Coral reefs protect coastlines from inundation and flooding and serve over 200 million people globally. Wave transformation has previously been studied on coral reef flats with limited focus on forereef zones where wave transformation is greatest during high‐energy conditions. This study investigates the role of forereef spur and groove (SaG) morphology in wave energy dissipation and transmission at the reef crest. Using XBeach on LiDAR‐derived bathymetry from One Tree Island in the southern Great Barrier Reef, we reproduced dissipation rates comparable to SaG field studies. We examined how wave energy dissipation differs between realistic bathymetry and those with SaG features removed, demonstrating an up to 40% decrease in dissipation when SaG features are absent. We then investigated changes to wave energy dissipation and wave transmission at the reef crest based on IPCC AR5 emission scenarios (RCP2.6 and RCP8.5) and a total disaster scenario (TD) for the year 2100. For RCP2.6, an increase in wave heights of 0.8 m and an increase in water level of 0.3 m resulted in a two‐fold increase in dissipation rates. For RCP8.5 and TD, with no increase in incident wave height, dissipation rates were 29% and 395% lower than RCP2.6. This resulted in increased wave transmission at the reef crest by 1.8 and 2.7 m for the RCP8.5‐ and TD based models, respectively, when compared to the RCP2.6‐based model. The results from our novel modeling approach of using long‐shore varying accurate bathymetry on forereefs show increased wave energy dissipation rates with implications for reducing coastal flooding and island inundation on reef‐lined coasts.

Wave energy dissipation on coral reefs provides protection from coastal hazards to over 200 million people globally (Ferrario et al. 2014). The mechanisms of wave energy dissipation over the complex morphologies of the fore-reef slope have typically been overlooked (Duce et al. 2016). Here we focus on wave attenuation by the elongated troughs and depressions typical of fore-reef slopes known as spurs and grooves (SaGs).

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.

Coral reefs protect coastlines from inundation and flooding, servicing over 200 million people globally. Wave transformation has previously been studied on coral reef flats with limited focus on forereef zones where wave transformation is greatest during high-energy conditions. This study investigates the role of forereef spur and groove (SaG) morphology on wave energy dissipation and overtopping on coral reefs. Using XBeach on LiDAR-derived bathymetry, we reproduced dissipation rates comparable to SaG field studies. Our results emphasize accurate bathymetries’ role in forereef wave energy dissipation models by including morphological features (e.g., groove sinuosity, irregular forereef slopes) that control the mode of wave energy dissipation (frictional and breaking). We then investigated changes to wave energy dissipation and wave overtopping based on IPCC AR5 low and high emission scenarios (RCP2.6 and RCP8.5) and a total disaster scenario (TD) for the year 2100 considering changes to SaG morphology, wave power and relative sea-level rise. For RCP2.6, an increase in wave heights of 0.8 m and an increase in water level of 0.3 m resulted in a two-fold increase in dissipation rates. For RCP8.5 and TD, with no increase in incident wave height, dissipation rates were 29% and 395% lower than RCP2.6. This resulted in increased overtopping at the reef crest by 1.8 m and 2.7 m for RCP8.5 and TD scenarios, respectively, when compared to RCP2.6. Decreased dissipation rates and increased wave overtopping in forecasted climate conditions suggest the need for strategies to promote coral growth to facilitate high dissipation rates in the future.

Coral reef hydrodynamics control the transport and distribution of sediments, nutrients, and coral larvae, consequently influencing all coral reef processes across broad spatio-temporal scales. Yet, in situ hydrodynamic data from the disparate morphological zones (e.g., forereef slope) are limited. Spurs and grooves (SaG) morphology, consisting of reef normal coral ridges alternating with grooves, has been documented globally across fringing reefs, barrier reefs and atoll reefs (da Silva et al. 2020). Here, we present field observations of wave and tidally driven flows through a forereef groove. Our data captures wave and current flow characteristics in shallow, highly turbulent SaG and provides insight into the geomorphic role and origin of SaG.