Discussion of "Submerged Flow over Barrage Weirs: A Computational Fluid Dynamics Model Study" by Tirtha Roy Biswas, Pankaj Singh, and Dhrubajyoti Sen

Barrage weirs, which are trapezoidal-sectioned sharp-crested weirs having finite crest length, are primarily used in the Indian subcontinent for diverting water for irrigation. This study presents an investigative study on submerged flows over barrage weirs, as occurs during floods, and establishes correlations for determining the structure's coefficient of discharge using a two-dimensional (2D) Reynolds averaged Navier Stokes-volume of fluid (RANS-VOF) model, coupled with the renormalization group (RNG) k-ϵ equations. The study specifically examines the piezometric gauging technique, commonly employed in Crump weirs, for estimating the discharge reduction factor of irrigation barrage weirs under submergence and compares the results with those obtained by the traditional tailwater gauging method. The results establish a unique correlation between the discharge reduction factor and the ratio of the piezometric head at a point immediately downstream of the weir crest and the total pressure head at a location on the upstream face of the weir. The method promises a more accurate estimate of the submerged discharge capacity for the barrage weirs than the conventional procedure. The study also suggests that a steeper upstream slope of a weir potentially leads to greater reduction in discharge under submergence. Further, for a given upstream weir face slope, the discharge reduction varies inversely with the relative crest length. The submergence rate at the modular limit is found to be in the range of 0.70-0.78. The modular limit, however, is found to be independent of the upstream face slope. The accuracy and reliability of the computed results are ensured by conducting relevant grid convergence analyses and validating the results of numerical simulations against experimental data. © 2021 American Society of Civil Engineers.