Beach and Backward Bragg Sea-Swell Wave Reflection Across Rocky and Sandy Shores

IF 3.3 2区地球科学 Q1 OCEANOGRAPHY Journal of Geophysical Research-Oceans Pub Date : 2024-11-18 DOI:10.1029/2023JC020177

Patrick Collins, Jamie MacMahan, Edward Thornton, Charlotte Benbow, Paul Jessen

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Abstract

An observational study comparing beach reflection characteristics for sea-swell waves along six rocky and three sandy shores, spanning from Monterey to Santa Cruz, CA, shed light on the previously unknown wave reflection along rocky shores. Wave reflection is derived from directional spectra acquired via several surface GPS-based wave buoys. Owing to the Lagrangian nature of the buoy, they were validated $(r^{2} = 0.7)$ against Eulerian beam velocities from a bottom-mounted acoustic Doppler current profiler, providing a spatial array for more degrees of freedom in resolving wave direction and now including adjustment for instrument tilt. Reflection varies across shoreline types: sandy shores ( $<$ 20%), rough, rocky shores ( $<$ 10%), rocky shores with cliffs ( $<$ 15%), and rocky platforms with cliffs ( $<$ 30%). Contrary to expectations, rocky shores exhibit minimal reflection. Existing reflection estimators developed for sandy shores are found inadequate for rocky shores $(r^{2} = 0.23; R M S E = 6.4)$ , necessitating a new estimator considering beach slope, wave steepness, and shoreline rugosity $(r^{2} = 0.63; R M S E = 0.05)$ . Beach reflection is limited by the dissipation of incident waves due to bottom friction and depth-limited wave breaking as well as by the reduction in reflected waves caused by wave trapping. Observed reflection increased offshore on all beaches attributed to sea-swell waves' resonant interaction with the rough rocky bottom, causing backward Bragg scattering. The Bragg scattering was less on larger-sloped bottoms, attributed to larger slopes hindering effective wave-bottom interactions.

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海滩和后向布拉格海浪在岩石海岸和沙质海岸上的波浪反射

一项观测研究比较了从加利福尼亚州蒙特雷到圣克鲁斯的六处岩石海岸和三处沙质海岸的海浪反射特征，揭示了岩石海岸以前未知的海浪反射情况。波浪反射是通过几个基于全球定位系统的海面波浪浮标获取的方向光谱得出的。由于浮标的拉格朗日性质，它们与安装在底部的声学多普勒海流剖面仪的欧拉波束速度进行了验证 r 2 = 0.7 $\left({r}^{2}=0.7\right)$，为解析波浪方向提供了空间阵列的更多自由度，现在还包括仪器倾斜的调整。不同海岸线类型的反射率各不相同：沙质海岸（< ${< } $ 20%）、粗糙的岩石海岸（< ${< } $ 10%）、有悬崖的岩石海岸（< ${< } $ 15%）和有悬崖的岩石平台（< ${< } $ 30%）。与预期相反，岩石海岸的反射率极低。Existing reflection estimators developed for sandy shores are found inadequate for rocky shores r 2 = 0.23 ; R M S E = 6.4 $\left({r}^{2}=0.23\mathrm{;}\,\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{E}=6.4\right)$ , necessitating a new estimator considering beach slope, wave steepness, and shoreline rugosity r 2 = 0.63 ; R M S E = 0.05 $\left({r}^{2}=0.63\mathrm{;}\,\mathrm{R}\mathrm{M}\mathrm{S}\mathrm{E}=0.05\right)$ .海滩反射受到入射波因海底摩擦和深度有限的破浪而消散以及反射波因波浪捕获而减少的限制。在所有海滩上观测到的近海反射都在增加，这归因于海涌波与粗糙的岩石底部的共振相互作用，引起了后向布拉格散射。在坡度较大的底部，布拉格散射较小，这是因为较大的坡度阻碍了有效的波底相互作用。

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