The Influence of Coral Reef Spur and Groove Morphology on Wave Transformation and Attenuation

Coral reefs are the most biodiverse and productive ecosystems on Earth. Changes to global climate will alter the conditions required for coral reef survival. In atoll reefs, coral species distribution and survival is determined by waves and tidally induced flows. The outermost part of atoll reefs, the forereef, presents a key morphological control over wave attenuation and consequently nutrient distribution at the scale of entire reefs. These high-energy environments provide protection from the effects of wave inundation to over 200 million people globally. Despite this, the mechanisms of wave transformation over the complex bathymetries of forereefs have received little attention. Here we focus on processes of wave transformation and attenuation by the elongated troughs and depressions typical of forereef slopes known as spurs and grooves (SaGs) and provide a better understanding of the morphodynamics of SaGs. We combine a quantitative morphometric analysis with wave transformation modelling and hydrodynamic field data from One Tree Reef (OTR) of the southern Great Barrier Reef, Australia. We used a 50 cm resolution LiDAR bathymetry dataset and novel methods of morphometric spectral analysis at swell wave exposed, semi-exposed, and protected locations. The wave transformation model, XBeach, was calibrated and validated with published SaG field data and wave data obtained from a 33-year analysis of satellite altimeters. The effects of SaGs on wave energy dissipation were examined under various forecasted climate change scenarios (RCP 2.6, RCP 8.5, and a disaster scenario) to elucidate their role in preventing future coastal hazards. Finally, field data collected from a fourth site on at OTR over a three-day period in October 2022 used an array of two acoustic doppler velocimeters and an 8 Hz pressure transducer. Current and wave measurements were taken in a long (140 m) and deep (max spur height = 5.3 m) groove under typical wave conditions (Hs = 0.8 m, Tp = 7 s). Findings from the numerical models suggest SaGs play a critical role in dissipating wave energy, increasing wave dissipation by bed friction by 75%. Our results demonstrate that a decrease in wave energy dissipation results in an exponential increase in wave overtopping with storm waves producing overtopping > 3 m under worst-case scenarios. Additionally, we found that high bathymetric gradients in SaGs increase dissipation by wave breaking by up to 52% under RCP 8.5, leading to a 71% increase in mean wave energy dissipation despite an 89% reduction in bed friction factor and a 1 m increase in relative sea level. Field observations demonstrate the wave and tide-dominated flow regimes through forereef grooves. Future work aims to investigate the form and process feedbacks in forereef hydrodynamics and consider global to regional scale climate forecasts. To facilitate this, we provide field data and analytical codes open source. Understanding the morphodynamics of forereefs is critical to forecasting reef health, wave transformation, and coastal hazard reductions under climate change conditions.