A. Y. Annie Lau

A preliminary study of the distribution, sizes and orientations of large reef-top coral boulders deposited by extreme waves at Makemo Atoll, French Polynesia

ABSTRACT Lau, A.Y.A., Etienne, S., Terry, J.P., Switzer, A.D., Lee, Y.S., 2014. A preliminary study of the distribution, sizes and orientations of large reef-top coral boulders deposited by extreme waves at Makemo Atoll, French Polynesia. In: Green, A.N. and Cooper, J.A.G. (eds.), Proceedings 13th International Coastal Symposium (Durban, South Africa), Journal of Coastal Research, Special Issue No. 70, pp. 272–277, ISSN 0749-0208. The history of extreme wave events on the Tuamotu Archipelago of French Polynesia in the central South Pacific remains poorly understood, even though huge wave deposited coastal boulders were identified in the area decades ago. Numerous large coral boulders deposited on the reef flats of Makemo Atoll (16.56°S, 143.73°W) were investigated in this study in an attempt to understand the characteristics of extreme wave events in the region. The positions, dimensions and orientations of 286 boulders were measured along over 15 km of the northern coastline of the atoll. The biggest clast measures over 130 m3 in size and it weighs more than 310 tonnes. The size-distribution of the Makemo boulders suggests that these huge clasts were transported by extreme storm waves. The long-axes of boulders are mostly aligned parallel to sub-parallel to the shoreline. However, a relationship between the boulder size and orientation was not found, suggesting the orientation of boulders is not representative of boulder transport mode.

Uncertainties and Continuing Challenges with Interpreting Coastal Boulders

The significance of coastal boulders for high-energy marine inundation (HEMI) studies relies on the primary control of two influential environmental parameters: mechanisms of boulder generation and the source of the boulders themselves. Uncertainties inherent to natural boulders can occasionally be overcome by examining ‘anthropogenic boulders’ sourced from engineered coastal defence structures. However, distinguishing the very nature of HEMI events, i.e. whether storm or tsunami in origin, is still a contentious issue. Similarly, the intrinsic ability of boulder deposits to exhaustively capture all inundation events is highly debatable: as a resilient object, a boulder can be (re)mobilised by several successive events, yet at the same time, its gradual degradation precludes it from accurately recording the oldest (original) event that emplaced it. Future progress should be achieved through an improvement and a standardisation in the collection and presentation of coastal boulder data.

Outlook for Boulder Studies Within Tropical Geomorphology and Coastal Hazard Research

Reef-platform coral boulders are produced, transported and deposited during high-energy marine inundation events such as large storms or tsunamis. Documented for centuries as extraordinary features of the coastal landscape, these enigmatic boulders have recently proven invaluable indicators for characterising and interpreting marine erosion and transport processes on shorelines. As such, the examination of boulder deposits has become increasingly applicable to coastal hazard and risk assessment studies, although a number of challenges remain unresolved. Future prospects are optimistic for improving boulder analysis, within the broader scope of developing multi-proxy approaches for investigating the impacts of high-magnitude inundation events on coasts.

The Scientific Value of Reef-Platform Boulders for Interpreting Coastal Hazards

Coral boulders are one of the signatures of high-energy marine inundation along tropical coastlines. Data derived from boulders may include age, event frequency, inundation direction and event intensity. The latter is approached via hydrodynamic transport equations, i.e. calculation of the minimum wave energy (linked to wave height or flow velocity) required for boulder transport. However, establishing hydrodynamic models for the transport of coastal boulders involves some simplification of wave properties and transport mechanisms. Accurate dating of HEMI events through coral boulder age-dating can be achieved with traditional dating techniques (air photos, radiocarbon, uranium-series, ESR), but it also raises several challenges linked to the fundamental nature of the boulder, specifically the age of death of the corals comprising the limestone fabric.

Case Study: Coral Boulder Fields on Taveuni Island Coasts, Fiji

The coastline of Taveuni Island in the southwest Pacific was struck by a category-4 tropical cyclone in March 2010. Post-storm field investigations of the coastal geomorphic impacts concentrated on an area in central Taveuni where protected fringing reefs and coastlines form part of the Bouma National Heritage Park. Here, a range of cyclone constructional imprints were found to have supplemented existing coastal sediments. Fresh coral boulders strewn across reef platforms indicate that TC Tomas had sufficient power to deliver new coral blocks, but that this material comprises a relatively minor component of pre-existing boulder fields. Comparison between the dimensions of fresh and older blocks reveals that unknown earlier events (storms or tsunamis) produced much larger debris and therefore presumably generated more energetic flow velocities across the fringing reefs than TC Tomas did. Analysis of calcarenite slabs quarried from in situ beachrock exposures was also particularly useful for calculating surging flow velocities at the shoreline.

Historical Review and Changing Terminology

Coastal boulders are often prominent features in the coastal landscape, sometimes mapped on nautical charts and named by local people. The vernacular names occasionally furnish vital clues to identify the original event that created the boulders. Coral boulders were first mentioned in the literature two centuries ago when scientists started to explore the Great Barrier Reef. The significance of coral boulders has long fuelled scientific debate, interpreted by some as remnants of former elevated reefs or by others as an inheritance from the action of past storm waves. For more than a century ‘negro-head’ was the seminal expression used to portray the emerged black-coloured rocks observed on reef platforms. Fortunately, this inappropriate choice became outmoded during the twentieth century and new terms have since been employed. Despite the existence of a specified grain-size scale to define large clasts, inconsistent nomenclature and the plethora of synonyms now in use causes some confusion.

Coastal Boulders: Introduction and Scope

Boulders represent a singular class of sediment, encountered in very diverse geomorphic systems. In coastal areas, they may appear as isolated exotic deposits or as a part of larger constructional features. Owing to their size, boulders are less easily reworked than finer sediments and their presence has become increasingly recognised as an important signature for high-energy marine inundation (HEMI) events. They therefore represent a valuable subject for natural hazard studies. This is especially evident along tropical coasts where coral reefs may provide large quantities of boulders that accumulate on the reef flat or farther inland. This volume explores the significance of a particular subset of coastal boulders, namely reef-platform coral boulders. While these geomorphological objects have been scrutinised in the last decade following catastrophic events like the 2004 Indian Ocean tsunami, they actually bear a much longer history in terms of scientific interest stretching back over two centuries.