LAUSR

ABSTRACT Duckbill weir is one type of long crested weirs which passes more flow with less head variation on the weir than standard weirs. Due to contraction and decreasing discharge along the weir, a gradual variation in the water surface is occurred such as spatially varied flow phenomena. Therefore, in this study by solving the spatially varied flow equation, the water surface profile of duckbill weir is obtained when the bottom slots were used along the weir. Furthermore, Flow-3D software is used to simulate the water surface profile along the weir. The results of dimensional analysis showed that the discharge coefficient of the weir is increased by increasing of length and opening of slots. The results also show that the discharge coefficient of the weir is more affected by slot opening rather than the slot length. By solving the spatially varied flow equation along the weir one can found that there is a threshold value of H/P (H is the head of water over the weir and P is weir height) for a given θ (the angle between side wall and flow direction of the weir) in which the water surface tends to drop by entering to the weir and then a gradual rise in the water surface as it approaches the downstream end of the weir. The results also show that the calculated water surface profile along the weir by using spatially varied flow equation and Flow-3D software is close to experimental results. The maximum errors in computation of water surface profile for solving spatially varied flow equation and Flow-3D software are 6.8% and 3.05%, respectively.