Journal of Coastal Research, Special Issue No. 65, 2013 606 Vila-Concejo, et al. Coral reef sediment dynamics: evidence of sand-apron evolution on a daily and decadal scale Ana Vila-Concejo†, Daniel L Harris†, Amelia M Shannon†, Jody M Webster†, Hannah E Power† †Geocoastal Research Group. School of Geosciences, The University of Sydney, Australia ana.vilaconcejo@sydney.edu.au daniel.harris@sydney.edu.au asha8429@uni.sydney.edu.au jody.webster@sydney.edu.au hannah.power@sydney.edu.au INTRODUCTION Coral reefs are highly complex systems where biological, physical, and chemical processes continually interact over a broad range of spatial and temporal scales (Hatcher 1990). Different branches of science take a differing view on the concept of stability on coral reefs: while ecologists maintain that coral reefs are fragile environments, geologists portray them as robust landforms (Done 1992; Perry et al. 2011a; Perry et al. 2011b). However, the concept of stability depends largely on the spatio- temporal domain used for analysis, as reefs can appear both as resilient landforms over geological timescales, or fragile systems due to rapid changes over ecological timescales (Leon and Woodroffe 2011). In this context, the need for an understanding of reef evolution at geomorphological scales is vital as it can bridge the temporal and spatial gap between long-term geological and short-term ecological processes (Buddemeier and Smith 1988; Leon and Woodroffe 2011). Coral reef platforms are mostly composed of sedimentary deposits: coral sand and rubble. One example of these deposits is the ubiquitous sedimentary features called sand aprons (or sand sheets) that prograde into back reef areas, smothering living corals and altering the configuration of associated lagoons. Lagoonal infill is the major constructional process in coral reefs once they attain a stable elevation with respect to sea level (Marshall and Davies 1982). Sand is produced on the reef crest and transported into the back reef area or lagoon by waves and currents. This sediment often forms sand aprons which are progradational features and on average cover 20% of the reef platform (Rankey and Garza-Pérez 2012). Even though there is an understanding of the forcing mechanisms that drive sand apron formation, attempts to predict sand apron dimensions using parameters such as wave characteristics have proven unsuccessful (Rankey and Garza-Pérez 2012). Coral reef response to sea level rise depends on how the rates of coral growth compare with the rate of sea level rise. Traditional qualitative and descriptive studies have led to development of the “leaky bucket” theory (Purdy and Gischler 2005) by which reef platforms are expected to infill continuously with small percentages of sediment leaking out of the reef system, i.e., lost to the deeper sea floor surrounding the reef. Morphodynamics and contemporary processes on coral environments have been the subject of few studies that have investigated topics including morphodynamic growth of reef islands (Barry et al. 2007, 2008), wave transformation on coral reefs (e.g., Gourlay 1994; Nelson 1994; Gourlay and Colleter 2005), the geomorphology of reef islands (e.g., Woodroffe et al. 1999; Kench et al. 2006; Kench et al. 2008; Perry et al. 2011a), and physical processes (e.g., Frith and Mason 1986; Kench 1998; Wolanski 2001; Ogston et al. 2004; Hoeke et al. 2011). This paper presents evidence of sand apron evolution on a decadal and daily scale. The decadal scale is studied through remotely sensed images with which the changes in the sand apron are quantified. Daily evolution is analysed using hydrodynamic ABSTRACT 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 In: Conley, D.C., Masselink, G., Russell, P.E. and O’Hare, T.J. (eds.), Proceedings 12 th International Coastal Symposium (Plymouth, England), Journal of Coastal Research, Special Issue No. 65, pp. 606-611, ISSN 0749-0208. This paper investigates sand apron progradation on decadal and daily scales on a platform reef (One Tree Reef, OTR) located in the southern Great Barrier Reef. The decadal scale is addressed by analysing sand apron progradation using remotely sensed images (aerial photos and satellite imagery) coupled with wind data and cyclone events. The daily scale is addressed through a field campaign that was undertaken in September-October 2011. The campaign consisted of hydrodynamic measurements in three stations over the southern sand apron in OTR. It was found that while there was a small overall progradation over the last 31 years, the progradation had not occurred continuously or consistently along the entire sand apron. Additionally, the effect of cyclones was not clear on the decadal scale. On the daily scale, it was found that currents are generally weak (<0.4 m/s) and that currents during conditions at which suspended sediment is maximized are ocean-ward directed on the central part of the sand apron and lagoon-ward directed on the easternmost end. As such, daily sediment transport does not represent a gross contribution to lagoon infilling by sand apron progradation. Our results show that sand apron progradation does not occur continuously on the decadal or the daily scale. ADDITIONAL INDEX WORDS: sand apron progradation, sand sheet, coral sand, tropical geomorphology, coral reef morphodynamics www.JCRonline.org ____________________ DOI: 10.2112/SI65-103.1 received 07 December 2012; accepted 06 March 2013. © Coastal Education & Research Foundation 2013 www.cerf-jcr.org