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SEDIMENT DYNAMICS UNDER REAL WAVES
Abstract
In order to improve sand transport models, the coastal engineering community needs data collected in natural conditions. In particular, sand transport processes in the wave boundary layer still need to be investigated, but the high temporal and spatial variability observed in this area make it complicated to monitor it properly. In order to address this issue, the first worldwide in situ deployment of UB-Lab 3C ©, based on Acoustic Concentration and Velocity Profiler technology, took place at Porsmilin beach, in 2022. First data collected with this instrument are processed, and averaged in order to obtain intra-wave velocity, concentration and flux for two categories of waves in the shoaling zone: low and high orbital velocity waves. Sediment concentrations under higher orbital velocity waves are stronger than under lower orbital velocity waves. This preliminary data set is promising, and further analysis should allow to extend knowledge on sand concentration and transport under real in situ waves in shallow environment.
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A large‐scale laboratory experiment was conducted to evaluate cross‐shore sediment transport and bed response on a sandbar under erosive and accretive field‐scale wave conditions (total of 11 cases). Unprecedented vertical resolution of sediment concentration was achieved through the use of conductivity concentration profilers alongside miniature fiber optic backscatter profilers. Observations were made of intrawave (phase‐averaged) and wave‐averaged cross‐shore sediment flux profiles and transport rates in the lower half of the water column on the crest of a sandbar. The net sediment transport rate was partitioned into suspended sediment (SS) and bed load (BL) components to quantify the relative contributions of SS and BL to the total sediment transport rate. Net SS transport rates were greater than net BL transport rates for the positive (wave crest) half‐cycle in 6 of 11 cases, compared to 100% (11 of 11) for the negative (wave trough) half‐cycle. Net (wave‐averaged) BL transport rates were greater, in magnitude, than net SS transport rates for 7 of the 11 cases. The dominant mode of transport was determined from the ratio of net BL to net SS transport rate magnitudes. The net transport rate was negative (offshore‐directed) when SS dominated and positive (onshore‐directed) when BL dominated. Net BL transport rate correlated well with third moments of free‐stream velocity (r² = 0.72), suggesting that energetics‐type quasi‐steady formulae may be suitable for predicting BL transport under the range of test conditions.
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.
A large series of field-scale experiments on turbulent sand-laden flows, conducted in preceding years in the LOWT and AOFT large oscillating water tunnels are reviewed and reanalysed. Using the combined experimental data sets, new insights are obtained on the detailed sand transport processes occurring in sheet-flow and ripple regime conditions. For sheet flow (i) new equations are presented relating maximum erosion depth and sheet-flow layer thickness to the maximum Shields parameter; (ii) detailed analysis of sediment flux data shows the dominance of the current-related flux in the sheet-flow layer and the different characters of the current-related flux for fine and medium sands; (iii) a RANS-diffusion type model is shown to reproduce important trends in net transport rate related to grain size, velocity and wave period and to predict the magnitude of net transport rate to within a factor 2 of measured values. For the ripple regime it is shown that (i) asymmetric waves generate negative ('offshore') streaming and the current-related suspended sediment flux associated with this streaming appears to be of the same order of magnitude as the wave-related suspended sediment flux; (ii) time-averaged near-bed transport and time-averaged suspended transport appear to be of about equal magnitude but of opposite sign, and are concentrated on the 'onshore' flank of the ripple for asymmetric wave conditions; (iii) near-bed transport along the onshore flank is generated by sand transported over the ripple crest during the 'onshore' half-cycle. Net sand transport under asymmetric waves can be 'onshore' directed or 'offshore' directed, depending on the degree of unsteadiness in the sand flux behaviour during the wave cycle. Dimensionless phase-lag parameters are presented, for sheet flow and ripples, which can discriminate between predominantly quasi-steady behaviour (resulting in 'onshore' transport) and predominantly unsteady behaviour (resulting in 'offshore transport').
1] The present study focuses on the fine‐scale flow and sand transport processes above onshore migrating ripples below skewed surface gravity waves in the shoaling zone. A set of acoustic instruments was deployed in the shoaling region of the large‐scale wave channel at Canal d'Investigacío i Experimatacío Marítima, Universitat Poltiècnica de Catalunya, Barcelona, Spain, in order to provide high‐resolution velocity and sediment concentration profiles with an acoustic concentration and velocity profiler (ACVP). Measurements are analyzed relative to the positions of the measured nonmoving sand bed and the interface separating the suspension from the near‐bed load layer. This interface is detected here by the application of a novel acoustic bed echo detection method. Furthermore, the use of the dual‐frequency inversion proposed in the work of Hurther et al. (2011) allows for the calculation of the sediment concentration profile across both the suspension and near‐bed load layers. The sand bed was covered by quasi‐two‐dimensional suborbital ripples migrating onshore. As proposed by O'Donoghue et al. (2006), the occurrence of quasi‐two‐dimensional ripples is attributed to the fine‐size sand of D 50 = 250 mm used in the present study under full‐scale forcing conditions. In order to determine the effect of shoaled wave skewness on the ripple vortex entrainment and sediment transport, the instantaneous and mean measurements of the flow, sediment concentration, and sediment flux along the ripple profile are discussed in terms of (1) the occurrence of ripple vortex entrainment on either side of the ripple crest; (2) the wave velocity phase lagging driven by the ripple vortex entrainment process and the turbulent bed friction effects in the wave boundary layer; (3) phase lagging between velocity and maximum concentration and sediment flux events; (4) the structure of bed friction and ripple‐driven turbulence across the suspension and the near‐bed load layers; and (5) the streaming components. The results on these aspects strongly support that the wave velocity skewness effect under shoaling waves is fairly similar to the one obtained in skewed oscillatory water tunnel flows. Furthermore, it is found that the onshore‐oriented net bed load sediment transport is at the origin of the onshore ripple migration. This flux is roughly twice as much as the opposite offshore‐oriented net suspension flux dominated by the ripple vortex entrainment processes. Citation: Hurther, D., and P. D. Thorne (2011), Suspension and near‐bed load sediment transport processes above a migrating, sand‐rippled bed under shoaling waves, J. Geophys. Res., 116, C07001, doi:10.1029/2010JC006774.
Over the past two decades the application of acoustics to the measurement of small-scale sediment processes has been gaining increasing acceptance within the sedimentological community. This has arisen because acoustics has the potential to measure non-intrusively, with high temporal and spatial resolution, profiles of suspended sediment size and concentration, profiles of flow, and the bedform morphology. In the present article we review the capability of acoustics to deliver on its potentiality to make a valuable and unique contribution to the measurement of small-scale sediment processes. The article introduces the reasons for using acoustics, the physics underlying the approach, a series of examples illustrating collected data, a discussion on some of the difficulties encountered when applying acoustics and finally a look to the future and possible new developments.
In wave-generated sheet flow conditions, oscillatory near-bed flow is large, ripples are washed out and sand is mainly transported in a thin layer, a few centimetres thick, close to and below the no-flow bed level. New experiments in the Aberdeen Oscillatory Flow Tunnel have produced a comprehensive dataset of transport processes for well-sorted and graded sands in large-scale sinusoidal and asymmetric oscillatory sheet flow conditions. In this paper, detailed time- and height-varying concentration measurements are analysed. The range and level of detail in the measurements make it possible to quantitatively analyse concentration behaviour more rigorously than before. A new empirical equation is presented which characterises the time-dependent concentration profile in the sheet flow layer. The equation involves two time-dependent parameters: erosion depth and reference concentration. The dependence of both parameters on flow and bed conditions is analysed. New results are presented which make it possible to estimate time-dependent erosion depth, reference concentration and, therefore, concentration profile. The effects of grading on sheet flow concentrations are analysed by comparing results from the large range of sand beds tested.
Field measurements of near-bed sediment concentrations and sediment diffusivity in the breaker zone and in the inner surf zone of natural beaches were obtained using stacks of optical and fiber-optical backscatter sensors under conditions with strongly plunging breakers as well as with spilling surf bores. The field data suggest that significant differences exist between these wave types in terms of near-bed sediment concentration levels and vertical mixing of sediment from the bed upward into the water column. When plotting mean sediment concentration profiles in log C-z space, profiles are upward concave for spilling surf bores, and sediment diffusivity increases approximately linearly upward from the bed. For plunging waves, however, mean sediment concentration profiles are linear in a log C-z plot indicating vertically constant sediment diffusivity. This suggests that under surf bores, sediment suspension from the bed occurs through diffusion processes while vertical mixing is mainly a convective process under plunging waves. Under surf bores, sediment is largely confined to the lower layers of the water column while it is lifted to significantly higher elevations under plunging breakers.
Ces travaux constituent la première étude fondamentale traitant de l’influence des forçages hydrodynamiques sur l’évolution morpho-sédimentaire des plages sableuses du littoral de la mer d’Iroise. Ils s’insèrent dans la démarche de l’Observatoire du Domaine Côtier (Observatoire des Sciences de l’Univers) qui supporte l’observation de la variabilité biologique, chimique et physique de ce littoral à long terme. Dans cette perspective, quatre sites ont été choisis pour illustrer la pluralité des morphologies et des conditions hydrodynamiques dans ce contexte macrotidal soumis à l’agitation des houles océaniques. La diversité des échelles spatiales et temporelles impliquées dans le fonctionnement hydro-sédimentaire des plages a nécessité la mise en oeuvre de méthodes d’observation in situ adaptées :
1- l’analyse diachronique de photographies aériennes anciennes à l’échelle pluridécennale ;
2- un suivi topométrique des plages à l’échelle pluri-annuelle ;
3- des campagnes ponctuelles de mesures hydrodynamiques et morphologiques à haute fréquence.
L’analyse des séries de profils de plage rend compte de la variabilité morpho-sédimentaire des estrans et de la dynamique des corps et figures sableuses à l’échelle événementielle, saisonnière et pluri-annuelle. On distingue deux groupes de plages, celles dominées par une variabilité transversale expérimentant une translation de leur profil à l’échelle annuelle (en progradation à Porsmilin, en recul aux Blancs Sablons) et celles dominées par une variabilité longitudinale, dont le bilan sédimentaire est relativement stable (Corsen, Tregana). Les principaux mécanismes contrôlant l’évolution à moyen terme des plages sont : l’exposition aux agents de forçage hydrodynamique, la capacité d’adaptation du système à la récurrence des tempêtes, celle-ci étant intimement liée à la disponibilité en matériel sédimentaire sur l’avant-côte.
Précisément, l’observation de la dynamique de la plage subtidale à Porsmilin par un suivi bathymétrique saisonnier a permis d’établir la profondeur de fermeture du profil (-3 m C.M.) et de souligner le caractère non conservatif du système cross-shore avec des flux longitudinaux entrants et sortants (respectivement en période estivale et hivernale). Ces résultats soulèvent la question de l’influence du transport longitudinal régional sur la disponibilité sédimentaire de certaines plages et sur leur évolution à l’échelle pluri-annuelle.
A une échelle de temps plus courte (du cycle de marée à la semaine), les campagnes de mesures intensives ont permis de caractériser l’agitation en zone intertidale et d’analyser l’influence de la marée sur la variabilité spatio-temporelle des processus hydrodynamiques. Deux signatures hydrodynamiques sont observées dans les petits fonds :
1- dans la zone subtidale, à l’extérieur de la zone de surf, les courants de marée, bien que peu intenses, dominent l’hydrodynamisme. Les courants induits par les vagues sont inexistants sauf pendant les tempêtes, durant lesquelles l’asymétrie des vagues génère une composante de courant dirigée vers la plage qui se superpose aux courants de marée.
2- dans la zone intertidale, les courants induits par les vagues sont les principaux agents du transport sédimentaire : le courant de retour de fond, d’une intensité maximale en zone de surf externe, transportant les sédiments vers le large ; l’obliquité des vagues au rivage produisant une dérive longitudinale.
Lors de conditions énergétiques, les ondes infragravitaires peuvent représenter jusqu’à 60 % de l’énergie totale des vagues dans la zone de déferlement et participer à la formation de barres sableuses sur les niveaux d’étale de la marée. Par ailleurs, une onde longue sub-harmonique et stationnaire favorise le développement des croissants de plage en plus des processus d’autoorganisation.
Enfin, des ondes courtes sont également générées et libérées au déferlement sur la plage de Corsen dans un contexte morphologique tout à fait inhabituel, en l’absence de barres sableuses. Ces processus de transfert d’énergie des vagues sont donc directement impliqués dans la formation des barres sableuses et des systèmes de croissants de plage, et plus largement dans la dynamique sédimentaire en zone intertidale.
The paper focuses on the numerical simulation of erosion of plane sloping beaches by irregular wave attack in three wave flumes of different scales. One of the prime objectives of the tests was to provide a consistent data set for the improvement of numerical beach profile models. A practical application of this research with wave attack on plane sloping beaches is the erosion of the plane beaches after nourishment. Three models (CROSMOR, UNIBEST-TC and DELFT3D) have been used to simulate the flume experimental results focusing on the wave height distribution and the morphological development (erosion and deposition) along the beach profiles. Overall, the model predictions for wave heights show consistent results. Generally, the computed wave heights (Hrms and H1/3) are within 10% to 15% of the measured values for all tests (under-prediction of the largest wave heights close to the shore). The three models can simulate the beach erosion of the wave flume tests (erosive tests) reasonably well using default values of the sand transport parameters. The model performance for the accretive tests is less good than that for the erosive tests. A practical field application of this research is the erosion of nourished beaches, as these beaches generally have rather plane beach slopes immediately after nourishment. Various graphs are given to estimate the beach erosion of nourished beaches.
A simple method is provided for calculating transport rates of not too fine (d50≥0.20 mm) sand under sheet flow conditions. The method consists of a Meyer-Peter-type transport formula operating on a time-varying Shields parameter, which accounts for both acceleration-asymmetry and boundary layer streaming. While velocity moment formulae, e.g.., , calibrated against U-tube measurements, fail spectacularly under some real waves (Ribberink, J.S., Dohmen-Janssen, C.M., Hanes, D.M., McLean, S.R., Vincent, C., 2000. Near-bed sand transport mechanisms under waves. Proc. 27th Int. Conf. Coastal Engineering, Sydney, ASCE, New York, pp. 3263–3276, Fig. 12), the new method predicts the real wave observations equally well. The reason that the velocity moment formulae fail under these waves is partly the presence of boundary layer streaming and partly the saw-tooth asymmetry, i.e., the front of the waves being steeper than the back. Waves with saw-tooth asymmetry may generate a net landward sediment transport even if , because of the more abrupt acceleration under the steep front. More abrupt accelerations are associated with thinner boundary layers and greater pressure gradients for a given velocity magnitude. The two “real wave effects” are incorporated in a model of the form Qs(t)=Qs[θ(t)] rather than Qs(t)=Qs[u∞(t)], i.e., by expressing the transport rate in terms of an instantaneous Shields parameter rather than in terms of the free stream velocity, and accounting for both streaming and accelerations in the θ(t) calculations. The instantaneous friction velocities u*(t) and subsequently θ(t) are calculated as follows. Firstly, a linear filter incorporating the grain roughness friction factor f2.5 and a phase angle ϕτ is applied to u∞(t). This delivers u*(t) which is used to calculate an instantaneous grain roughness Shields parameter θ2.5(t). Secondly, a constant bed shear stress is added which corresponds to the “streaming related bed shear stress” . The method can be applied to any u∞(t) time series, but further experimental validation is recommended before application to conditions that differ strongly from the ones considered below. The method is not recommended for rippled beds or for sheet flow with typical prototype wave periods and d50<0.20 mm. In such scenarios, time lags related to vertical sediment movement become important, and these are not considered by the present model.