CE01 - Terre fluide et solide

Natural Breaking WavEs and Sediment Transport during beach recovery – WEST

Natural Breaking WavEs and Sediment Transport during beach recovery

WEST project focuses on Wave Boundary Layer and sediment transport at the land-sea interface, specifically on sandy beaches. It relies on the combination of observations and modeling approaches to understand the variability of the coastal environment and aims to fill the gap between theory and in situ measurements of nearshore dynamics.

interactions between nearshore hydrodynamics and sediment transport

While offshore transport is strongly linked to current, accretion is linked to wave-generated fluxes occurring in the WBL, generated by steady streaming and wave shape streaming mainly via bedload transport. Previous experiments and numerical studies show that wave boundary layer dynamics are induced by various hydrodynamic and morphological factors (nearshore currents, streaming, wave dynamics, wave non-linearities, bed slope and roughness, turbulence…). During calm periods, all these processes contribute to the net sediment transport over one wave cycle with non-negligible contributions. The two scientific issues raised by the literature are: the lack of knowledge on the role played by each process in the accretion mechanism and their relative contribution, and the difficulty to obtain accurate time-resolved measurements of bed shear stress and sand transport in WBL in situ. <br />The overall aim of WEST is to evaluate the interactions between waves, hydrodynamics and bed slope leading to onshore net sediment transport under waves in the surf zone. The overall objectives are to clarify the relative contributions of physical processes leading to onshore net sediment transport and to evaluate the net transport over one wave cycle according to hydrodynamic and morphological factors. At the time of project completion, WEST will provide the world first dataset allowing to address the influence of wave non-linearities across the water column down to the WBL and their influence on the bed shear stress variability. WEST will also make it possible to quantify the sediment transport in suspension as well as in bedload, which was not possible until now via time-resolved measurements. WEST will provide accurate-enough dataset of wave shape and hydrodynamics, and process-based numerical modeling to quantify the influence of bed slope. These results will enhance our understanding of beach recovery mechanisms, leading to <br />improvements in morphodynamic models commonly used by <br />engineers and coastal practitioners for coastal zone <br />management. The first objectives of WEST will be to study the impact of free surface deformation on water column hydrodynamics, especially in the WBL. Then, the interaction between current and sediment transport (via bedload and suspension) will be investigated in a second objective, as well as, in a lesser extent, the possible influence of seabed micro-mechanics. The sediment transport will induce bed erosion or accretion and thus will modify the bed slope. The third objective of WEST address then the retroaction of the bed slope on the wave non-linearities and the breaking.

Future progress on Wave Boundary Layer (WBL) processes and sediment transport depends on our ability to study WBL in situ under natural waves. First, to resolve WBL transport processes, measurements should be done at high temporal (O(0.1 s)) and spatial (O(0.001 m)) resolutions. A recent system, called the ACVP (Acoustic Concentration and Velocity Profiler), matches these requirements. It consists in a bistatic acoustic profiler with a large angle between the emitter and the receivers, allowing to accurately measure quasi-instantaneous and co-located velocity and sediment concentration over a 10 to 20 cm vertical profile. New acoustic inversion techniques have also been developed, increasing the reliability of acoustic measurements in terms of sediment concentration and size. Secondly, measuring wave non-linearities with sufficient accuracy implies to be able to reconstruct the sharp shape of the free-surface close to breaking. Latest development in stereo-video systems and remote sensing such as LiDAR, allowing complex surface to be recorded in 3D, now makes measuring non-linear waves shape a reality. numerical models allow an in-depth investigation of the relative contributions of each process which is one of the overall objectives of WEST. Modeling a cross-shore profile is now accessible via Computational Fluid Dynamics methods. Waves2Foam allows to correctly model WBL dynamics over a fixed bed considering traditional turbulence models (k-e or k-?). SedFoam 2.0, an advanced two-phase model in the framework of OpenFOAM, allows for using either Kinetic Turbulent Granular Flow or dense granular rheology for the calculation of particle stresses. Recently, a three-dimensional two-phase flow model for sediment transport in sheet flow conditions has been introduced. The Large Eddy Simulation (LES) approach was adopted to model turbulence and turbulence-particle interaction. Since the influence of the free-surface deformation on the WBL dynamics has been demonstrated, it is crucial to be able to model wave field when addressing accretion. SedWaveFoam is a new model able to concurrently resolve free surface deformations, WBL, and sediment transport processes throughout the entire water column, integrating SedFoam 2.0, and Waves2Foam, in the OpenFOAM framework. SedWaveFoam is validated with a large wave flume dataset for sheet-flow-driven monochromatic non-breaking waves. Waves2Foam and SedWaveFoam are the most advanced process-based modeling tools allowing to address WBL dynamics and associated sediment transport under waves. Both will be used to address WEST objectives.

The overall aim of WEST is to evaluate the interactions between waves, hydrodynamics and bed slope leading to onshore net sediment transport under waves in the surf zone. WEST proposes an in-depth study of WBL processes: its hydrodynamics, sediment transport and breaking-induced turbulence as well as the free-surface deformation toward breaking using innovative measurement techniques and numerical methods. Thus, using sound and original approaches, WEST proposes to address key scientific and technologic issues via original approaches. At the time of project completion, WEST will enhance our understanding of beach recovery mechanisms, leading to improvements in morphodynamic models commonly used by engineers and coastal practitioners for coastal zone management. To reach the long-term goal of improving commonly used numerical models for environmental studies, the comparison of phase resolved shallow water equations model (e.g. Xbeach NH) to the LES and RANSE/VOF-type models that will be implemented in WP3 is critical in order to detect, and address, models’ limitations. Since models used by engineers make use of empirical formulations (in order not to solve each process and to have acceptable calculation time), the aim will be to improve their parametrization thus accuracy, in agreement with our results, by a better understanding of the physical processes at play and their relative contributions. The WP2 results will provide important knowledge on waves non-linearities’ variability during their propagation over a sandy beach and breaking processes. This will be particularly useful to help quantify wave impact load on structures. Additionally, it could help quantify longshore variability of free-surface, which is a supposed mechanism linked to many nearshore processes (flash rip, edge wave…), but never observed so far. This will be made possible by our novel and accurate 3D free-surface deformation measurements.

WEST project is a first step. It constitutes a missing brick in fundamental research and technological advances before being able to carry on future advances in coastal dynamics studies.
Two projects are envisioned in the future. The first would involve researchers from the SNO DYNALIT and would deal with the impact of bottom roughness on wave transformation and WBL hydrodynamics. Seabed roughness, via the wave excursion to roughness length ratio, is shown to impact the boundary layer thickness and the flow regime. Increasing roughness implies a shift onshore of the streaming profile. Instruments (including UB-Lab3C©) would be installed at different geographic sites of DYNALIT exhibiting particular roughnesses (rough fixed bed at cliff toe, dissipative to reflective beaches…). A first attempt to gather data of roughness and wave transformation on different study sites is in progress, with already many researchers in Geography, Geology, Physics having expressed their interest and sent datasets. Thus, the number of study sites and researchers involved could be important. Another possible project would imply our whole team, who are from complementary scientific domains (Physics, Paleontology, Geomorphology, Sedimentology). Gathering all these skills to address sustainability of a complex coastal system would be very valuable. A study case could be in West Africa, where our team has already many collaborations. Barbarie tongue, classified as a World Heritage site by Unesco, is at the mouth of Senegal river estuary and is submitted to a strong littoral drift and energetic swell. A 4 meters breach was cut in the tongue in 2003 to help counter possible flooding. However, the breach quickly widened and the tongue keeps on loosing sediment. 17 years after, the breach has widened to 6 km and the sea has claimed a large band of land and has caused the loss of villages in addition to changes in the flora and fauna. Using Holocene data to understand the past variability and using WEST advances to investigate short term variability will allow quantifying an accurate sediment budget and bringing reliable breakthrough in the tongue variability understanding. This project would also involve geographers/economists experts in risk assessment and physicists experts in estuarine fluxes, addressing sustainability via multi-disciplinary approaches.
These are two hypothetic projects, but the advances done in WEST will give many opportunities to elaborate a large panel of projects. They could bring first world dataset to address questions that are of interest: flash rip processes and induced sediment transport in Benin, or the impact of waves on cliffs. The technical advances, and associated SPOC, will benefit the whole scientific and technical community and will foster new international collaborations.

International conferences
1. Suspended sediment concentration and sediment transport measurements with a two components ultrasonic profiler.
H. Guta , M. Burckbuchler, S. Fischer (Ubertone) – THESIS conference, Les Houches, Juin 2022
2. In situ high frequency measurement of sediment transport in the surf zone. N. Fritsch, G. Fromant, F. Floc’h, S. Fischer – Coastal Sediment, New Orleans, April 2023

National conferences
1. Drones et capteurs embarqués de l'Institut Universitaire Européen de la Mer. J. Ammann, 1ere édition de journées Drones & Caps, Oléron, 28 - 30 septembre 2021
2. Mesure hydrosédimentaire en zone intertidale : conception d’un mouillage modulaire portable, F. Floc’h, E. Droniou, M. Huchet, N. Fritsch. Journée du Low’Coast, Plouzané, Mai 2022
3. Mesure 4D de vagues par stéréo-vidéo, Augereau E., Jaud M., Bertin S., Journée du Low’Coast, Plouzané, Mai 2022
4. Journée du Low’Coast, Plouzané, Mai 2022
5. Stéréo-GoPro, Stéphane Bertin, Marion Jaud, Emmanuel Augereau, Charles Poitou, Aelaïg Cournez, France Floc’h, Workshop sur la stéréo-vidéo, Plouzané, avril 2022
6. Mesure de la dynamique hydrosédimentaire dans la zone de déferlement par mesure in situ : conception d’un mouillage modulaire dédié. N. Fritsch, F. Floc’h, G. Fromant, E. Droniou, and S. Fischer. ASF, Brest, septembre 2022
7. Développement et évaluation d’un système de stéréo-vidéo pour la mesure de vagues en zone de déferlement. M. Jaud, S. Bertin, E. Augereau, C. Poitou, A. Cournez, N. Fritsch, F. Floc’h – GCGC, Chatou, Octobre 2022
8. Mesure de la dynamique hydrosédimentaire dans la zone de déferlement par mesure in situ : conception d’un mouillage modulaire dédié. N. Fritsch, F. Floc’h, G. Fromant, E. Droniou, and S. Fischer. GCGC, Chatou, Octobre 2022

Popularization articles
1. Floc’h F., Le retour du sable sur les côtes, juillet-Aout 2021, Sciences Ouest n.390 www.espace-sciences.org/sciences-ouest/390/actualite/le-retour-du-sable-sur-les-cotes

Coasts, as land-sea interface, are among the most dynamic and fragile environments on Earth. Sandy coastlines are particularly vulnerable, a reason being coastal hazards, which impact sediment transport and sand budgets. Hydrodynamics forces play a critical role driving beach change, being increasingly responsible for major perturbations of coastal socio-ecosystems. Furthering research on the interactions between nearshore hydrodynamics and sediment transport is crucial to evaluate the future response of coastal systems Over the past two decades, significant research efforts have been dedicated to the understanding and modeling of sediment transport processes under realistic non-linear waves in the nearshore zone. The erosion mechanisms during extreme events being well described in the literature, the novelty of WEST is to initiate new research on sediment transport during accretive periods, leading to beach recovery. In fact, storm erosion can have short- to medium-term impacts on coasts, depending on the post-storm recovery mechanisms and timescales. To predict coastal evolution, both erosion and accretion mechanisms should be accurately defined. During beach recovery, several physical processes interplay with no predominant factor controlling the sediment transport. The overall aim of WEST is to evaluate the interactions between waves, hydrodynamics and bed slope leading to onshore net sediment transport under waves in the surf zone, during post-storm or seasonal recovery. The overall objectives are to clarify the relative contributions of physical processes leading to onshore net sediment transport and to evaluate the net transport over one wave cycle according to hydrodynamic and morphological factors. The first objectives of WEST will be to study the impact of free-surface deformation on water column hydrodynamics, especially in the Wave Boundary Layer (WBL). Then, the interaction between current and sediment transport (via bedload and suspension) will be investigated in a second objective, as well as, in a lesser extent, the possible influence of seabed micro-mechanics. The sediment transport will induce bed erosion or accretion and thus will modify the bed slope. The third objective of WEST addresses then the retroaction of the bed slope on the wave non-linearities and the breaking. The novelty of WEST lies also in the first attempt to obtain accurate time-resolved in situ measurements in unison of sediment transport and free-surface deformations at the critical zone: towards wave breaking. For the first time, the whole water column towards breaking will be monitored in the natural environment. Many research point out the lack and thus the need of fields observations to determine the relative importance of physical processes. Monitoring natural waves in their physical environment is very challenging yet crucial if we aim to make new advances in physic-based numerical modeling and improved predictions of future shoreline position. WEST will build on the most recent research in wave dynamics and sediment transport, implementing cutting-edge technology to produce the most comprehensive measurements of WBL under natural breaking waves, as well as using the most recent improvements in process-based numerical methods. At the time of project completion, WEST will provide the world first dataset allowing to address the influence of wave non-linearities across the water column down to the WBL and their influence on the bed shear stress variability. WEST will also make it possible to quantify the sediment transport in suspension as well as in bedload. These results will enhance our understanding of beach recovery mechanisms, leading to improvements in morphodynamic models commonly used by engineers and coastal practitioners for coastal zone management.

Project coordinator

Madame France Floc'h (Université de Bretagne Occidentale (UBO), Laboratoire Géosciences Océan (LGO))

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

UBO-LGO Université de Bretagne Occidentale (UBO), Laboratoire Géosciences Océan (LGO)

Help of the ANR 289,440 euros
Beginning and duration of the scientific project: December 2020 - 48 Months

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