Wave climate and sand apron development on the southern Great Barrier Reef

• Introduction

Sand aprons are ubiquitous depositional sedimentary features that offer insights into the sediment dynamics of coral reef environments. Global studies found that the extent of sand aprons are not related to reef platform size and their widths are a function of environmental factors such as swell period and height, tidal amplitude, latitude, and exposure to wind and waves (Rankey and Garza-Perez 2012).  Recent studies using numerical modelling have found that sand aprons in reef flats attain a critical water depth resulting in constant depth (Ortiz and Ashton 2019), and that lagoon infilling through sand apron progradation is a self-limiting process (Rankey 2021). Sand apron progradation is an eco-morphodynamic process and climate change, including intensification and increased frequency of marine heatwaves, ocean and coastal acidification, and changes in wave and tropical storm climates are triggering changes that need to be understood to inform sound management of coral reefs.

This paper presents data on the Holocene evolution of the sand aprons on 21 offshore platform reefs located on the southern Great Barrier Reef and how it can be used to infer past wave climates. We then present the recent wave climate for the study area (Smith et al. 2022) and analyse sand apron evolution accordingly.

• Methods

The sand aprons on 21 reefs located on the Capricorn Bunker Group (Southern Great Barrier Reef) were assessed from high-resolution satellite imagery, obtaining digital bathymetric models and digitizing the reef and lagoon contours. We then measured reef area and lagoon area to calculate the percentage of lagoon infilling. The wave climate for the study area was obtained from satellite altimetry using an open-source Python tool (Smith et al. 2020).

• Preliminary findings

Our results showed that the most important factor for lagoon infilling was the size of the reef, with larger reefs typically appearing less infilled than smaller reefs. Wave incidence seemed to be unimportant: the three reefs with less than 50% infill were all medium-sized and exposed to incident waves while all six protected reefs had infilling above 50%. While some authors had pointed out at relative sea-level changes to explain current sand apron stability (Harris et al. 2015), our results show that the self-limiting nature of the sand apron progradation, combined with relative sea-level changes, is a better explanation for sand apron stability. In any case, the extent of the sand apron can be used to infer wave climate at the time of sand apron progradation. For example, for One Tree Reef, one can argue the sand apron could had stopped prograding 4 ka BP because of the self-limiting sediment transport and remained stable until the sea-level fell.

The wave climate on the Southern Great Barrier Reef is characterized by significant wave heights (Hs) of 1.7 m and has been stable for the past 33 years (Smith et al. 2022). Future changes in the wave climate, storm frequency, increases in sea level and changes to sediment availability caused by anthropogenic climate change will modify the eco-morphodynamics of the sand aprons and the percentage infilling of the lagoons.