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An efficient method to calculate depth-integrated, phase-averaged momentum balances in non-hydrostatic models

July 13, 2021

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 nonlinearity and energy transfers are important.

Publication Year 2021
Title An efficient method to calculate depth-integrated, phase-averaged momentum balances in non-hydrostatic models
DOI 10.1016/j.ocemod.2021.101846
Authors Renan F. da Silva, Dirk P. Rijnsdorp, Jeff E. Hansen, Ryan J. Lowe, Mark L. Buckley, Marcel Zijlema
Publication Type Article
Publication Subtype Journal Article
Series Title Ocean Modelling
Index ID 70222449
Record Source USGS Publications Warehouse
USGS Organization St. Petersburg Coastal and Marine Science Center