Acoustic measurements of boundary layer flux profiles over a sandy rippled bed under regular waves

The study of boundary layer sediment transport processes requires contemporaneous measurements of the bedforms, the flow and the sediment movement. Obtaining these three parameters, at the required temporal-spatial resolutions, has been traditionally difficult, especially within a few centimetres of the bed. To circumvent some of the deployment of an acoustic backscatter system, ABS, an acoustic ripple profiler, ARP, and an acoustic Doppler velocity profiler, ADVP, to measure sediment entrainment processes above a rippled bed under regular waves. High resolution acoustic observations of the suspend sediment concentration, flow and bedforms have been collected. Here we report on some of the initial results obtained from this study

Turbulent transport in the outer region of rough-wall open-channel flows: the contribution of large coherent shear stress structures (LC3S)

Acoustic Doppler velocity profiler (ADVP) measurements of instantaneous three-dimensional velocity profiles over the entire turbulent boundary layer height, δ, of rough-bed open-channel flows at moderate Reynolds numbers show the presence of large scale coherent shear stress structures (called LC3S herein) in the zones of uniformly retarded streamwise momentum. LC3S events over streamwise distances of several boundary layer thicknesses dominate the mean shear dynamics. Polymodal histograms of short streamwise velocity samples confirm the subdivision of uniform streamwise momentum into three zones also observed by Adrian et al. (J. Fluid Mech., vol. 422, 2000, p. 1). The mean streamwise dimension of the zones varies between 1δ and 2.5δ. In the intermediate region (0.2<z/δ<0.75), the contribution of conditionally sampled u'w' events to the mean vertical turbulent kinetic energy (TKE) flux as a function of threshold level H is found to be generated by LC3S events above a critical threshold level Hmax for which the ascendant net momentum flux between LC3S of ejection and sweep types is maximal. The vertical profile of Hmax is nearly constant over the intermediate region, with a value of 5 independent of the flow conditions. Very good agreement is found for all flow conditions including the free-stream shear flows studied in Adrian et al. (2000). If normalized by the squared bed friction velocity, the ascendant net momentum flux containing 90% of the mean TKE flux is equal to 20% of the shear stress due to bed friction. In the intermediate region this value is nearly constant for all flow conditions investigated herein. It can be deduced that free-surface turbulence in open-channel flows originates from processes driven by LC3S, associated with the zonal organization of streamwise momentum. The good agreement with mean quadrant distribution results in the literature implies that LC3S identified in this study are common features in the outer region of shear flow

Near-bed hydrodynamics and turbulence below a large-scale plunging breaking wave over a mobile barred bed profile

Funded by The research presented in this paper is part of the SINBAD project. Grant Number: STW (12058) and EPSRC (EP/J00507X/1, EP/J005541/1)Peer reviewedPublisher PDFPublisher PD

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

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

Turbulent transport in the outer region of rough-wall open-channel flows : the contribution of large coherent shear stress structures (LC3S)

Author Posting. © Cambridge University Press, 2007. This article is posted here by permission of Cambridge University Press for personal use, not for redistribution. The definitive version was published in Journal of Fluid Mechanics 574 (2007): 465-493, doi:10.1017/S0022112006004216.Acoustic Doppler velocity profiler (ADVP) measurements of instantaneous three-dimensional velocity profiles over the entire turbulent boundary layer height, δ, of rough-bed open-channel flows at moderate Reynolds numbers show the presence of large scale coherent shear stress structures (called LC3S herein) in the zones of uniformly retarded streamwise momentum. LC3S events over streamwise distances of several boundary layer thicknesses dominate the mean shear dynamics. Polymodal histograms of short streamwise velocity samples confirm the subdivision of uniform streamwise momentum into three zones also observed by Adrian et al. (J. Fluid Mech., vol. 422, 2000, p. 1). The mean streamwise dimension of the zones varies between 1δ and 2.5δ. In the intermediate region (0.2<z/δ<0.75), the contribution of conditionally sampled u'w' events to the mean vertical turbulent kinetic energy (TKE) flux as a function of threshold level H is found to be generated by LC3S events above a critical threshold level Hmax for which the ascendant net momentum flux between LC3S of ejection and sweep types is maximal. The vertical profile of Hmax is nearly constant over the intermediate region, with a value of 5 independent of the flow conditions. Very good agreement is found for all flow conditions including the free-stream shear flows studied in Adrian et al. (2000). If normalized by the squared bed friction velocity, the ascendant net momentum flux containing 90% of the mean TKE flux is equal to 20% of the shear stress due to bed friction. In the intermediate region this value is nearly constant for all flow conditions investigated herein. It can be deduced that free-surface turbulence in open-channel flows originates from processes driven by LC3S, associated with the zonal organization of streamwise momentum. The good agreement with mean quadrant distribution results in the literature implies that LC3S identified in this study are common features in the outer region of shear flows.The study was supported by the Swiss National Foundation for Scientific Research for the experimental part (grant 2100 050739) and the French National Center for Scientific Research (CNRS) for the data analysis and interpretation

Suspended sediment transport around a large-scale laboratory breaker bar

The authors wish to thank the staff of CIEMLAB (Joaquim Sospedra, Oscar Galego and Ricardo Torres) and Mick Poppe from the University of Twente for their contributions to the experiments. We are also grateful to fellow SINBAD researchers and to prof. Peter Thorne for their feedback on preliminary results and to two anonymous reviewers and prof. dr. Suzanne J.M.H. Hulscher for their feedback on the draft manuscript. The research presented in this paper is part of the SINBAD project, funded by STW (12058) and EPSRC (EP/J00507X/1, EP/J005541/1). We further acknowledge the European Community’s FP7 project Hydralab IV (contract no. 261520) for funding the accompanying SandT-Pro experiments and the ACVP development by CNRS-LEGI (D. Hurther, P.-A. Barraud, J.-M. Barnoud).Peer reviewedPostprin

Suspended and bedload transport in the surfzone : implications for sand transport models

ACKNOWLEDGMENTS The research presented in this paper is conducted within the SINBAD project, funded by STW (12058) and EPSRC (EP/J00507X/1, EP/J005541/1), and received additional funding through the European Community’s FP7 project Hydralab IV (contract no. 261520).Publisher PD

On the study of boundary layer sediment transport processes using new developmental acoustic techniques

This study aims at presenting a set of novel acoustic flow and sediment measuring tools used in a complementary way for the investigation of fine-scale flow and sediment transport processes above a rippled sand bed under waves. Measurements were conducted in the 100m long wave channel at UPC-LIM with surface gravity waves generating migrating ripples on a mobile sand bed. The set of acoustic instrumentation consisted of an Acoustic 3D Ripple Profiler (3D-ARP), a novel Bedform And Suspended Sediment Imager (BASSI) and three Acoustic Concentration and Velocity Profilers (ACVP). Here we assess the ACVP’s and the BASSI.Postprint (published version

Wave Boundary Layer Hydrodynamics and Sheet Flow Properties under Large-Scale Plunging-Type Breaking Waves

The authors wish to thank the staff of CIEMLAB, in particular Joaquim Sospedra, Oscar Galego and Ricardo Torres, for their hospitality and hard work during the experimental campaign. This research was funded by the European Community’s Horizon 2020 Programme through the Integrated Infrastructure Initiative HYDRALAB+ COMPLEX (no. 654110), the French DGA funded ANR Astrid Maturation project MESURE (no. ANR-16- ASMA-0005-01) and the SINBAD project funded by STW (12058) in the Netherlands and by EPSRC (EP/J00507X/1, EPJ005541/1) in the UK. Data of this study are available at U. Twente repository doi (10.4121/uuid:753f1d84-36e5-47fa-b74b-55c288545b9b). Comments and remarks raised by the two anonymous reviewers helped to improve the quality of the paper.Peer reviewedPublisher PD