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

• Published in: Aeolian research 2019-08, Vol.39, p.47-65
• Main Authors: Hilton, M.J., Borrie, D.R., Konlechner, T.M., Wakes, S.J., Lane, T.P., Kench, P.S., Kennedy, D.M., Aslam, Mohamed
• Format: Article
• Language: English
• Subjects: Aeolian sedimentation; Island formation; Maldives; Nebkha; Sand cay
• ISSN: 1875-9637; 2212-1684
• DOI: 10.1016/j.aeolia.2019.04.006

•The first high-frequency measurement of incident and near-surface wind flow over a lagoon sand cay in the Maldives.•The first to document aeolian sand transport on an equatorial atoll.•Dune development increased coral cay elevation over a 12 month period.•Aeolian sedimentation occurs during episodes of surface-based density currents under cumulonimbus clouds.•Aeolian sedimentation is enhanced by flow acceleration over beach-scarps. 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.