Improving the hydraulic performance of single step broad-crested weirs

Abstract

In this study the hydraulic performance of single step broad-crested weir was improved. Through analyzing the parameters that have effect on the shape of the step and its influence on the flow characteristics, and energy dissipation percent (E%) downstream (D/S) of the weir. The differential equation of gradually varied flow for the water surface profile over the weir was solved analytically. Furthermore, empirical relations for E% and discharge coefficient (C d) due to the affecting factors were derived .The results showed that the weir model when the ratio of the length of D/S step to the length of the weir L 2 /L 1 =0.5 gives a higher E% in comparison with other weir models. Three types of flow regimes were observed, nappe flow below 350 cm 3 /s/cm, transition flow 350-700 cm 3 /s/cm and skimming flow upper than 700 cm 3 /s/cm. The comparison between calculated values by the differential equation of gradually varied flow and experimental values gives a good agreement, the maximum difference is about 7%. Two empirical relations were obtained, the first to estimate Cd in terms of the ratio for upstream U/S water head to U/S weir height h/P 1 and L 2 /L 1 . While the second relation to estimate E% in terms of the ratio for D/S water head to U/S weir height h/P 1 , L 2 /L 1 and the Froude number Fr 2 with a high correlation coefficient.