Acoustic Doppler Velocity Profiler Research Articles

Analyzing sediment transport in dam-break-driven swash processes: Insights from laboratory experiments using a high-speed camera

Dam-break-driven experimental tests are conducted to explore the intricate dynamics of water and sediment movement during a swash process. Utilizing a combined approach of a high-speed camera and an acoustic doppler velocity profiler, detailed records of flow velocity and sediment movements are captured. This study offers novel insights regarding flow shear stress, layer thickness and velocity of sediment transport in the backwash process, as well as behaviors of individual sand particles. Linear relationships serve to depict alterations in bed shear stress over time. The changing rate of bed shear stress can be influenced by the factors such as the beach slope, sediment grain size, and location on the beach. Increasing layer thickness of sediment transport corresponds with increasing outer layer velocity, exhibiting a linear-exponential relationship. A tentative interpretation of the inflection point (of linear and exponential relationship) designates it as a division between sheet load and a conjunction of sheet load and suspended load. With the onset of a bore and under the influence of turbulence and bubbles, sediment predominantly moves upwards, leading to sediment pickup. However, during both the late uprush phase and the backwash phase, the primary direction of sediment movement reverses downwards, favoring sediment settling. Empirical equations are formulated to estimate the parameters on the bed shear stress and the sediment transport layer. This study enhances understanding of water and sediment movement in swash processes, supplying validation data for numerical models.

Measurement of Yangtze River Flow Based on Coastal Acoustic Tomography

River flow is an important hydrological element for river management, and it can be used to detect the profile of river water resources. Coastal acoustic tomography technology, is a novel flow measurement technology, can continuously observe flow changes compared to the acoustic Doppler velocity profiler (ADCP) flow measurement. In this article, our experiment has arranged two acoustic tomography stations on both banks near the Datong Hydrological Station in the Yangtze River. Each station is equipped with a coastal acoustic tomography system. The depth of the transducer placed underwater is about 7 meters, the frequency of the transmitted signal is 50kHz. Knowing the distance between the two stations, calculate the average flow velocity between the stations according to the reciprocal propagation time difference between the stations, and then obtain the average velocity of the section according to the geometric relationship between the flow velocity between the stations and the section velocity. Finally the cross-sectional velocity multiplied by the cross-sectional area is the cross-sectional flow, and the calculation result of the river flow is compared with the official data. Experimental results prove that coastal acoustic tomography technology has broad prospects in monitoring river flow.

Exploring Wind-Driven Subsurface Water Flow with an Acoustic Doppler Velocity Profiler

AbstractIn this study, we report on experiments carried out in a large wind-wave tank to investigate the potential of the acoustic Doppler velocity profiler for determining the structure of the subsurface water boundary layer. This flow located just beneath the air–water interface forms whenever wind blows. The profiler is first tested for a steady flow generated by pumps beneath a flat water surface. Measurements of the velocity field at different stages of development of the wind-induced shear flow, from laminar to fully turbulent, are then analyzed. The best way to obtain reliable data under these flow conditions is thoroughly examined. Despite the inherent difficulty of seeding acoustic tracers homogeneously in such a boundary layer, the profiler has the major advantage of providing records of the instantaneous profiles of the subsurface velocity field referenced to surface elevation. This feature makes it possible to estimate statistical properties of the water motions at various scales in a wave-following coordinate system, and thus greatly increases the physical significance of the measured quantities. The variation with fetch of the main characteristics of the mean drift current, orbital wave motions, and turbulent flow disturbances estimated in this coordinate system is then presented and discussed in detail.

Experimental investigation of the effect of salt wedge on concentration of suspended non-cohesive particles in turbid flow

To have an insight of the effects of salt wedge on suspended sediment concentration (SSC), we carried out experiments of intruding salt wedge in flume with fresh turbid water of non-cohesive particles and salt water free of sediment. Two sets of Acoustic Doppler Velocity Profiler (ADVP) were used to record instantaneous velocity profiles in the saline affected zone near the bed. The acoustic back scattered signal amplitudes (AMP) in the measurements were used to estimate SSC at the same point and the same time. The results show that around the interface shear of exchange flow is remarkable and SSC increases noticeably. Flow analysis indicates that large scale turbulence from interfacial instability is the main power source to enhance sediment carrying capacity of the flow. It weakens and retards the particles in the upper turbid layer sinking to the lower saline zone. However, the energy dissipation doesn’t show much change.

OBSERVATIONS OF CANOPY FLOW STRUCTURE AND FRICTIONAL VELOCITY OF CORAL COLONIES IN DONGSHA ATOLL

Understanding the hydrodynamics is important for biological, ecological, and biogeochemical processes in coral reef systems. The near-bed flow motion affects the benthic organism distributions, morphological evolution, larvae settlement, and nutrient uptake. The near-bed flow structures have been characterized as planar boundary-layer flows when the bottom roughness scale created by benthic organisms is much smaller than the water depth. On the other hand, when the bottom roughness scale becomes much larger, the resistance drag forces caused by these canopy elements should be considered (Rosman and Hench, 2011). The form drag of the multiple coral colonies generates turbulent wakes, enhances turbulent mixing, and changes the flow structure (Huang, 2015). Many laboratory and modeling studies have reported the drag parameterization and the flow structure for unidirectional flows through submerged canopy or vegetation (e.g., Finnigan, 2000; among many others). However, the vertical flow structures of the canopy layer caused by coral colonies (bommies) are rarely reported in fields. Here we present field measurements of flow structure over coral colonies using acoustic Doppler velocimetry (ADV) and pulse-coherent Doppler velocity profiler (PCADP) techniques. The measured current profiles and turbulence are used to study the flow dynamics in the canopy-layer created by coral colonies.

Large-scale coherent flow structures in rough-bed open-channel flow observed in fluctuations of three-dimensional velocity, skin friction and bed pressure

ABSTRACTThe dynamics of large-scale coherent structures (LSCS) are investigated in a rough-bed flow over the whole water column and longitudinal and transverse turbulent fluxes of momentum are compared. The relation between LSCS and fluctuations of skin friction and near-bed surface pressure is examined in order to understand the effects of LSCS on the bed. The study was carried out in an open channel with a non-moving permeable bed of screeded loose-mixed gravel (D50 = 1.5 cm) at Reynolds numbers ranging from 3.19 × 104 to 1.08 × 105. Quasi-instantaneous full-depth three-dimensional (3D) velocity profiles with high spatial and temporal resolution measured with an acoustic Doppler velocity profiler (ADVP) are combined with the signals of a hot-film sensor mounted on the top of the gravel and a piezoresistive pressure transducer placed in the top layer of the bed particles at the centre of the channel. Vertical turbulent momentum flux is clearly dominated by ejections and sweeps over most of the water column. Near the bed, contributions of sweeps are larger than those of ejections. This is reversed at the mid-depth (z/h = 0.5). No preferential quadrant orientation is seen for the transverse momentum flux. A correlation between LSCS dynamics in the water column and data from the sensors on the bed is made evident. Sweeps correlate with positive fluctuations of bottom pressure and skin friction while ejections correlate with negative ones.

Comparison of velocity and turbulence profiles obtained with a Vectrino Profiler and PIV

Laboratory experiments were carried out in a small openchannel hydraulic flume at the Université de Sherbrooke. A PIV and an acoustic Doppler velocity profiler (Vectrino II (VII)) were used to measure high frequency velocities in profiles along the centreline of a small openchannel flume. Two background turbulence levels were tested. Comparisons were made of mean and turbulent statistics obtained with the two measurement techniques. The results show reasonable agreement between mean streamwise and lateral velocities measured with the PIV and VII near the “sweet spot” of the VII. In contrast, mean vertical velocities deviate substantially between the two measurement methods. Turbulence statistics have somewhat similar profile shapes, yet significant bias is observed between the two measurement methods.

Measuring Bedload Sediment Transport with an Acoustic Doppler Velocity Profiler

Acoustic Doppler velocity profilers (ADVP) measure the velocity simultaneously in a linear array of bins. They have been successfully used in the past to measure three-dimensional turbulent flow and the dynamics of suspended sediment. The capability of ADVP systems to measure bedload sediment flux remains uncertain. The main outstanding question relates to the physical meaning of the velocity measured in the region where bedload sediment transport occurs. The main hypothesis of the paper, that the ADVP measures the velocity of the moving bedload particles, is validated in laboratory experiments that range from weak to intense bedload transport. First, a detailed analysis of the raw return signals recorded by the ADVP reveals a clear footprint of the bedload sediment particles, demonstrating that these are the main scattering sources. Second, time-averaged and temporal fluctuations of bedload transport derived from high-speed videography are in good agreement with ADVP estimates. Third, ADVP based estimates of bedload velocity and thickness of the bedload layer comply with semi-theoretical expressions based on previous results. An ADVP configuration optimized for bedload measurements is found to perform only marginally better than the standard configuration for flow measurements, indicating that the standard ADVP configuration can be used for sediment flux investigations. Data treatment procedures are developed that identify the immobile-bed surface, the layers of rolling/sliding and saltating bedload particles, and the thickness of the bedload layer. Combining ADVP measurements of the bedload velocity with measurements of particle concentration provided by existing technology would provide the sediment flux.

On local velocity measurement in gravity-driven flows with intense bedload of coarse lightweight particles

A broad dataset of sheet-flow experiments with granular material under steady and uniform conditions is presented in this paper with the focus on measurements of streamwise velocity component. Three different lightweight sediment fractions were used during 128 experimental runs across a wide range of sheet-flow modes which were represented by dimensionless Shields parameter from 0.3 up to 2.3. The local velocity information was obtained using three independent methods: Prandtl probe (PT), Ultrasonic Velocity Profiler (UVP) and Acoustic Doppler Velocity Profiler (ADVP). The measurement methods were compared to each other using results in absolute and dimensionless velocity magnitudes. Limitations of the methods are discussed. The results were consistent for all experimental runs and are further used for a description of the observed flow internal structure. The capability of individual measuring methods is demonstrated here on an identification of a transport layer of a linear velocity distribution and of a basal sublayer at the transition between the stationary bed and the transport layer.

Use of the velocity defect law to assess sand transport rates

Abreu, T., Silva, P.A., Sancho, F. and Michallet, H., 2013. Use of the velocity defect law to assess sand transport rates.This work uses a simple method based on the defect law that is able to reproduce the horizontal velocities within the wave bottom boundary layer, using a limited number of parameters. The theory is checked against the measured velocity profiles gathered with a high resolution Acoustic Doppler Velocity Profiler. The velocities were collected during an experimental project performed at the Large Oscillating Water Tunnel of Deltares under flat-bed/sheet flow conditions, allowing the analysis of the effects of wave nonlinearities and of a net current on the sediment transport processes. The velocity estimates are used to infer the time-varying bed shear stress, using the momentum-integral method. Secondly, the bed shear stresses are incorporated in a quasi-steady bed load formulation and the transport rate predictions are compared with the net transport rate measurements obtained in the same facility. The results show that the methodology presented in this work provides a suitable estimation of the horizontal near-bed sediment transport rates under non-linear oscillatory flows.

Propeller Jet–Induced Suspended-Sediment Concentration

AbstractWhen subjected to a strong swirling jet flow induced by a rotating ship’s propeller, the bed of a waterway may undergo significant changes. These changes will not only cause instability to quay structures and damage the protection work of the beds/banks of rivers and canals, but also resuspend the bed sediment. The latter likely will pollute the water column and affect the aquatic life. In this study, the propeller-jet velocity and sediment-concentration profiles downstream of a propeller are measured using an acoustic Doppler velocity profiler and a suction device, respectively. The estimates obtained by using formulas for the axial velocity and four published methods for calculating the reference near-bed concentration of suspended sediment are compared with the experimental data. Moreover, the behavior underlying the temporal and spatial variations in seabed scour and sediment suspension is also investigated. A new set of equations for estimating the sediment concentration profiles is developed...

One-dimensional velocity profiles in open-channel flow with intense transport of coarse sediment

The paper deals with laboratory experiments in open-channel flows with intense transport of model sediment (coarse plastic particles) in our new tilting flume. The major objectives of the paper are: 1. to discuss applied measuring methods, 2. to analyze measured velocity profiles. Ad 1. A profile of the longitudinal component of local velocity was measured across the vertical axis of symmetry of a flume cross section using three independent measuring methods (Prandtl tube, Ultrasonic Velocity Profiler, Acoustic Doppler Velocity Profiler). Due to strong stratification of the flow in the flume, parts of the profile are measured in regions of very different local concentrations of sediment (from virtually zero concentration to the maximum concentration of bed packing). This makes measurements complicated, particularly for ultrasonic measuring techniques. Profiles measured using the different techniques are evaluated and mutually compared. Ad 2. The layered character of the flow causes that shapes of velocity profiles tend to be different in the transport layer (rich on transported particles) above the bed and in the solids-free region between the top of the transport layer and the water surface. Shapes of the profiles are analyzed. Particular attention is paid to the logarithmic profile in the solids-free region of the flow cross section. The profile can be handled using the law of the hydraulically-rough wall. In the law, the eroded top of the bed with the transport layer is supposed to be the rough boundary and appropriate values are sought for its variables.

Detection and reconstruction of large‐scale coherent flow structures in gravel‐bed rivers

ABSTRACTLarge‐scale flow structures (LSFS) in the streamwise direction are important features of gravel‐bed river flows, because they may contribute to sediment transport and gas exchange. In the present study, these structures are detected using Huang's empirical mode decomposition and reconstructed with phase‐averaging techniques based on a Hilbert transform of the velocity signal. The analysis is based on the fluctuating component of 15 quasi‐instantaneous velocity profiles measured with a three‐dimensional (3D) acoustic Doppler velocity profiler (ADVP) in an armoured gravel‐bed river with a low relative submergence of 2.9 (ratio between flow depth and bed grain diameter). LSFS were identified in most of the measured profiles and consistently showed similar features. We were able to characterize the geometry of these large‐scale coherent structures: the front has a vertical linear shift in the time domain and a vertical profile corresponding to a first quarter moon with the apex situated at z/h ≈ 0.4. In the vertical, the front scales with flow depth h, and in the streamwise direction, LSFS scale with three to seven times the mean flow depth. On the bed, the effect of LSFS is a periodic non‐linear variation of the friction velocity on average between 0.90 and 1.10 times the mean value. A model for the friction velocity cycle resulting from LSFS oscillation is presented. Copyright © 2014 John Wiley & Sons, Ltd.

Shear velocity estimates in rough‐bed open‐channel flow

ABSTRACTShear velocity u* is an important parameter in geophysical flows, in particular with respect to sediment transport dynamics. In this study, we investigate the feasibility of applying five standard methods [the logarithmic mean velocity profile, the Reynolds stress profile, the turbulent kinetic energy (TKE) profile, the wall similarity and spectral methods] that were initially developed to estimate shear velocity in smooth bed flow to turbulent flow over a loose bed of coarse gravel (D50 = 1·5 cm) under sub‐threshold conditions. The analysis is based on quasi‐instantaneous three‐dimensional (3D) full depth velocity profiles with high spatial and temporal resolution that were measured with an Acoustic Doppler Velocity Profiler (ADVP) in an open channel. The results of the analysis confirm the importance of detailed velocity profile measurements for the determination of shear velocity in rough‐bed flows. Results from all methods fall into a range of ± 20% variability and no systematic trend between methods was observed. Local and temporal variation in the loose bed roughness may contribute to the variability of the logarithmic profile method results. Estimates obtained from the TKE and Reynolds stress methods reasonably agree. Most results from the wall similarity method are within 10% of those obtained by the TKE and Reynolds stress methods. The spectral method was difficult to use since the spectral energy of the vertical velocity component strongly increased with distance from the bed in the inner layer. This made the choice of the reference level problematic. Mean shear stress for all experiments follows a quadratic relationship with the mean velocity in the flow. The wall similarity method appears to be a promising tool for estimating shear velocity under rough‐bed flow conditions and in field studies where other methods may be difficult to apply. This method allows for the determination of u* from a single point measurement at one level in the intermediate range (0·3 < h < 0·6). Copyright © 2013 John Wiley & Sons, Ltd.

Fine sediment dynamics in unsteady open-channel flow studied with acoustic and optical systems

In order to simulate fine sediment dynamics over an armored bed in a tidal river, unsteady accelerating, then steady open-channel flow over a movable (but not moving) coarse gravel bed (D50=5.5mm) was studied. A layer of fine sediment (D50=120μm) was placed on the coarse gravel bed. The thickness of the fine sediment layer on the gravel bed was varied between 4 and 6mm, but it was found that the thickness of the layer had no effect on the results. Quasi-instantaneous profiles of velocity and sediment concentration were taken simultaneously and co-located. An Acoustic Doppler Velocity Profiler (ADVP) was combined with Particle Tracking Velocimetry (PTV) for suspended sediment particle tracking. Measurements resolved turbulence scales. During the final phase of the accelerating flow range, fine sediment suspension from the bed started in packets and rapidly created a ripple pattern that remained nearly stationary. Thereafter, vortex shedding produced most of the sediment suspension into the water column in the form of events or packets, making suspension intermittent. Simultaneously, sediment particles rolled along the bed following the ripple structure, thus slowly advancing the ripple pattern in the direction of the flow without altering ripple geometry. Fine sediment particles and hydrogen bubbles were used individually or combined as flow tracers in the acoustic measurements. When used individually, hydrogen bubbles provided full depth flow and backscattering information, whereas sediment particles traced only the lower layers of the flow, indicating sediment suspension. When both tracers were combined, hydrogen bubbles could only be distinguished from sediment particles when results at two different acoustic carrier frequencies were compared. The intermittency was observed in the backscattering of the acoustic system. The event structure in fine sediment suspension is seen by the PTV method. PTV velocity vectors varied in speed and orientation, were organized in large coherent packets, mainly in the near bed layers, but also extended well above the bed. The two methods provide complementary information. ADVP measurements allow long time series analysis, whereas most of the spatial details seen in the PTV results cannot be resolved in the ADVP measurements.

Secondary Currents and Corresponding Surface Velocity Patterns in a Turbulent Open-Channel Flow over a Rough Bed

River and open-channel flows present complex three-dimensional (3D) structures in the water column and on the surface as a result of the existence of secondary currents driven by either centrifugal force or turbulence. This paper experimentally investigates secondary currents in a straight, turbulent, rough-bed open-channel flow; the effect of different channel width to flow depth ratios (12.25, 15, and 20) on secondary currents; and surface boils associated with secondary currents at moderate Reynolds numbers. Nearly instantaneous profiles of three components of flow velocity and turbulence characteristics in the water column were measured by using an acoustic Doppler velocity profiler (ADVP). Simultaneously, large-scale particle image velocimetry was used to measure the water surface velocities and turbulence structures. Mean velocity patterns in the water undulate across the channel, indicating the presence of secondary currents in the long-term average flow structure. Secondary currents affect the distribution of bed shear stress, Reynolds stress, and turbulence intensities across the channel. The aspect ratio determines the number of secondary cells in the water column. A mean multiband undulating surface velocity pattern in the transverse direction correlates with the secondary cell distribution in the water column below. The instantaneous position of the upwelling and downwelling regions on the surface may deviate from their long-term mean position, indicating a meandering of the surface pattern. Vortex structures are detected from instantaneous surface velocity maps, and vortex boil lines are identified. Boil vortices mainly occur in upwelling areas with high vorticity. The simultaneous detailed velocity measurements in the water column and on the free surface have shown a good agreement between the secondary cell patterns obtained by the two methods.

Improving the accuracy of four‐receiver acoustic Doppler velocimeter (ADV) measurements in turbulent boundary layer flows

Acoustic Doppler instrument measurements suffer from random spikes and Doppler noise. Using a four‐receiver ADV (acoustic Doppler velocimeter; Vectrino manufactured by Nortek) that allows recording beam velocities, we combine a spike‐removal procedure on the beam velocities with a noise‐reduction method on the flow velocities to improve turbulence measurements. We compare the results with those obtained from ADVP (acoustic Doppler velocity profiler) measurements under the same conditions, i.e., in turbulent open‐channel flow over a coarse‐grained bed. It is shown that spikes are best removed from ADV beam velocity data before calculating flow velocities, thereby correcting all three flow velocity components at the source. Spikes in beam velocities do not correlate with low correlation values. ADVP data generally have few spikes and do not need spike removal treatment, showing that spikes are instrument related. The noise reduction method is based on the decorrelation of the Doppler noise terms contained in two vertical velocities redundantly sampled in the same volume. The combined ADV data treatment is sufficient to significantly extend the resolved frequency range in the velocity spectra. It reduces RMS values by up to a factor of 2, and the corrected values agree with ADVP results and theoretical predictions, indicating that both treatments are needed. Owing to spatial averaging effects over the ADV sample volume, a sampling frequency limit of close to 50 Hz is determined by the deviation of the spectra from the −5/3 slope.

Experimental analysis of cross‐sectional flow motion in a large amplitude meandering bend

AbstractFlow in meandering bends is characterized by the formation of a large cross‐sectional central‐region circulation cell. The width‐to‐depth ratio is one of the most important parameters affecting the entity of the cross‐circulation motion. In steep outside bends, beside the central‐region cell, a counter‐rotating circulation cell often forms in the upper part of the outer‐bank. In spite of its practical importance, the evolving mechanisms of both the circulation cells and their role on boundary shear stress distribution in bends are not yet fully understood. The aim of the present paper is to gain some insight into how cross‐sectional flow motion evolves along meandering bends. Experiments have been carried out in a laboratory meandering channel of large amplitude, over a deformed‐rigid bed, for two values of the width‐to‐depth ratio. The three‐dimensional flow velocity field has been measured in detail at five cross‐sections, almost equally spaced along the channel reach between two consecutive apex sections. The measurements have been carried out on a fine grid by an acoustic Doppler velocity profiler. The distributions of the cross‐sectional flow (e.g. cross‐sectional flow velocity, net transversal flux) and turbulent kinetic energy are analyzed in each investigated section. Measurements show that the counter‐rotating circulation cell is evident only in the case of ‘small’ width‐to‐depth ratio. Such circulation cell begins at the bend entrance and it is fully developed at the bend apex; then it decays. At the bend apex, the core of maximum velocity is found near the bed at about the separation between the central and the outer‐bank circulation cells. Moreover, the presence of the counter‐rotating circulation cell allows the bank shear stress to maintain low values in the outer‐side of the bend. Copyright © 2010 John Wiley & Sons, Ltd.

Flow Fields in front of a Cylindrical Obstacle

An experimental investigation, conducted in two different flows Reynolds number, was carried out to study the structure of the flow field upstream of a cylindrical obstacle. An Acoustic Doppler Velocity Profiler (ADVP) was used to obtain instantaneously the three directions of the mean velocity. Results of the experiments show the longitudinal velocities, , decrease approaching the cylinder, their distribution becomes more uniform and close to the bed a reverse flow is noticeable with increasing importance. The downward velocity component is clearly shown, continuing with the return flow near the bed, forming a vortex. At positions where the vortex appears upstream from the cylinder, a large increase of the three components of the turbulence intensities is remarked. Approaching the cylinder, one observes the shear stress decreases, having the opposite direction at positions close to the cylinder. A zero value of shear stress should be at the separation point.

On the structure of shear stress and turbulent kinetic energy flux across the roughness layer of a gravel-bed channel flow

This study examines the structure of shear stress and turbulent kinetic energy (TKE) flux across the roughness layer of a uniform, fully rough gravel-bed channel flow (ks+ ≫ 100, δ/k = 20) using high-resolution acoustic Doppler velocity profiler measurements. The studied gravel-bed roughness layer exhibits a complex random multi-scale roughness structure in strong contrast with conceptualized k- or d-type roughness in standard rough-wall flows. Within the roughness layer, strong spatial variability of all time-averaged flow quantities are observed affecting up to 40% of the boundary layer height. This variability is attributed to the presence of bed zones with emanating bed protuberances (or gravel clusters) acting as local flow obstacles and bed zones of more homogenous roughness of densely packed gravel elements. Considering the strong spatial mean flow variability across the roughness layer, a spatio-temporal averaging procedure, called double averaging (DA), has been applied to the analysed flow quantities. Three aspects have been addressed: (a) the DA shear stress and DA TKE flux in specific bed zones associated with three classes of velocity profiles as previously proposed in Mignot, Barthélemy & Hurther (J. Fluid Mech., vol. 618, 2009, p. 279), (b) the global and per class DA conditional statistics of shear stress and associated TKE flux and (c) the contribution of large-scale coherent shear stress structures (LC3S) to the TKE flux across the roughness layer. The mean Reynolds and dispersive shear structure show good agreement between the protuberance bed zones associated with the S-shape/accelerated classes and recent results obtained in standard k-type rough-wall flows (Djenidi et al., Exp. Fluids, vol. 44, 2008, p. 37; Pokrajac, McEwan & Nikora, Exp. Fluids, vol. 45, 2008, p. 73). These gravel-bed protuberances act as local flow obstacles inducing a strong turbulent activity in their wake regions. The conditional statistics show that the Reynolds stress contribution is fairly well distributed between sweep and ejection events, with threshold values ranging from H = 0 to H = 8. However, the TKE flux across the roughness layer primarily results from the residual shear stress between ejection and sweep of very high magnitude (H = 10–20) and of small turbulent scale. Although LC3S are seen to penetrated the interfacial roughness layer, their TKE flux contribution is found to be negligible compared to the very energetic small-scale sweep events. These sweeps are dominantly produced in the bed zones of local gravel protuberances where the velocity profiles are inflexional of S-shape type and the mean flow properties are of mixing-layer flow type as previously shown in Mignot et al. (2009).