increases relative to the bed friction velocity. Van Rijn suggested that this is due to centrifugal forces that tends to throw inertial particles out of the turbulent vortices leading to an enhanced particle dispersion compared to momentum. In the present contribution, we use high-resolution experimental data and turbulence resolving two-phase flow simulations that directly resolve the turbulent momentum and particle fluxes and the flow turbulence to investigate the different terms appearing on the mass balance mentioned above. Both the experimental and the numerical results show that the actual turbulent Schmidt number based on the resolved sediment flux is higher than unity meaning that turbulent dispersion efficiency of « heavy particles » is reduced. This contradicts van Rijn’s prediction model of the Schmidt number. One plausible explanation is that the settling velocity of particles is reduced in highly turbulent flows. Using the experimental and numerical results, the actual settling velocity in the turbulent flow is retrieved from the mass balance at steady state. It is found that it is significantly retarded compared with the value in quiescent water (10 to 40%). This result is in good agreement with the one obtained in recent experiments performed in a turbulent grid at KIT (Germany) using the same particles (Akutina et al., 2020). The authors found a settling retardation of 16% for the same turbulent intensities as in the present experiments. The results presented herein completely change the paradigm for turbulent suspension load modeling and open new perspectives on the development of new, physical process-based, parametrizations required for large-scale models. This, of course, will require to extend the proposed methodology to a wider range of flow and sediment conditions.

We present the hydroacoustic inversion tool HYDRAC. This versatile open-source software, developed in Python 3, offers the capability to read hydroacoustic data from widely-used instruments (eg. ABS, ADCP,…) and perform acoustic inversions following several advanced methods found in the literature to estimate the Suspended Particulate Matter mass concentration and particle size characteristics. This software, designed for long-term community-based developments, includes a specific module for modelling the SPM scattering properties, from organic to mineral particles. The originality of this tool lies in its high technological readiness level (TRL 5), towards the emergence of a performant SPM technology for operational use.