Missions

During storms, the nearshore dynamics is primarily governed by depth-induced wave breaking processes. As waves break, they dissipate energy, generating intense currents that control the morphological response of beaches, including the erosion of beachfaces and dunes. Through these dissipative processes, breaking waves also contribute to storm surges by sur-elevating mean water levels at the coast (wave setup). Despite its critical importance for predicting coastal hazards, the wave-breaking process remains coarsely parametrised in spectral wave models, in particular because the frequency distribution of dissipated energy remains unknown. The standard modelling approach follows the work of Eldeberky and Battjes (1996), in which a linear relationship is postulated between the energy dissipated at the wave angular frequency ω and the spectral energy at that frequency. Recently, Bonneton (2023) used physical analogies between Burgers turbulence and “saw-tooth” inner surf zone waves to develop, and validate using lab data, a new theoretical model for inner surf zone waves spectra. This theoretical spectrum is composed of 2 subranges: the inertial subrange, with the commonly-observed ω^(-2) tendency, and a diffusive subrange at high-frequencies, with an exponential decay of the energy. Critically, Bonneton (2023) provides strong evidence that the parametrisation of Eldeberky and Battjes (1996) incorrectly predicts the frequency distribution of energy dissipation in inner surf zone waves.

The universal energy spectrum of inner surf zone waves developed in Bonneton (2023) is controlled by 3 physical variables: the mean wave period T_m, the total energy E_∞ and the diffusive frequency ω_ν. The diffusive frequency ω_ν, introduced for the first time within this framework, is a statistical variable related to the characteristic temporal width of wavefronts. Since the variables T_m and E_∞ can be computed from wave energy spectra, we believe that a better understanding of the « unknown » ω_ν holds the key for more robust and physics-based parametrisations of wave breaking in spectral wave models.

The main objective of this postdoc is to better quantify and predict ω_ν, in particular by exploring the spatio-temporal characteristics of surface rollers, which are proxies for energy dissipation in the inner surf zone (Martins et al., 2018). For this, a robust characterisation of the diffusive subrange is critical, which can only be achieved through precise, high-frequency measurements of the free surface elevation. Here, we will characterize ω_ν with infrared lidar scanners, which have the capacity to directly measure the free surface elevation at high spatial (O(cm)) and temporal resolution (typically 10 Hz). The research activities developed by the recruited postdoc will contribute to two larger collaborative projects dedicated to advancing the comprehension and predictive capability of the physical processes underlying coastal submersion hazards: the ANR JCJC IMPASTO, which funds this project, and the Nouvelle-Aquitaine Region-funded project CORALI. As such, the work will require regular exchanges and collaboration with researchers from the different partner institutions.


Relevant references :
Bonneton, P. (2023). Energy and dissipation spectra of waves propagating in the inner surf zone. Journal of Fluid Mechanics 977, A48.
Bonneton, P., & Martins, K. (2024). Caractérisation in situ du comportement spectral des vagues en zone de surf. In Journées Nationales Génie Côtier Génie Civil, Juin 2024, Anglet. Doi : 10.5150/jngcgc.2024.004.
Eldeberky, Y., & J. A. Battjes (1996). Spectral modeling of wave breaking: Application to Boussinesq equations, Journal of Geophysical Research: Ocean 101(C1), 1253 – 1264.
Martins, K., Blenkinsopp, C. E., Deigaard, R., & Power, H. E. (2018). Energy dissipation in the inner surf zone: New insights from LiDAR‐based roller geometry measurements. Journal of Geophysical Research: Oceans 123(5), 3386 - 3407.
Martins, K., Brodie, K. L., Fiedler, J. W. et al. Seamless nearshore topo-bathymetry reconstruction from lidar scanners: a Proof-of-Concept based on a dedicated field experiment at Duck, NC. Coastal Engineering 199, 104748.

Activités

Principal tasks of the postdoctoral researcher:
In collaboration with the PI, organise and perform the lidar acquisition at the inner surf zone/swash zone boundary during the campaigns planned as part of the IMPASTO (main project funding this postdoctoral contract) and the CORALI projects. The objective will be to create a comprehensive dataset covering a wide range of beach and wave conditions, with selection criteria including the availability and accuracy of bathymetric data, the robustness of lidar measurements (single vs multi-beam, sampling rate, and spatial resolution), and incident wave conditions. Past datasets collected by the PI will also be included when criteria are met.
Analyse the sensitivity of ω_ν estimates using a range of approaches. Bonneton and Martins (2024) noted that the spectral fitting techniques as done in Bonneton (2023) is rather sensitive to the amount of energy in the inertial range of frequencies. Here, we will explore promising alternatives such as physics-informed neural networks (Raissi et al., 2019), which seem particularly powerful to analyse Burgers-like equations, which are at the basis of the theoretical framework of Bonneton (2023). This approach allows to « discover » variables in pre-defined partial differential equations on series of data (time-domain technique). Recent tests conducted with both lidars and wave gauges at the prototype-scale in the GWK wave flume (Hannover, Germany), could also be used to analyse the sensitivity and robustness of lidar processing techniques on the description of high-frequency spectrum shape.
Analyse the diffusivity frequency ω_ν across the entire lidar dataset as a function of other wave parameters. Starting from the law ω_ν∼(g / h_m )^0.5 observed in small-scale laboratory experiments (Bonneton, 2023), we will search for an accurate parametrisation of ω_ν as a function of the mean water depth h_m, E_∞ and T_m. We will explore its relations with statistics of the geometrical properties of surface rollers (e.g., length and height), which are at the core of seminal roller-based parametrisations of breaking waves energy dissipation (Duncan, 1981). This work will form the basis for a robust theoretical and practical framework for the parametrisation of energy dissipation by breaking in both phase-resolving and spectral wave models.

Relevant references :
Duncan, J. H. (1981). An experimental investigation of breaking waves produced by a towed hydrofoil. Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences 377(1770), 331 - 348.
Raissi, M., Perdikaris, P., & Karniadakis, G. E. (2019). Physics-informed neural networks: A deep learning framework for solving forward and inverse problems involving nonlinear partial differential equations. Journal of Computational physics 378, 686-707.

Compétences

The ideal candidate will have a strong background in nearshore and coastal ocean processes, and should hold a PhD in coastal hydrodynamics, physical oceanography, or a related field. Candidates holding a PhD in more fundamental physics fields, whose expertise could be relevant to the present topic, are also encouraged to contact the PI or directly apply, provided they can demonstrate a strong motivation to engage with nearshore hydrodynamics. The following skills and qualifications are typically expected:

· Proficiency in signal processing of timeseries, including for spectral analysis, and in numerical analysis tools (e.g. Python, MATLAB, or equivalent).
· Ability to interpret and synthesize complex datasets with scientific rigor.
· Familiarity with field measurements, instrumentation, or numerical wave modelling is desired but not essential, as long as the motivation to participate in the planned field work is demonstrated.
· Capacity to work independently while actively collaborating within a multidisciplinary research team.
· Strong communication and writing skills in English, including the preparation of scientific manuscripts and reports.

Contexte de travail

This postdoctoral project is funded by the ANR-JCJC 2025 project IMPASTO (PI: Kévin MARTINS), which aims to advance the understanding and prediction of surf-zone wave spectral transformation and unravel the driving mechanisms of wave runup, including during storm conditions. The IMPASTO consortium brings together researchers from LIENSs (CNRS – La Rochelle University), EPOC (CNRS – University of Bordeaux), and INRIA (Bordeaux Center).
The postdoctoral researcher will be hosted in the « Dynamique Physique du Littoral » (DPL) team in LIENSs (https://lienss.univ-larochelle.fr/Equipe-DPL), which studies coastal dynamics and sea-level variability across multiple spatial and temporal scales, using laboratory and field experiments, remote sensing, and numerical modelling. The team aims to improve our understanding of the physical processes shaping littoral environments. In addition to collaborating with junior and senior researchers at LIENSs (e.g. for field data collection and processing), the postdoctoral researcher will work closely with Philippe Bonneton (EPOC) on the theoretical aspects of the new inner surf zone spectrum, and with Maria Kazolea and Martin Parisot (INRIA) on the parameterization of breaking processes in phase-resolving numerical models.
The position is initially offered for 12 months, with a possible 12-month extension (funding already secured). Please note that the position is located in a sector under the protection of scientific and technical potential (PPST), and therefore requires, in accordance with the regulations, that your arrival is authorized by the competent authority of the Ministry of Higher Education and Research (MESR).

Contraintes et risques

Fieldwork activities may involve irregular working hours depending on tidal and weather conditions. Appropriate safety measures will be implemented to minimize risks for both personnel and equipment.