A first evaluation of the contribution of aeolian sand transport to lagoon island accretion in the Maldives

Abstract

Aeolian sedimentation and dune development have not been reported from coral atolls at equatorial latitudes. This study presents high-frequency measurements of incident and near surface wind flow and aeolian sand transport on a lagoon sand cay (Maaodegalaa) in the Maldives. Sonic anemometers and Wenglor™ particle counters were operated at 1 Hz for 8 days during the Iruvai monsoon in February 2018. Sand traps were deployed to estimate sand flux and island topography and vegetation cover were surveyed using UAV (un-manned aerial vehicle) photogrammetry and a laser level (in 2017 and 2018). Flow over beach scarps is modelled using computational fluid dynamics. Maaodegalaa sand cay reaches just 0.9 m above the highest spring high tides. Nebkha, between 0.10 and 0.40 m high, are widespread and are associated with Scaevola taccada and Cyperus conglomeratus. Between 2017 and 2018 the eastern section of the sand cay accreted 0.3 m following Cyperus colonisation. Reptation and aeolian ripple development occurred during fieldwork when near-surface flows exceeded 6 ms−1. Saltation occurred at higher wind speeds (8 ms−1). The highest rates of sand transport occurred during north-east incident winds of 12 ms−1 (at 6 m), that were probably generated by surface-based density currents under cumulonimbus clouds. Spatially, higher rates of sand transport were recorded downwind of a beach scarp, probably forced by flow acceleration. We propose a conceptual model of lagoon island formation, with both over-wash and aeolian sedimentation contributing to island accretion. A period of aeolian sedimentation may be critical to the emergence of sand cays.