Elsevier

Aeolian Research

Volume 39, August 2019, Pages 47-65
Aeolian Research

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

https://doi.org/10.1016/j.aeolia.2019.04.006Get rights and content

Highlights

  • 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.

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.

Introduction

The Maldives is host to approximately 1200 islands located on reef platform surfaces, of which 200 are inhabited (Kench, 2011). The islands are found in two distinct depositional contexts: on the peripheral reef rim of atolls, where the largest islands are situated, and on atoll lagoon reef platforms. All islands are composed of carbonate sand and gravel derived from the surrounding reef. They are typically small in aerial extent and have a mean elevation of less than 1 m above sea-level. Studies of the formation of reef islands in the Maldives have primarily focussed on lagoon islands and have shown the islands are mid-Holocene in age, having accreted vertically as the reef platform grew (Kench et al., 2005, Perry et al., 2012). The formation of the larger vegetated islands occurred during the latter stages of Holocene sea-level rise and its subsequent fall to present level (Kench et al., 2005, East et al., 2018). However, the processes that lead to the emergence of lagoon sand cays above the limit of tides, and the formation of stable islands, both in the Maldives and atolls elsewhere, have not been resolved.

The precondition for island formation, the accumulation of sand as a subtidal sand cay, results from wave processes that transport sediment across the reef flat to a nodal depocentre (Gourlay, 1988, Mandlier and Kench, 2012). Swash processes subsequently control the vertical limit of island building in many reef locations worldwide (e.g. McKoy et al., 2010). In the Maldives, sediment transport fluxes are modulated by seasonal energy gradients, with rapid morphological adjustments occurring in response to monsoonal reversals in wind and wave patterns (Kench and Brander, 2006). At the event scale, extreme waves also impact the islands. For example, the 2004 Sumatran tsunami promoted minor island erosion, but also transferred sediments from beaches to island surfaces. This overwash was able to vertically build the margins of reef islands by up to 0.3 m (Kench et al., 2006b, Kench et al., 2006a).

The potential for aeolian processes to contribute to island formation in the Maldives has not been recognised or assessed. In general, the role of aeolian processes and their potential contribution to island formation in the humid tropics has been underestimated (Hesp, 2008). At lower latitudes, in the southern Indian Ocean, aeolian sedimentation has been shown to contribute to coral island topography, including the Glorioso Islands (Bayne et al., 1970); Tromelin Island (Marriner et al., 2010); the Chagos Group (especially Diego Garcia; Stoddart and Taylor, 1971); and the Cocos (Keeling) Islands (Woodroffe and McLean,1994). A variety of dune forms are reported, including transgressive and sheets, parabolic forms and nebkha. Nebkha are low dunes, convex in profile, formed around vegetation (Melton, 1940). Larger, transgressive dunes reach 11 m in elevation on South Island in the Cocos (Keeling) Islands (Woodroffe, 2008). Parabolic dunes and blowouts are also found in the western Indian Ocean mid-latitude (10°–25° S) and these are closely aligned with the Southeast Trade winds that persist throughout the year (Schotte and McCreary, 2001). On Tromelin Island wind speeds exceeding 8 m s1 are contained in a narrow directional window between 100° and 140° (Marriner et al., 2010). In contrast, the equatorial region of the Indian Ocean, including the Maldives, is an area of relatively low mean wind stress and aeolian sedimentation has not been reported. Indeed, several conditions combine to lower expectations of aeolian activity including the equatorial climate (high humidity, high rainfall, low reported wind speeds); low topography and narrow beach width; and dense vegetation cover on many established islands.

This study arose from the observation of aeolian ripples, shadow dunes and nebkha on Maaodegalaa sand cay by the authors during fieldwork in February 2017. Considering most lagoon sand cays are little more than 1 m above the reach of spring tides, any dune development may significantly contribute to island relief. Remarkably, the key process mechanisms that build islands above sea-level, including the development of a stable cay surface, which can then be colonised by plants, remain to be investigated. Sand cays and islands occur in a range of forms in the atoll lagoons of the Maldives, from submerged sand cays, to recently emerged cays colonised by early successional vegetation, to stable and forested islands. The current paper reports the first measurements of aeolian sedimentation in the Maldives on a low-lying sand cay (Maaodegalaa) in Huvadhoo Atoll. We aim to (i) measure incident and near-surface wind flow and associated aeolian sand transport over a recently emerged sand cay; (ii) examine spatial variations in patterns of sedimentation, particularly processes of flow acceleration and sediment transport over beach scarps; and (iii) consider the implications of aeolian sedimentation for island formation.

Section snippets

Regional setting

The Maldives comprises a double chain of atolls which extends almost 900 km, from 6° 57′ N latitude, to just south of the equator (0° 34′ S latitude). Huvadhoo atoll, just north of the equator, is the largest atoll in the Maldives (Fig. 1), with an area of 3279 km2 and maximum dimensions of 80 km (north–south) and 60 km (west-east). The rim of the atoll is defined by reef platforms and islands, broken by multiple deep channels. The atoll lagoon, which attains water depths of 80 m, contains (i)

Materials and methods

The current study reports observations and measurements of aeolian sedimentation on Maaodegalaa sand cay, Huvadhoo Atoll, over an 8-day period in February 2018 during the northeast monsoon and island accretion and dune development over a 12 month period (January 2017 – February 2018). The lagoon islands of the Maldives are comprised of biogenic materials (Liang et al., 2016) but there was no existing information on the textural characteristics of Maaodegalaa. Surface sediment samples (0–0.05 m)

Sediments

The island is composed of fine to medium, moderately-well sorted, reef carbonates. Surface samples from the reef platform, obtained approximately 5 m from the toe of the beach, are relatively coarse and less well sorted (mean grain size = 0.42 +/- 0.65 phi units), compared with the intertidal beach at mean tide level (0.96 +/- 0.57 phi) and the supratidal island surface (1.34 +/-0.51 phi). Visual inspection of the sediments indicated they were composed of almost entirely of coral species, with

Discussion

The current paper presents the first high-frequency observations of wind flow and aeolian sedimentation in the Maldives during the Iruvai Monsoon. In general, these winds did not generate near-bed flows of sufficient speed to transport sand. However, the experimental period was punctuated by two high wind speed events that generated near-bed speeds in excess of the sediment threshold and we measured saltation and observed the formation of aeolian ripples and shadow dune forms. We hypothesize

Conclusions

Aeolian sand transport of fine to medium, well-sorted, coral sands was recorded on Maaodegalaa sand cay during two wind events. This is the first high-frequency measurement of this process in the Maldives and in an equatorial lagoon island setting. It is likely this process occurs frequently and in a range of geomorphic settings in Huvadhoo Atoll and in other atolls, including low sand terraces on established islands. Reptation and aeolian ripple development probably occurs frequently when

Funding sources

University of Otago, New Zealand, University of Auckland, New Zealand.

Acknowledgements

Permission to conduct research in the Republic of the Maldives was kindly granted by the Ministry of Agriculture and Fisheries. Mr Chris Garden executed the cartography with patience and care. The first author thanks the School of Geography, University of Melbourne, for hospitality and laboratory facilities during his recent sabbatical during which this manuscript was prepared. The University of Otago provided financial support for fieldwork in the Maldives in 2017 and 2018. Finally, shukuriyaa

References (42)

  • P.L. Woodworth

    Have there been large recent sea-level changes in the Maldive Islands? Global Planet

    Change

    (2005)
  • C.J. Bayne et al.

    Geography and ecology of cosmoledo atoll

    Atoll Res. Bull.

    (1970)
  • J.A. Church et al.

    Estimates of the regional distribution of sea-level rise over the 1950–2000 period

    J. Climate

    (2004)
  • H.K. East et al.

    Coral Reef Island initiation and development under higher than present sea levels

    Geophys. Res. Lett.

    (2018)
  • Z. Feng et al.

    Mechanisms of convective cloud organization by cold pools over the tropical warm ocean during the AMIE/DYNAMO field campaign

    J. Adv. Model. Earth Syst.

    (2015)
  • P.G. Flood et al.

    Coral cay instability and species-turnover of plants at Swain Reefs, southern Great Barrier Reef, Australia

    J. Coast. Res.

    (1986)
  • Gourlay, M.R., 1988. Coral cays: products of wave action and geological processes in a biogenic environment. In:...
  • S.A. Harangozo

    Flooding in the Maldives and its implications for the global sea-level rise debate. Sea level changes: determination and effects

    Geophys. Mono.

    (1992)
  • P.A. Hesp

    Coastal dunes in the tropics and temperate regions: location, formation, morphology and vegetation processes

  • P.A. Hesp et al.

    Storm wind flow over a foredune, Prince Edward Island, Canada

    J. Coastal Res. SI

    (2009)
  • P.A. Hesp et al.

    Jet flow over foredunes

    Earth Surf. Proc. Land.

    (2016)
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