Beach development on an uplifted coral atoll: Niue, south west Pacific

- Beaches can accumulate on uplifted coral atolls such as Niue and generally form within embayments.
- Beaches form at the rear of erosional shore platforms and sit on top of abrasion ramps.
- The potential for sediment transport is high with beaches affected by waves each tidal cycle.
- The benthic carbonate communities on the platform immediately in front of the beach are the sediment source.
- These beaches tend to be ephemeral over a decadal scale under current climatic conditions.

Niue is an uplifted coral atoll in the south western Pacific characterised by erosional terraces on its coastal margin. Beaches are found around the island located in pockets at the rear of erosional shore platforms. The beaches in Niue are < 100 m long, < 25 m wide and generally less than 0.5 m thick. The beaches sit on top of an abrasion ramp that dips seaward at a similar angle to the beach. The morphology, stability and sedimentology of these beaches are investigated through laser surveying, aerial photo analysis and petrographic techniques. Surveying was undertaken in 2008 and 2010 with data compared to previous work conducted in the 1990s in order to assess the controls on sediment deposition on uplifted coral atolls. There is a high potential for sediment transport on the island. The beaches are entirely removed during tropical cyclone events and even under calm conditions sediment is mobile. The restriction of beaches to pockets along the rocky coast suggests that these areas temporally interrupt sediment transport allowing beaches to form. All the beaches are composed of a typical chlorozoan assemblage of carbonate grains dominated by coral (20-50%), coralline algae (18%) and foraminifera (up to 81%). These sediments are produced on the platforms in the immediate vicinity of the beaches with little longshore transport between embayments being evident. The close relationship between source and depositional zones, combined with the high transport potential across the platforms indicates that the beaches are highly vulnerable to any change in either energy conditions or sediment supply.