Dynamics of Carbonate Sands and Morphodynamics of Coral Environments

ARC FT100100215

Coral reef platforms are mainly composed of sedimentary deposits- coral sand and rubble. These deposits are mobile and their changing morphologies can influence the characteristics of the living regions of coral reefs and other biota. One example of these deposits is the sand aprons, also known as sand sheets, that prograde (advance) into back reef areas, smothering living corals and altering the configuration of back reef lagoons. Although little is known about the mechanisms and rates of sand movement, it is highly likely that climate change will impact on these patterns. Understanding how and why sediments move on the reefs is crucial to managing and predicting the effects of climate change, and will allow us to prepare mitigation measures.

Rubble coral islands are also sedimentary deposits; while it is commonly accepted that they were formed as storm deposits, the magnitude of the events creating them remains unknown. Rubble coral islands are dynamic features that respond to wave energy, understanding the events that modify them will inform future management under changing climate.

More than 90% of the wave energy dissipates before reaching the lagoons and structures such as spur and groove (SaG) are believed to play a crucial role. While some studies exist on SaG, their formation and evolution and relation to wave climate is far from clear.

Who are we?

We are a team from the Geocoastal Research Group of the School of Geosciences. The research is led by Dr Ana Vila-Concejo and Dr Jody Webster with a group of research students and collaborators including Dr Dan Harris. Ms Steph Duce is currently doing her PhD studies on the morphodynamics of spur and groove.

Objectives

  • To investigate why sand aprons are ubiquitous features that are not always oriented in the direction of prevailing forcing, and what are the other control mechanisms.
  • To analyse the hydrodynamic forcing mechanisms that control the morphodynamics of sand aprons, with particular attention to the role of high-energy episodes.
  • To determine whether sand aprons reach dynamic equilibrium, are continuously infilling, or undergo shorter-term cycles of erosion/accretion.
  • To adapt existing sediment transport equations developed for siliciclastic (quartz/continental) sediments to represent the transport of coral sand.
  • To provide an interpretative basis for the processes associated to sandy facies in coral reef cores using the modern analogue.

One Tree Reef

This is the first of our study sites, we expect to study other sand aprons on other reefs in the near future. The University of Sydney operates a Research Station in One Tree Reef: One Tree Island Research Station. More information on the site can be found here.

One Tree Reef is a platform reef located in the southern Great Barrier Reef. It is located only 20 km landward of the edge of the continental shelf; it is therefore considered a high-energy reef. There are three lagoons and our studies so far have been restricted to the largest one, First lagoon.

First lagoon (Figure 1) has two main prograding sedimentary features:

  • On the eastern flank there is a well-developed rubble flat and a subtidal sand apron.
  • On the southern flank there is a large subtidal sand apron, larger and shallower than the one on the eastern side.

Spur and groove features are present all around the reef but are more prominent on the eastern flank. One Tree Island is a rubble island on the southeastern corner of the reef.

Satellite image from 2009 showing One Tree Reef and studied sites.

Figure 1: Satellite image from 2009 (World View 2) showing One Tree reef, First lagoon and its main sedimentary deposits.

Publications

Peer-review papers

  1. Vila-Concejo, A., Harris, D.L., Power, H.E., Shannon, A.M., Webster, J.M., Sediment transport and mixing depth on a coral reef sand apron. Geomorphology (in press, available online since 18.Oct.2013).
  2. Harris, D.L., Vila-Concejo, A., Webster, J.M., Geomorphology and sediment transport on a submerged back-reef sand apron: One Tree Reef, Great Barrier Reef. Geomorphology (in press 13.03.2014).
  3. Duce, S., Vila-Concejo, A., Hamylton, S., Eleanor, B.,Webster, J.M, Spur and groove distribution and relationship to relative wave exposure, Southern Great Barrier Reef, Australia. Journal of Coastal Research, SI(66) (in press 4.Jan.2014)
  4. Shannon, A.M., Power, H.E., Webster, J.M. and Vila-Concejo, A., 2013. Evolution of coral rubble deposits on a reef platform as detected by remote sensing. Remote Sensing, 5(1): 1-18.
  5. Vila-Concejo, A., Harris, D.L., Shannon, A.M., Webster, J.M. and Power, H.E., 2013. Coral reef sediment dynamics: evidence of sand-apron evolution on a daily and decadal scale. Journal of Coastal Research, SI(65): 606-611.
  6. Harris, D.L. and Vila-Concejo, A., 2013. Wave transformation on a coral reef rubble platform. Journal of Coastal Research, SI(65): 506-510.
  7. Harris, DL, Webster, JM, de Carli, EV, Vila-Concejo, A, 2011. Geomorphology and morphodynamics of a sand apron, One Tree Reef, Southern Great Barrier Reef. Journal of Coastal Research SI64: 760-764.

Theses and internal reports (available under request)

  1. Harris, DL, 2013. Physical processes and morphodynamics of coral reefs. Ph.D. Thesis, The University of Sydney, Sydney, NSW, Australia.
  2. Thornborough, KJ, 2012. Rubble-Dominated Reef Flat Processes and Development: Evidence from One Tree Reef, Southern Great Barrier Reef. Ph.D. Thesis, The University of Sydney, Sydney, NSW, Australia.
  3. Nott, L, 2012. Comparison of sediment analyses to characterise carbonate sediments: Implications for sediment transport and distribution on coral reef environments. Honours Thesis, School of Geosciences, University of Sydney.
  4. Shannon, A, 2012. Application of remote sensing to establish the geomorphic evolution of a coral reef platform on a decadal scale: A case study of One Tree Reef. Honours Thesis, School of Geosciences, University of Sydney.
  5. Dougherty, S, 2011. Sediment dynamics of two carbonate sand sheets on One Tree Reef: a comparison of the sediment composition and taphonomy of foraminifera on the eastern and southern sand sheets. Internal Report. Geocoastal Research Group, The University of Sydney.
  6. Parry, AH, 2010. Sediment dynamics on One Tree Reef: Investigation of the sediment variability of the eastern sand sheet. Internal Report. Geocoastal Research Group, The University of Sydney.
  7. Carli, E, 2010. Sediment dynamics on One Tree Reef, Australia: Investigating the controls behind sediment variability and evolution of the southern sandsheet. Honours Thesis, School of Geosciences, University of Sydney.

Fieldwork Campaigns

2010

This was our preliminary year. The main campaign took place in April-May 2010 and the focus was to obtain a baseline topographic and bathymetric map, surficial sediment samples and vibro cores on the southern sand apron (Figure 2). This was also the year that we took our first hydrodynamic measurements on the sand apron. Later on that year more surficial sediment samples were also obtained on the eastern sand sheet. Figure 3 shows a summary of the measurements and samples taken in 2010.

Fieldwork photos from 2010. Coring.

Figure 2: Fieldwork photos from 2010, vibro-coring.

2011

Our main campaign of 2011 was in September-October, coincident with the largest tidal ranges. We wanted to analyse the effect of maximal tidal currents and wave propagation over those conditions. Aside from continuing with topographic and bathymetric measurements, we established three hydrodynamic stations where currents and waves were simultaneously measured; one of the stations was also fitted with OBS instruments measuring suspended sediment. We also measured mixing depth next to each of the stations. Hydrodynamic measurements also included transects of wave sensors to study wave propagation both on the southern and the eastern flanks simultaneously and an ADCP with waves capability that was deployed in the lagoon for over 6 months. During this campaign there was also a lot of satellite imagery groundtruthing work and collection of foraminifera samples on the southern algal rim. A summary of the hydrodynamic measurements and samples obtained in 2011 is in Figure 3; photos can be found in Figure 4.

GIS image of OTI showing a summary of the locations for hydrodynamic measurements and sampling.

Figure 3: Summary of the locations of the hydrodynamic measurements and sampling undertaken during fieldwork in 2010 and 2011. Background image from 2009 (WorldView 2).

6 photos taken during fieldwork: bathymetry, groudntruthing, LBF, hydro, arriving back.

Figure 4: Fieldwork photos taken in 2011.

2012

The campaign of 2012 was again mostly focused on the southern sand apron. We deployed two Sontek Hydras (waves, currents and suspended sediment) and set up two mixing depth experiments next to each of the Hydras. One mixing depth experiment was open and the other was surrounded by wire to avoid bioturbation by holothurians. Pressure transducers were also deployed across-shore in line with the locations of the Hydras and also along-shore in the southern and eastern flanks.

2013

No major intensive campaigns were carried out in 2013 (as Vila-Concejo was on maternity leave). Steph Duce undertook hydrodynamic measurements, waves and currents, in some of the spur and grooves around One Tree Reef. Jody Webster and Steph Duce also did some remote sensing observations with a camera fitted to a kite; this was done in collaboration with Dr Mitch Bryson from the ACFR, Sydney University.

2014

So far this year, we have measured hydrodynamics on the spur and groove (Steph Duce).

We plan to visit OTIRS in December for another intensive experiment.

Acknowledgements

  • Australian Research Council Future Fellow FT100100215 - Vila-Concejo.
  • Women in Science Re-entry Fellowship, Faculty of Science, The University of Sydney - Vila-Concejo.
  • ANSTO grants - Harris.
  • SIMS (Sydney Institute of Marine Science) - ADCP loan.
  • Tom Baldock (University of Queensland) - Hydra loan.
  • Moninya Roughan and Jason Middleton (UNSW) - FSI loan.
  • Aurelien Leroy and Brewen Lecuyer - help with sediment analyses.