Abstract Cross-shore and alongshore velocities were measured over five high tide cycles in the swash and inner surf zones of a dissipative beach (significant wave height between 0.5 and 1.65 m; water levels between 0 and 0.87 m) using acoustic and electromagnetic current meters. Measurements are used to determine the importance of alongshore motions relative to cross-shore motions for bed shear stress; a parameter required for many sediment transport formulations. Velocities and water depths are infragravity dominated with running average (5 min) cross-shore and alongshore velocities being of similar magnitude near 0.25 m/s. Significant coherence squared between cross-shore velocity and water depth is found in an infragravity frequency range (0.0078 Hz–0.024 Hz). A narrower infragravity frequency range (0.012 Hz–0.022 Hz) of significant coherence squared is found between alongshore velocity and water depth. Near bed velocity profiles at high spatial resolution indicate the mean profile of cross-shore or alongshore velocity for different relative cross-shore positions are nearly depth uniform, suggesting, on average, a well-mixed water column. Time-averaged onshore-directed depth-averaged velocities are nearly constant as a function of mean water depth, whereas time-averaged offshore-directed depth-averaged velocity magnitudes decrease with an increase in mean depth. In contrast, mean alongshore depth-averaged velocity magnitudes increase with depth regardless of the corresponding cross-shore motion and were always southerly-directed. These variations result from changes in breaking wave forcing location (i.e. relative cross-shore position). Near instantaneous velocity profile data are used to estimate friction coefficients and the bed shear stress through the Law of the Wall assumption. Mean friction coefficients for alongshore flows are similar to those obtained for cross-shore flows and values of ~0.02 are appropriate for all phases of flow. Alongshore bed shear stresses are the dominant bed shear stress component for over 27% of the samples implying an enhancement of total bed shear stress through incorporation of alongshore motions. Neglecting these alongshore processes (even if alongshore uniform) in cross-shore sediment transport models will lead to errors in predicted sediment transport rates.

中文翻译：

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沿岸流对耗散海滩内海浪和冲刷带流体动力学的作用

摘要 使用声学和电磁电流计测量了耗散海滩（显着波高在 0.5 到 1.65 m 之间；水位在 0 到 0.87 m 之间）的五个高潮周期内的跨海岸和沿岸速度。测量用于确定沿岸运动相对于跨海岸运动对床剪应力的重要性；许多沉积物输送公式所需的参数。速度和水深以次重力为主，平均（5 分钟）跨岸和沿岸速度在 0.25 m/s 附近具有相似的幅度。在超重力频率范围 (0.0078 Hz–0.024 Hz) 中发现了跨岸速度和水深之间的显着相干性平方。较窄的次重力频率范围 (0.012 Hz–0. 022 Hz) 的显着相干平方被发现在沿岸速度和水深之间。高空间分辨率下的近床速度剖面表明，不同相对跨岸位置的跨岸或沿岸速度的平均剖面几乎是深度均匀的，这表明平均水体混合良好。作为平均水深的函数，时间平均陆上定向深度平均速度几乎是恒定的，而时间平均离岸定向深度平均速度幅度随着平均深度的增加而减小。相比之下，平均沿海岸深度平均速度大小随深度增加而不管相应的跨海岸运动如何，并且总是向南。这些变化是由破浪强迫位置（即相对跨岸位置）的变化引起的。近瞬时速度剖面数据用于通过壁面定律假设估算摩擦系数和床层剪切应力。沿岸流动的平均摩擦系数与跨岸流动获得的摩擦系数相似，~0.02 的值适用于流动的所有阶段。沿岸床剪切应力是超过 27% 样品的主要床剪切应力分量，这意味着通过结合沿岸运动增强了总床剪切应力。在跨岸沉积物迁移模型中忽略这些沿海岸过程（即使沿海岸是均匀的）将导致预测的沉积物迁移率出现错误。沿岸流动的平均摩擦系数与跨岸流动获得的摩擦系数相似，~0.02 的值适用于流动的所有阶段。沿岸床剪应力是超过 27% 样品的主要床剪应力分量，这意味着通过结合沿岸运动增强了总床剪应力。在跨岸沉积物迁移模型中忽略这些沿海岸过程（即使沿海岸是均匀的）将导致预测的沉积物迁移率出现错误。沿岸流动的平均摩擦系数与跨岸流动获得的摩擦系数相似，~0.02 的值适用于流动的所有阶段。沿岸床剪应力是超过 27% 样品的主要床剪应力分量，这意味着通过结合沿岸运动增强了总床剪应力。在跨岸沉积物迁移模型中忽略这些沿海岸过程（即使沿海岸是均匀的）将导致预测的沉积物迁移率出现错误。