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
For the last twenty years or so, scientists have been trying to gain a better understanding of how waves transport sand to coastal areas. Until now, research has focused mainly on beach erosion during storms. We now know fairly well how these phenomena occur. But one question remains unanswered: how do beaches recover after a storm? This is where the WEST project comes in. Its aim is to study the mechanisms that enable beaches to recover after a period of erosion. After a storm, a beach may take more or less time to return to its original shape. To better predict coastal change, we need to understand both erosion and beach recovery. Erosion is well studied: it occurs when the balance between the beach and sea conditions is disrupted, particularly as a result of the power of waves and currents. But reconstructing beaches is more complex, because several physical phenomena come into play at the same time, with no single one clearly dominating. We know that the transport of sand towards the open sea is mainly due to currents, while its return to the beach depends on the movements of the water near the seabed, generated by the waves. The problem is that we still lack precise information about these mechanisms. Researchers are finding it difficult to measure precisely how sand moves under the effect of waves and currents, mainly because of the complex interactions between turbulence, the slope of the seabed and the shape of the waves. Until now, most studies have been carried out in the laboratory or by numerical simulation. The challenge of the WEST project is to observe these phenomena directly in a natural environment, under real waves. To do this, we need very precise measuring instruments capable of capturing the slightest variations in the seabed, sometimes on scales of a millimetre and in a tenth of a second. In addition, to fully understand the impact of waves, we also need to be able to measure their exact shape when they are about to break. The WEST project is using innovative measurement techniques to meet this challenge. In addition, the researchers are using an advanced computer model, called CROCO, to simulate these processes and verify their observations. This work will help to improve our understanding of the natural phenomena that shape our beaches, with practical applications for coastal protection in the face of storms and climate change.
To fully understand how sand moves under the effect of waves, we need ultra-precise measurements, both in time (every tenth of a second) and in space (to the nearest millimetre). A recent device called ACVP (Acoustic Concentration and Velocity Profiler) makes it possible to achieve this precision. It uses an advanced acoustic system that simultaneously measures water velocity and sediment concentration over a height of 10 to 20 cm. Thanks to a collaboration with Ubertone, a portable version of this device has been developed. It uses improved technology to obtain even more reliable measurements of the quantity and size of sand grains in suspension.
Another challenge is to measure the shape of waves as they approach the shore and begin to break. This is a complex phenomenon, as these waves have irregular and changing shapes. Today, thanks to advances in stereo-video cameras and remote sensing technologies such as LiDAR, it is possible to obtain accurate 3D images of the water surface. In the WEST project, tests were carried out using a low-cost video system and LiDAR on board a drone. This was the first time that wave measurements using 16 LiDAR beams had been tested from a drone, enabling the advantages and limitations of these different methods to be compared.
Finally, numerical modelling plays a key role in better understanding the interactions between waves and sediments. Once validated, these models can be used to assess the impact of each phenomenon individually. The advanced version of the CROCO software has been chosen to model these processes to an accuracy of around 2 mm. It can simulate turbulence, waves, sediment transport and changes in the shape of the seabed. Although other tools such as Waves2Foam and SedWaveFoam were considered, they proved too computationally intensive and difficult to install on the project's servers. CROCO, supported by the Brest scientific community, proved to be the best option for efficiently simulating the processes studied in WEST. The next research project will therefore focus even more on this tool.
The research carried out as part of the WEST project has highlighted a major problem: the computer models used to date do not take proper account of the reconstruction of beaches after a storm. Thanks to our work, we have been able to measure the water surface in 3D and record, with very high precision, the speed and concentration of moving sand in the thin layer of water at the bottom of the beach.
One of our objectives was to analyse how the sand moves under the waves, distinguishing two zones: where the waves start to rise and where they break. To compare our results with laboratory experiments using simple, regular waves, we had to select the waves observed at sea so that they were representative in terms of power and shape. This methodological work was detailed in Noémie Fritsch's thesis and in an article currently being published.
Another key point of the project was to study how the shape of waves influences underwater currents, particularly in the layer of water close to the seabed. We analysed three aspects: the evolution of wave distortions (non-linearities), the offset between the main current and the near-bottom current, and the thickness of the bottom layer.
Our results show that breaking waves have a marked effect on this layer, a phenomenon already described by Berni et al. (2013). We also observed a shift of around 20 degrees between surface and near-bottom currents, but this remains difficult to predict accurately. As for the thickness of the moving sand layer, we have shown that existing definitions are not sufficient to correctly describe its evolution as the waves pass over it.
We then studied how currents influence the movement of sediments. We measured the quantity of sand transported and compared our data with the classic formulae used in modelling. The result: these formulas underestimate the transport towards the coast, notably because they do not take sufficient account of the sand that rolls along the seabed. We also found that natural waves transport more sediment than the artificial waves used in the laboratory, such as those simulated in the SANTOSS model.
Finally, we looked at the effect of beach slope on waves and wave breaking. Our analyses show that when the slope is too steep, waves do not have time to adapt to local depth conditions, a phenomenon studied by Diouf et al (2024) and which is currently being studied in greater depth by numerical simulations using the CROCO-NH model.
To conclude this project, we organised an international workshop, bringing together experts in the field to discuss the progress made and future research.
The WEST project is a first step. It represents a missing brick in the fundamental research and technological advances required before future advances can be made in coastal dynamics studies. Now that the tools have been developed, two projects are being prepared. One of these proposes to tackle directly the problems highlighted at the end of WEST, by building on the international network created during WEST. The idea is to succeed in predicting the key parameters describing the hydrodynamics of the boundary layer (thickness,
current phase shift and non-linearities) as well as the induced sediment transport, as a function of the spectral width of the incident waves, the interactions between frequencies leading to the
and the slope of the beach. For this project, we will rely on existing data and on new in situ and laboratory data (in collaboration with the University of Catalonia, which has a full-scale channel). The second project will be based in particular on our developments in 3D measurement of the
free surface. The idea of the project is to gain a better understanding of the dynamics of macro-rugosities on beaches (such as large wrinkles) and to assess the potential of pebble strips as a Nature-based Solution against coastal erosion. The originality of the project lies in understanding the influence of the longshore variability of the surf on currents, which is possible in the case of a pebble beach.
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 coordination
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.
Partnership
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
