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Comment on “An Efficient and Stable Hydrodynamic Model With Novel Source Term Discretization Schemes for Overland Flow and Flood Simulations” by Xilin Xia et al.
Author(s) -
Lu Xinhua,
Mao Bing,
Dong Bingjiang
Publication year - 2018
Publication title -
water resources research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.863
H-Index - 217
eISSN - 1944-7973
pISSN - 0043-1397
DOI - 10.1002/2017wr021563
Subject(s) - discretization , spurious relationship , shallow water equations , riemann solver , flow (mathematics) , mathematics , term (time) , machine epsilon , simple (philosophy) , computer science , algorithm , mathematical analysis , mechanics , physics , statistics , geometry , finite volume method , quantum mechanics , philosophy , epistemology
Xia et al. (2017) proposed a novel, fully implicit method for the discretization of the bed friction terms for solving the shallow‐water equations. The friction terms contain h −7∕3 ( h denotes water depth), which may be extremely large, introducing machine error when h approaches zero. To address this problem, Xia et al. (2017) introduces auxiliary variables (their equations (37) and (38)) so that h −4∕3 rather than h −7∕3 is calculated and solves a transformed equation (their equation (39)). The introduced auxiliary variables require extra storage. We implemented an analysis on the magnitude of the friction terms to find that these terms on the whole do not exceed the machine floating‐point number precision, and thus we proposed a simple‐to‐implement technique by splitting h −7∕3 into different parts of the friction terms to avoid introducing machine error. This technique does not need extra storage or to solve a transformed equation and thus is more efficient for simulations. We also showed that the surface reconstruction method proposed by Xia et al. (2017) may lead to predictions with spurious wiggles because the reconstructed Riemann states may misrepresent the water gravitational effect.