Wave height to water depth ratios for coral reef flats from different coral reef surf zones

Wave breaking and transformation on coral reef flats is an important process protecting tropical coastlines and regulating the energy regimes of coral reefs. However, the high hydraulic roughness, shallow water, and steep bathymetries of coral reefs may confound common surf zone assumptions, such as a depth-limited and saturated surf zone with a constant wave height to water depth ratio (γ). Here, we examine wave transformation across a coral reef flat, during three separate swell events, on both a time-averaged and a wave-by wave basis. We use the relationship between significant wave height and water depth (γs) to examine the change in surf saturation across the reef flat and compare the measured wave height decay to results of modelled wave energy dissipation in the surf zone. Our results show that γs was not cross-reef constant and varied according to location on the reef flat and local water depth. On average, γs was greatest at the outer reef flat, near the reef crest, and progressively reduced towards the inner reef flat, near the reef lagoon. This was most pronounced in shallow water with large γs values (γs > 0.85) at the outer reef flat and small γs values (γs < 0.1) at the inner reef flat. This indicates that there is an increase in wave energy dissipation in shallow water, most likely due to increased breaker and bed frictional dissipation. The measured wave energy dissipation across the entire reef flat could, on average, be modelled accurately; however, this required location specific calibration of the free parameters, the wave friction factor (fw) and γ, and further suggests that there is no value for either parameter that is universally applicable to coral reef flats. Despite model calibration inaccuracies were still observed, primarily at the outer reef flat. These inaccuracies reflected the observed cross-reef variation of γ on the reef flat and potentially the limitations of random wave breaker dissipation models in complex surf zones. Our results have implications for the use of wave energy dissipation models in predicting breaker dissipation and subsequent benthic community change on coral reef flats, and uggest that careful consideration of the free parameters in such models (such as fw and γ) is required.

珊瑚礁坪（coral reef flats）上的波浪破碎与演化过程，是保护热带海岸、调控珊瑚礁能量格局的关键环节。然而，珊瑚礁极高的水力粗糙度、浅水环境与陡峭的水下地形（bathymetries），可能会打破碎波带（surf zone）的经典假设——即深度受限且饱和的碎波带具备恒定的波高水深比（γ）。本研究基于时间平均与逐波两种分析尺度，针对三次独立涌浪事件（swell events），探究了珊瑚礁坪上的波浪演化过程。本研究采用有效波高（significant wave height）与水深的比值（γs），分析了礁坪上碎波饱和状态的空间变化，并将实测波高衰减结果与碎波带内波浪能耗散的模拟结果进行对比。研究结果表明，γs并非跨礁坪恒定，而是随礁坪位置与局地水深发生变化。平均而言，γs在靠近礁顶（reef crest）的外礁坪达到最大值，并朝向靠近礁湖（reef lagoon）的内礁坪逐渐减小。这一特征在浅水区域最为显著：外礁坪处γs值较大（γs>0.85），而内礁坪处γs值较小（γs<0.1）。这表明浅水区域的波浪能耗散有所增强，其原因大概率是破波作用与床面摩擦能耗散的加剧。整体礁坪的实测波浪能耗散结果，平均而言可被精准模拟；但这需要针对不同位置对自由参数——波浪摩擦因子（wave friction factor, fw）与γ进行率定，进一步说明不存在可普遍适用于珊瑚礁坪的参数取值。尽管仍存在模型率定误差，且该误差主要出现在外礁坪区域。这些误差既反映了礁坪上γ的跨礁坪空间变化，也可能体现了复杂碎波带内随机破波能耗散模型的局限性。本研究结果对利用波浪能耗散模型预测珊瑚礁坪上的破波能耗散及后续底栖生物群落变化具有重要指导意义，同时表明需谨慎考量此类模型中的自由参数（如fw与γ）。