Rectangular sharp-crested weir calibration for low head and clinging flow regime

Abstract

The classical head–discharge relation for rectangular sharp-crested weirs is applicable only to free flow conditions, not valid at low head when flow becomes clinging. Based on experimental data for eight sharp-crested rectangular weirs of different sizes, a new method for calculating discharge at the low head bistable and clinging flow regime was proposed in this study. In the bistable zone, the head–discharge relationship can be covered partly by the classical weir–discharge equations of free flow. The discharge coefficient is quite similar to Rehbock’s equation for free flow with a surface tension term. In the clinging flow regime, the head had to be transformed into an equivalent head obtained by regression. All the regression parameters were about the same for the weirs of different sizes. In the clinging zone, discharge was also directly proportional to weir width, while unit width discharge was directly proportional to the square of head, independent of weir height. The errors of model-predicted discharge were less than 10 % for more than 90 % of the data points. Therefore, the proposed method for discharge of clinging flow is applicable to rectangular sharp-crested weirs at very low head, while the classical formulas often fail under these flow conditions.