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 (Cd) 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 L2/L1=0.5 gives a higher E% in comparison with other weir models. Three types of flow regimes were observed, nappe flow below 350 cm3 /s/cm, transition flow 350-700 cm3/s/cm and skimming flow upper than 700 cm3/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/P1 and L2/L1. While the second relation to estimate E% in terms of the ratio for D/S water head to U/S weir height h/P1, L2/L1 and the Froude number Fr2 with a high correlation coefficient . Key words: Hydraulics; weirs ; performance ; dissipation energy.
One of the most important hydraulic structures which are used in irrigation systems are the broad crested weirs, which can be defined as a vertical barrier in a canal or tank side in which the water may flow freely. This weir can be used to measure the discharge and to raise the water level in the upstream. In this study the effect of longitudinal section on streamlined flow over broad crested weir has been investigated. Loss in energy, working efficiency and the coefficient of discharge have been considered. Experimental study carried out using (25) samples with six different hydraulic heads. The analysis of results indicate that the coefficient of discharge (Cd) increases with increasing Froud Number (Fr) and the most effective slope of upstream and downstream faces of the weir wear (45°) and (60°) respectively, at which the coefficient of discharge demonstrated was (0.644