Chronologie des événements de submersion marine extrêmes dans les Tuamotu. Intérêt de l'étude des blocs récifaux.

Numerous large carbonate boulders up to 164 tonnes in mass were investigated on the reef flat and beaches of Makemo Atoll in the Tuamotu Archipelago of French Polynesia to reveal the past occurrence and to anticipate the future potential threat of extreme wave events, possibly generated by tropical cyclones and tsunamis. The modern reef edge and emerged mid-Holocene coastal landforms were identified as sources of boulders mobilized during extreme wave events in the past. The minimum flow velocities produced by extreme wave events were estimated to exceed 5.4–15.7 m/s at the reef edge on different parts of the atoll. Comparison of uranium–thorium ages of boulder coral fabric with written historical records indicates that two large boulders (77 and 68 tonnes) were possibly emplaced on the reef flat by a powerful cyclone in February 1878. Although most boulder dates are older than the earliest historical cyclone and tsunami records in French Polynesia, their ages concur with the following: (a) periods of “storminess” (i.e. increased cyclone activity compared to today) in the central South Pacific over the last millennium; and (b) periods of high sea-surface temperature (SST) at the Great Barrier Reef, possibly associated with higher-than-normal SSTs Pacific-wide that facilitated the generation of cyclones affecting the central South Pacific Ocean. None of the boulders on Makemo were dated younger than CE1900, implying that the last century has not experienced extreme waves of similar magnitude in the past. Nevertheless, the findings suggest that waves of comparable magnitudes to those that have transported large boulders on Makemo may recur in the Tuamotus and threaten island coasts across the central South Pacific in the future.

A.Y. Annie Lau, James P. Terry, Alan D. Ziegler, Adam D. Switzer, Yingsin Lee, Samuel Etienne. 2016. Understanding the history of extreme wave events in the Tuamotu Archipelago of French Polynesia from large carbonate boulders on Makemo Atoll, with implications for future threats in the central South Pacific. Marine Geology. 22 April 2016.
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Conférence - Risques de submersion, archives sédimentaires, mémoire collective. LabEx Corail

Samuel ETIENNE et Rémy CANAVESIO - La connaissance du risque de submersion marine dans l'archipel des Tuamotu: complémentarité des approches géomorphologiques et ethno-géographiques. Colloque du Laboratoire d’Excellence CORAIL

Paris, Aquarium de la Porte Dorée, 7-9 septembre 2015 

Mégablocs coralliens d'origine cyclonique à Anna, Tuamotu.

Publication - 13th International Coastal Symposium - Durban, Afrique du Sud, 13-19 avril 2014

Les résultats de l'étude menées sur les dépôts de blocs coralliens sur le platier récifal de Makemo seront présentés au congrès ICS2014 qui se tient à Durban. Ces travaux ont été financés dans le cadre du Contrat de projet Etat-Territoire de la Polynésie française (programme RINALPOF). L'origine cyclonique des blocs est privilégiée au regard de leur distribution spatiale. Des datations U/Th permettent de montrer qu'au moins une dizaine de cyclones importants ont touché Makemo au cours du dernier millénaire. Le détail des analyses et interprétations est publié dans Journal of Coastal Research. 

Article : ici

Publication - Les blocs coralliens des platiers récifaux.

L'éditeur Springer vient de publier un ouvrage co-écrit avec James Terry et Annie Lau portant sur l'étude des blocs coralliens que l'on rencontre parfois sur les platiers récifaux des littoraux tropicaux.

Reef-Platform Coral Boulders. Evidence for High-Energy Marine Inundation Events on Tropical Coastlines

The primary goal here is to present a treatise on the significance and value of coarse clastic carbonate sediments (i.e. large coral boulders) on tropical coastlines for understanding both modern and pre-historical (Holocene) high-magnitude marine inundation events.  There has been a rapid groundswell of interest in large carbonate blocks on tropical coasts over the last decade, yet it is not widely appreciated that such features were observed and recorded back in the early explorations of Matthew Flinders on the Great Barrier Reef in the 1800s. This book will illuminate how various characteristics of datable carbonate blocks torn up from coral reefs and deposited on reef platforms yield importance evidence about the storms and tsunamis that emplaced them over decadal and centennial timescales.  No comprehensive review has so far been published.  A need now exists for a ‘definitive reference’ on coral boulder research, which details the earliest observations, changing terminology, sedimentology, and relevance for coastal hazard research in the tropics.  A wide range of examples will be incorporated from across Asia, Australia, the Pacific and the Americas, as well as a full up-to-date review of the existing literature.
Préface de James Goff.

Voir la fiche sur le site de l'éditeur Springer

Un bloc corallien posé sur le platier d'un récif frangeant à Sumatra.
Il s'agit de l'un des plus volumineux blocs arrachés et transportés par le tsunami de l'océan Indien du 26 décembre 2004.

Contenu de l'ouvrage

Chapter 1  Coastal Boulders: Introduction and Scope1.1 Types of Coastal Boulders
1.2  Association with High-Energy Marine Inundation
1.3  Rationale for This Book
1.4  References 

Chapter 2 Historical Review and Changing Terminology
 2.1 Named Coastal Landmarks
 2.2  Earliest Scientific Observations of Coral Boulders
 • Box 1.  Matthew Flinders’ Observations of Coral Boulders on the Great Barrier Reef in 1802
2.3 The Eruption of Krakatau Volcano in 1883
2.4  Reef Remnants versus Storm Deposits: Competing Ideas, Early 1900s
2.5 Varying Expressions for Coral Boulders
2.6  Perspectives on Sediment Clast Size
2.7  References 

Chapter 3 The Scientific Value of Reef-Platform Boulders for Interpreting Coastal Hazards3.1  Introduction
3.2  Coastal Sedimentology Within Marine Inundation Research
3.3  Wave Energy Estimation
3.3.1  Boulder Transport Equations
• Box2. Hydrodynamic Equations for Coastal Boulder Transport (Nott, 2003)
• Box 3.  Revised Hydrodynamic Equations  (Nandasena et al., 2011)
• Box4. Boulder Displacement: Equations for Estimating Minimum Current Velocity and Wave Height (Frohlich et al., 2011)
3.3.2 Assumptions and Difficulties
3.4    Inundation Direction
3.5    Boulder Mapping
3.6    Dating Prehistorical Marine Inundations
3.6.1  Boulder Age-Dating
3.6.2   Obstacles to Accurate Dating
3.7       References 

Chapter 4  Uncertainties and Continuing Challenges with Interpreting Coastal Boulders     

4.1 Introduction
4.2 Mechanisms of Coral Boulder Generation
4.3  Identifying Original Sources for Carbonate Boulders on Reefs
4.4  ‘Anthropogenic Boulders’: Advantages of Studying Boulders Sourced from Rip-Rap
4.5  Distinguishing Between Storm and Tsunami Boulders
• Box 5: Equation to Distinguish Storm from Tsunami Boulders (Lorang, 2011)
4.6 Undetectable Marine Inundation Events
4.7  Boulder Reworking by Backwash or Subsequent Events
4.8  Longevity of Boulders
4.9  Issues with Data Collection and Presentation
4.9.1 Volumetric Calculations4.9.2  Inconsistent Data
• Box 6: Nature of Boulder Data
4.10  References 

Chapter 5  Case Study: Coastal Boulder Fields on Taveuni Island Coasts, Fiji5.1 Introduction and Aims
5.2 Background to the Study Area
5.3  Features of Tropical Cyclone Tomas, March 2010
5.4  Field Procedures and Observations
5.5  Results and Discussion
5.3  Boulder Quarrying and Remobilisation
5.4  Transport Mechanisms and Flow Velocities
5.5   Caveats to Findings
5.6   Conclusions
5.7   References 

Chapter 6  Outlook for Boulder Studies Within Tropical Geomorphology and Coastal Research6.1  Brief Summary: Current Understanding, Guiding Questions
6.2  Future Prospects and Recommendations
6.3  References

RINALPOF 1 - Mission à Makemo, archipel des Tuamotu

La première mission du programme RINALPOF a eu lieu à Makemo, archipel des Tuamotu, du 8 au 15 décembre en compagnie d'Annie LAU (doctorante N.U.S.) et Ying Sin LEE (E.O.S.). L'objectif principal était une cartographie des champs de blocs cyclopéens (certains dépassent largement les 100 m3) présents sur les platiers récifaux de la moitié est de l'atoll. Le traitement des données permettra d'estimer la hauteur et la vitesse des vagues ayant déferlé sur cet atoll.