An examination of point-particle Lagrangian simulations for assessing time-resolved hydroacoustic particle flux measurements in sediment-laden flows

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 sedi�ment 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 condi�tions 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

Wave boundary layer hydrodynamics and sheet flow properties under large-scale plunging-type breaking waves

Wave boundary layer (WBL) dynamics are measured with an Acoustic Concentration and Velocity Profiler (ACVP) across the sheet flow-dominated wave-breaking region of regular large-scale waves breaking as a plunger over a developing breaker bar. Acoustic sheet flow measurements are first evaluated quantitatively in comparison to Conductivity Concentration Meter (CCM+) data used as a reference. The near-bed orbital velocity field exhibits expected behaviors in terms of wave shape, intrawave WBL thickness, and velocity phase leads. The observed fully turbulent flow regime all across the studied wave-breaking region supports the model-predicted transformation of free-stream velocity asymmetry into near-bed velocity skewness inside the WBL. Intrawave concentration dynamics reveal the existence of a lower pickup layer and an upper sheet flow layer similar to skewed oscillatory sheet flows, and with similar characteristics in terms of erosion depth and sheet flow layer thickness. Compared to the shoaling region, differences in terms of sheet flow and hydrodynamic properties of the flow are observed at the plunge point, attributed to the locally enhanced wave breaker turbulence. The ACVP-measured total sheet flow transport rate is decomposed into its current-, wave-, and turbulence-driven components. In the shoaling region, the sand transport is found to be fully dominated by the onshore skewed wave-driven component with negligible phase lag effects. In the outer surf zone, the total net flux exhibits a three-layer vertical structure typical of skewed oscillatory sheet flows. However, in the present experiments this structure originates from offshore-directed undertow-driven flux, rather than from phase lag effects.Peer ReviewedPostprint (published version

Hydrodynamics under Large-Scale Waves Breaking over a Barred Beach

This paper shows preliminary results of experiments obtained in a large-scale wave flume under monochromatic waves plunging over a fixed bar. Velocity measurements were conducted using acoustic and optical instruments at 22 cross-shore locations ranging from the final part of the shoaling zone up to the inner surf zone. The measurements included the bottom boundary layer and the lower part of the water column and provided insights on the mean velocity distribution, turbulent velocity fluctuations and Reynolds stresses. The mean velocity is generally seaward directed. Magnitudes of the mean velocity are small in the shoaling region and increase above the bar crest, especially in the higher part of the water column, while magnitudes in the boundary layer are relatively small. Fluid from the inner surf zone is transported offshore by the undertow and pushed up near the shoreward face of the bar, thus largely feeding the onshore mass transport above trough level. As a result a large recirculation cell located just above the trough of the bar is generated where currents and turbulent velocity fluctuations are strong

Measurement Suspended Particulate Matter (SPM) in the water column by combining acoustic and optical methods

La mesure de Matières En Suspension (MES) est cruciale autant pour comprendre les transferts sédimentaires que pour les études des écosystèmes marins. Elles sont classiquement mesurées ponctuellement par des prélèvements d’eau in situ, ou à partir des propriétés optiques de l’eau. Mais depuis plusieurs décennies, les appareils acoustiques, ont montré leur capacité à mesurer ces MES sur de plus grands volumes. Ces mesures, en particulier celles de la concentration, s’appuient sur les propriétés de rétrodiffusion des particules. Mais ces mesures demeurent peu représentatives dans la mesure où le contenu en MES dans la colonne d’eau varie à différentes échelles de temps et d’espace. Dans un premier temps, ces travaux de thèse visent à montrer à travers des mesures in situ réalisées dans l’estuaire de l’Aulne qu’il est possible d’étendre spatialement la mesure par inversion des données issues de sondeurs multifaisceaux (SMF). Un modèle de rétrodiffusion adapté à la suspension d’intérêt, constituée dans cette étude d’agrégats estuariens, est d’abord élaboré. Puis grâce à des observations issues d’un profileur multifréquences, la concentration massique en MES, distribuée par classes de tailles, a pu être déterminée par la résolution d’un problème inverse. Ces informations permettent de déterminer les rayons équivalents de la suspension, grâce auxquels les données issues du SMF, au préalable calibrées par une méthode innovante, peuvent être inversées de manière directe. Une étude des incertitudes attachées aux valeurs de concentrations estimées est par la suite proposée afin de qualifier la pertinence des résultats. Puis dans un second temps, les conditions nécessaires à l’établissement d’un protocole de mesure des MES par système multi-capteur sont identifiées. Ce dernier permet la caractérisation en continu des MES à différentes échelles de temps et d’espace, en exploitant la complémentarité des mesures issues des différents instruments.Measuring Suspended Particulate Matter (SPM) is essential to better understand sediment transport and marine ecosystems. SPM is traditionnaly estimated through in situ water samples analysis, or based on the optical properties of water. Yet for several decades, acoustical devices have shown their capability to measure SPM on larger volumes. These measurements (especially in terms of SPM concentration) are based on the backscattering properties of the particles. However, these measurements remain limited, since the SPM content in the water column is subjected to variations on both spatial and temporal scales.As a first step, this work aims at showing that it is possible to increase the degree of spatialization of the SPM measurements by inverting MultiBeam EchoSounder (MBES) data, through in situ measurements acquired in the Aulne macrotidal estuary. A backscattering model was first designed to describe the backscattering properties of the suspension of interest, consisting in this study in estuarine agregates. Then, thanks to multifrequency observations, the SPM mass concentration sorted by size classes was retrieved through the the resolution of the inverse problem. This kind of information allows to determine the equivalent spherical radius of the the whole suspension, through which the MBES data, calibrated beforehand using an original method, can be directly inverted. Subsequently, a study of the uncertainties attached to the final concentration estimate is proposed in order to qualify the relevance of the results.In a second step, the necessary conditions for establishing a measurement protocol of the SPM are identified. The latter allows continuous characterization of SPM at different spatial and temporal scales, by exploiting the complementarity of the measures delivered by different instruments

Mesure de Matières En Suspension (MES) dans la colonne d'eau par combinaison de méthodes acoustiques et optiques

Measuring Suspended Particulate Matter (SPM) is essential to better understand sediment transport and marine ecosystems. SPM is traditionnaly estimated through in situ water samples analysis, or based on the optical properties of water. Yet for several decades, acoustical devices have shown their capability to measure SPM on larger volumes. These measurements (especially in terms of SPM concentration) are based on the backscattering properties of the particles. However, these measurements remain limited, since the SPM content in the water column is subjected to variations on both spatial and temporal scales.As a first step, this work aims at showing that it is possible to increase the degree of spatialization of the SPM measurements by inverting MultiBeam EchoSounder (MBES) data, through in situ measurements acquired in the Aulne macrotidal estuary. A backscattering model was first designed to describe the backscattering properties of the suspension of interest, consisting in this study in estuarine agregates. Then, thanks to multifrequency observations, the SPM mass concentration sorted by size classes was retrieved through the the resolution of the inverse problem. This kind of information allows to determine the equivalent spherical radius of the the whole suspension, through which the MBES data, calibrated beforehand using an original method, can be directly inverted. Subsequently, a study of the uncertainties attached to the final concentration estimate is proposed in order to qualify the relevance of the results.In a second step, the necessary conditions for establishing a measurement protocol of the SPM are identified. The latter allows continuous characterization of SPM at different spatial and temporal scales, by exploiting the complementarity of the measures delivered by different instruments.La mesure de Matières En Suspension (MES) est cruciale autant pour comprendre les transferts sédimentaires que pour les études des écosystèmes marins. Elles sont classiquement mesurées ponctuellement par des prélèvements d’eau in situ, ou à partir des propriétés optiques de l’eau. Mais depuis plusieurs décennies, les appareils acoustiques, ont montré leur capacité à mesurer ces MES sur de plus grands volumes. Ces mesures, en particulier celles de la concentration, s’appuient sur les propriétés de rétrodiffusion des particules. Mais ces mesures demeurent peu représentatives dans la mesure où le contenu en MES dans la colonne d’eau varie à différentes échelles de temps et d’espace. Dans un premier temps, ces travaux de thèse visent à montrer à travers des mesures in situ réalisées dans l’estuaire de l’Aulne qu’il est possible d’étendre spatialement la mesure par inversion des données issues de sondeurs multifaisceaux (SMF). Un modèle de rétrodiffusion adapté à la suspension d’intérêt, constituée dans cette étude d’agrégats estuariens, est d’abord élaboré. Puis grâce à des observations issues d’un profileur multifréquences, la concentration massique en MES, distribuée par classes de tailles, a pu être déterminée par la résolution d’un problème inverse. Ces informations permettent de déterminer les rayons équivalents de la suspension, grâce auxquels les données issues du SMF, au préalable calibrées par une méthode innovante, peuvent être inversées de manière directe. Une étude des incertitudes attachées aux valeurs de concentrations estimées est par la suite proposée afin de qualifier la pertinence des résultats. Puis dans un second temps, les conditions nécessaires à l’établissement d’un protocole de mesure des MES par système multi-capteur sont identifiées. Ce dernier permet la caractérisation en continu des MES à différentes échelles de temps et d’espace, en exploitant la complémentarité des mesures issues des différents instruments

HYDRAC, a novel hydroacoustic inversion software for Suspended Particulate Matter measurements

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Circular estimation of the flow velocity using coherent Doppler sonar

International audienceThe problem of coherent Doppler frequency estimation is addressed here applied to the domain of active hydroacoustics in a context of high Signal to Noise ratios. Pulse-to-pulse coherent Doppler sonars allow non-intrusive high-resolution velocity measurements that are often delicate to analyse, due to the nature of the return signal. We present here a novel pulse-to-pulse coherent Doppler estimator using signal processing techniques defined in the circular domain aiming to better reconstruct the noisy phase of the Doppler signal at high temporal resolutions. This Linear Circular estimator was tested on hydroacoustics synthetic measurements and its robustness to noise was demonstrated compared to the standard pulse-pair algorithm. The study suggests its high potential for time-resolved quasi-instantaneous velocity estimation, and turbulence statistics evaluation. The Linear Circular estimator's robustness to noise also suggests potential interests in applying such methods for incoherent scattering evaluation

In situ measurement of hydrosedimentary dynamic in the surf zone : a versatile dedicated mooring

International audienceSandy beaches represent 39% of French coasts. They are submitted to erosion, notably because of the energetic wave conditions observed during extreme events (storms). On the contrary, when moderate wave conditions are observed, the sand tends to be brought back on the beach. Processes controlling erosion are quite well known thanks to numerous studies on the topic. As a consequence, erosion events are quite well predicted by models (VAN RIJN et al., 2011). It is not the case when it comes to accretion; indeed, sand quantities transported onshore tend to be underestimated in predictive models (DALY et al., 2017). In order to improve models, parameters controlling accretion need to be clarified. Onshore transport occurs mainly through bedload in the bottom boundary layer. One challenge still to overcome in coastal dynamic studies is to obtain in situ measurements allowing to observe the whole water column with enough resolution. The new mooring built in the frame of ANR WEST is versatile, easy to deploy in all kinds of areas, solid and allows to insonify the whole water column so as to obtain co-localized current, concentration and granulometry measurements. Thanks to the instrument positions, no acoustic interference is observed. Hydrodynamic parameters can be computed and agree with previous studies. The newly presented mooring is therefore well adapted to surf zone data collection

Sedimentary Dynamics and Decadal-Scale Changes in the Macrotidal Aulne River Estuary, Brittany, France

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