Vagues extrêmes et mouvement de blocs cyclopéens / Extreme Nearshore Wave Events and Coastal Boulder Transport

Séminaire organisé par Rónadh Cox (Williams College, Williamstown, USA) et Frederic Dias (University College Dublin, Ireland) du 22 au 27 octobre 2018

Participants

Fabrice Ardhuin, Ronan Autret, Jörn Behrens, Nicole Beisigel, Mary Bourke, Clément Calvino, Rónadh Cox, Frédéric Dias, Claire Earlie, Daniel Giles, Kazuhisa Goto, James Herterich, Andrew B. Kennedy, Christian Kharif, Raphaël Paris, Alison Raby, Dimitra Salmanidou, Pal Schmitt, Serge Suanez, Robert Weiss

Résumé

De récentes observations sur le terrain ont prouvé que de très larges blocs cyclopéens peuvent être déplacés par des vagues lors de puissantes tempêtes. Ces observations sont à contre courant d’une hypothèse majeure partagée par la communauté scientifique, à savoir que seules les vagues d’un tsunami seraient capable de déplacer de tels blocs. Avec l’intensification attendue des tempêtes en bord de mer comme conséquence du réchauffement climatique, il est essentiel d’approfondir nos connaissances sur le mouvement de ces blocs pour mieux quantifier les états de mer extrêmes. Le but du séminaire a été de mettre en commun des connaissances provenant de différents domaines scientifiques pour identifier les zones d’ombre à éclaircir. Le séminaire a réuni vingt scientifiques de six nationalités différentes autour de disciplines comme la géologie, la géomorphologie, la géophysique, l’océanographie, l’ingénierie côtière, la mécanique des fluides, l’informatique appliquée et les mathématiques appliquées. Plusieurs axes prioritaires de recherche ont été identifiés, dont notamment la nécessité d’obtenir plus de données expérimentales et plus de données sur le terrain, en particulier sur le déplacement de ces très larges blocs par des vagues. Obtenir des données de meilleure qualité est fondamental pour fournir une base fiable sur laquelle les modèles théoriques et numériques peuvent s’appuyer pour quantifier les diverses dynamiques mises en jeu.

 

Compte-rendu (en anglais)

What is known and what is unknown in boulder transport dynamics?

Field measurements

Extensive observations at boulder sites, before and after storm or tsunami events, are important to identify the conditions under which boulders are dislodged and moved (Serge Suanez, Raphaël Paris). The accumulation of boulders to form organised ridges, which has previously been interpreted as a sign of tsunami activity, is observed at many sites and is more likely to be the result of storms (Ronan Autret). Tracking large numbers of individual boulders permits derivation of key characteristics and relationships, which can help with the prediction of boulder movement (Kazuhisa Goto). In relation to the Aran Islands, the last documented large tsunami in the Atlantic Ocean basin is the Lisbon 1755 event (Daniel Giles), and in a period devoid of tsunamis, very large boulders have been shown to be moved by storm waves (Rónadh Cox).

A major difficulty in understanding CBD history is the inability to accurately date the boulder deposits, or even to relate boulder movements to specific storms. Even at well-studied sites, the time of boulder extraction/movement is constrained to within a season at best. The gold standard would be identification of the specific wave or set of waves responsible for the boulder movement. Field experiments with small (few kg) boulders (Mary Bourke) illustrate the small-scale dynamics of clasts on high-energy coasts. Rapid-response deployment of strategically placed cameras (Claire Earlie) reveals both the power and the chaos of a full Atlantic storm hitting the shore. It is this energy and unpredictability that make instrumentation and direct measurement of storm hydrodynamics so difficult.

Hydrodynamic modelling

Boulder extraction and emplacement involves three phases; dislodgement from bedrock (James Herterich), initiation of movement (Robert Weiss) and transportation. Mathematical treatments, however, require assumptions about some of the controling factors. Lack of appropriate consideration of the uncertainties in parameters such as coefficients of lift and drag, however, has resulted in problematic interpretations that persist in the literature. Simplistic equations, based on assumptions about the Froude characteristics of tsunami and storm waves, have produced unreliable measures of their relative transport competencies (Andrew Kennedy). Application of these equations has led to a perception that storm waves are incapable of transporting large boulder masses. But it is clear from the Aran Islands data alone that these interpretations are incorrect.

Essential to understanding transport hydrodynamics is proper characterisation of the fluid state. Complex interactions between waves may occur, and also between the waves and the shoaling bathymetry. These will control whether the wave is broken or not when it impacts the platform (Alison Raby). The onset of breaking waves has been extensively studied, and certain relations between local wave steepness and breaking have been derived (Frédéric Dias). Further, complex non-linear interactions resulting in rogue waves (Christian Kharif) in the near-shore need also be considered. Without in-situ measurements (videos) of the boulders it is difficult to decipher the important hydrodynamical regimes, but recent wave-tank experiments confirm that storm waves can produce such deposits (Rónadh Cox), and placing robust pressure sensors in the intertidal zone may help our understanding of flow depths in relation to offshore wave characteristics (Pal Schmitt).

Numerical modelling

The numerical modelling community has made great strides in modeling the sea states for storms (Fabrice Ardhuin, Nicole Beisiegel) and tsunami (Jörn Behrens), and can now also produce probabilistic hazard maps (Dimitra Salmanidou). Current models are well adapted for working at multiple spatial scales (Jörn Behrens). However, it is numerically difficult to bridge the gap from offshore waves to onshore inundation, and capturing the physics of wave breaking is also still a challenge. Validation test cases involving inundation and breaking are essential for improvement of numerical models and confidence in their results.

Future work

This multidisciplinary group convened to synthesise the state of the art, and to identify both the knowns and unknowns of boulder transportation by waves. Greater collaboration across the disciplines is essential. Outstanding questions remain in all areas, but the single most pressing issue is the lack of high quality field and experimental data, collected seasonally or in response to individual storms. Such data are fundamental to most of the issues raised above, from constraining time of extraction and movement to introduction of a numerical validation test suite. The group plans continued collaboration focused on the pertinent questions raised, which will improve the understanding of boulder motion, past wave climates, and coastal risk assessments.

Acknowledgements

The group sincerely thanks the Foundation for facilitating this excellent week. The setting was ideal for inducing discussions and collaborations and everyone gained immensely from the experience. The group is also very grateful to Christophe Morhange and François Sabatier of CEREGE for taking the time to lead a field excursion to boulder deposits in nearby Martigues. It was an excellent and most informative trip, especially because the modellers were finally able to see ‘boulders’, for real. 

Communications

SESSION I. COASTAL BOULDER DEPOSITS

• Serge Suanez – 40 years of research on boulder dynamics along the Brittany coast
• Ronan Autret – Boulder transport by wind-generated waves in SW Iceland
• Raphaël Paris – Supratidal boulders on historical lava deltas of the eastern coast of Reunion Island.
• Robert Weiss – Towards probabilistic inversions for wave conditions from boulder deposits
• Rónadh Cox – Using coastal boulder deposits to understand how wave energies are translated inshore

SESSION II. WAVE BEHAVIOUR AT COASTS

• Fabrice Ardhuin – Extreme sea states in coastal regions: how large can the waves be?
• Alison Raby – Modelling of extreme waves in the coastal zone
• Christian Kharif – Extreme sea waves in the vicinity of a vertical wall
• Pal Schmitt – Experimental extreme wave measurements in the intertidal zone
• Claire Earlie – The importance of incorporating short-term morphology and hydrodynamics into the longer-term context of cliff erosion

SESSION III. MODELING WAVES AND SURGES

• Jörn Behrens – Adaptive numerical methods for efficient coastal inundation simulations
• Frederic Dias – Wave breaking onset and strength
• Nicole Beisiegel – Numerical simulation of storm surges in Ireland
• Daniel Giles – Numerical study on the impact of the Lisbon 1755 tsunami on the Aran Islands
• Dimitra Salmanidou – Statistical emulation for uncertainty quantification of tsunami hazard

SESSION IV. TRANSPORT DYNAMICS

• Andrew Kennedy – Knowns, unknowns, and uncertainties in boulder transport dynamics
• Kazuhisa Goto – Boulders moved and not moved by the 2011 Tohoku-oki tsunami
• James Herterich – Hydrodynamic models of cliff-top boulder creation and transport
• Mary Bourke – Clast transport dynamics on an intertidal rock shore platform

More info:

https://www.frontiersin.org/articles/10.3389/fmars.2020.00004/full

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