# Wave Boundary Layer and Sediment Transport Observations in the Field

Noémie Fritsch, Guillaume Fromant, France Floc’H

► To cite this version:

Noémie Fritsch, Guillaume Fromant, France Floc’H. Wave Boundary Layer and Sediment Transport Observations in the Field. ISEH 2024 - 10th International Symposium on Environmental Hydraulics, IAHR Technical Committee on Fluid Mechanics; University of Aberdeen; 29008, Jun 2024, Aberdeen, United Kingdom. hal-04974837

HAL Id: hal-04974837

https://hal.science/hal-04974837v1

Submitted on 3 Mar 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Velocity reduction and drag in submerged canopies under oscillatory flow conditions

Noémie Fritsch¹*, Guillaume Fromant², France Floc'h¹

¹Université de Bretagne Occidentale, Laboratoire GéOcéan, Brest, France.

²Université du Littoral Côte d'Opale, Laboratory LISIC, Calais, France.

*Corresponding author: guillaume.fromant@univ-littoral.fr

**Keywords:** ACVP, sand transport, wave bottom boundary layer, phase-averaging.

## Introduction

The ability to obtain accurate predictions of sand transport on beaches represent high social, environmental and economical stakes. In order to improve parametric sand transport models, the link between the free stream velocity and sand transport processes in the Wave Boundary Layer (WBL) need to be further investigated. The present work reports on the first deployment of a UB-Lab 3C ©Ubertone (based on Acoustic Concentration and Velocity Profiler technology, Hurther et al., 2011) in situ at Porsmilin Beach, Brittany, France, in 2023, with the aim to study Wave Boundary Layer hydrodynamics and sediment transport properties under real coastal waves.

## Methods

<figure><img src="image_2.png" /><figcaption>Figure 1: Location of Porsmilin beach, France.</figcaption></figure>

<caption>Figure 2: Mooring frame deployed in the inter tidal zone of the beach</caption>

A UB-Lab 3C was deployed on Porsmilin Beach (see Figure 1, the red dot depicts the location of the platform), in Western Brittany, France, in the inter tidal zone of the beach, shown by the red dot on the map on Figure 1. The instrument was deployed during one tidal cycle on a mooring frame (Figure 2). This instrument combines incoherent acoustic particle scattering theory for sand concentration profiling and pulse-to-pulse coherent Doppler velocity estimation for 1D vertical profiling of 3C (three components u,v,w) velocity (Hurther et al., 2011, Fromant et al. 2019) at a temporal resolution of 25 to 50 Hz. Wave separation has been applied using the wave velocity data recorded by the UB-Lab 3C, which further allowed to obtain intrawave velocity and concentration profiles after phase-averaging. In the present case, phase-averaging of these irregular waves has been performed independently of their hydrodynamic parameters, over 100 waves of similar period. This procedure allowed to compute key Wave Boundary Layer hydrodynamic parameters, as well as sand flux.
## Results

<figure><img src="image_3.png" /><figcaption>Figure 3: rms (a,b), free stream (c,e) and intrawave (d,f) cross-shore velocity profiles computed from two sets of 100 waves at low (a,c,d) and high tide (b,e,f). Associated non-linearities evolutions are displayed on panels g and h.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 4: Quasi-instantaneous flux profiles (top panel) and associated free stream cross-shore velocity (lower panel).</figcaption></figure>

On Figure 3, intrawave velocity profiles for two mean waves taken at low (Fig. 3d), and high tide (Fig. 3f) of the experiment. Corresponding root mean square (rms) profiles are displayed (Fig. 3a, b), allowing to appreciate the nearbed velocity overshoot. The grey curves on Fig. 3d,f mark the velocity zero-crossings, showing near-bed velocity phase leads of the order of 20° compared to the free-stream cross-shore velocity. The superimposed black dots represent the intrawave evolution of the WBL thickness. Finally, panels g and h represent the evolution of the asymmetry Asy (in black) and skewness Sk (in red) values, that appear consistent with previous studies (eg. Berni et al., 2013). Finally, the UBLab3C data allowed to estimate quasi-instantaneous sand fluxes (Fig. 4) and nearbed level evolutions.

## Conclusions

The first field deployment of the UB-Lab 3C, based on the ACVP technology, is presented. The deployment has been successfully carried out in Porsmilin beach, allowing to observe key hydrodynamic parameters at the intrawave scale, as well as sand fluxes in the wave boundary layer. In the near future, we hope to be able to deepen our analysis on measured in situ transport rate, and to underline the necessity to collect more data in situ in order to validate practical sand transport model outputs in the field.

## Acknowledgements

This work is part of ANR project WEST (ANR-20-CE01-0009), and supported by the French directorate of Armament (DGA) and ISblue project (ANR-17-EURE-0015).

## References

* Fromant, G., Hurther, D., van der Zanden, J., van der A, D. A., Cáceres, I., O'Donoghue, T., & Ribberink, J. S. (2019). Wave boundary layer hydrodynamics and sheet flow properties under large-scale plunging-type breaking waves. *Journal of Geophysical Research: Oceans*, vol. 124.
* Hurther, D., Thorne, P. D., Bricault, M., Lemmin, U., & Barnoud, J. M. (2011). A multi-frequency Acoustic Concentration and Velocity Profiler (ACVP) for boundary layer measurements of fine-scale flow and sediment transport processes. *Coastal Engineering*, vol 58, pp. 594-605.

* Berni, C., Barthélemy, E., & Michallet, H. (2013). Surf zone cross-shore boundary layer velocity asymmetry and skewness: An experimental study on a mobile bed. *Journal of Geophysical Research: Oceans*, 118(4), 2188-2200.