Analysis of the mean (wave-averaged) momentum balance is a common approach used to explain the physical forcing driving wave set-up and mean currents in the nearshore zone. Traditionally this approach has been applied to phase-averaged models but has more recently been applied to phase-resolving models using post-processing, whereby model output is used to calculate each of the momentum terms. While phase-resolving models have the advantage of capturing the nonlinear properties of waves propagating in the nearshore (making them advantageous to enhance understanding of nearshore processes), the post-processing calculation of the momentum terms does not guarantee that the momentum balance closes. We show that this is largely due to the difficulty (or impossibility) of being consistent with the numerical approach. If the residual is of a similar magnitude as any of the relevant momentum terms (which is common with post-processing methods as we show), the analysis is largely compromised. Here we present a new method to internally calculate and extract the depth-integrated, mean momentum terms in the phase-resolving non-hydrostatic wave-flow model SWASH in a manner that is consistent with the numerical implementation. Further, we demonstrate the utility of the new method with two existing physical model studies. By being consistent with the numerical framework, the internal method calculates the momentum terms with a much lower residual at computer precision, combined with greatly reduced calculation time and output storage requirements compared to post-processing techniques. The method developed here allows the accurate evaluation of the depth-integrated, mean momentum terms of wave-driven flows while taking advantage of the more complete representation of the wave dynamics offered by phase-resolving models. Furthermore, it provides an opportunity for advances in the understanding of nearshore processes particularly at more complex sites where wave non-linearity and energy transfers are important.

平均（波浪平均）动量平衡分析是一种常用的方法，用于解释近岸区域的物理强迫驱动波设置和平均水流。传统上，这种方法已应用于相位平均模型，但最近已应用于使用后处理的相位解析模型，其中模型输出用于计算每个动量项。虽然相位解析模型具有捕捉在近岸传播的波浪非线性特性的优势（使其有利于增强对近岸过程的理解），但动量项的后处理计算并不能保证动量平衡关闭。我们表明，这主要是由于难以（或不可能）与数值方法保持一致。如果残差与任何相关动量项的大小相似（这在我们展示的后处理方法中很常见），则分析在很大程度上会受到影响。在这里，我们提出了一种新方法，以与数值实现一致的方式在相位分辨非静水波流模型 SWASH 中内部计算和提取深度积分的平均动量项。此外，我们通过两个现有的物理模型研究证明了新方法的实用性。通过与数值框架一致，内部方法以计算机精度计算具有低得多的残差的动量项，与后处理技术相比，大大减少了计算时间和输出存储要求。这里开发的方法可以准确评估深度积分，波浪驱动流动的平均动量项，同时利用相位解析模型提供的更完整的波浪动力学表示。此外，它为了解近岸过程提供了一个机会，特别是在波浪非线性和能量转移很重要的更复杂的地点。