Investigation of flow characteristics above trapezoidal broad-crested weirs Mohamad Reza Madadi a,n , Ali Hosseinzadeh Dalir b,1 , Davood Farsadizadeh b,2 a Department of Water Engineering, Young Researchers Society, Shahid Bahonar University of Kerman, P.O.Box 76169-133, Kerman, Iran b Department of Water Engineering, University of Tabriz, Tabriz, Iran article info Article history: Received 12 January 2014 Received in revised form 24 April 2014 Accepted 11 May 2014 Available online 16 May 2014 Keywords: Trapezoidal broad-crested weir Discharge coefficient Flow separation Velocity profile Critical depth abstract In this study the effect of upstream face slope of a trapezoidal broad-crested weir on discharge coefficient and water surface profile was investigated using the laboratory models. The velocity and pressure distribution profile were determined. The location of the critical section above the weir was specified. The dimensions of flow separation zone were also measured for different upstream face slopes. The results showed that decreasing the upstream face slope prevents development of separation zone. In this case, the flow was passed through the weir more regularly and the water surface and pressure drop were decreased. Decreasing the upstream face slope to 211, increased the discharge coefficient up to 10% and reduced the separation relative length and height up to 80% and 95% respectively. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Flow measurement structures are one of the main categories of hydraulic structures which are generally designed to act as a control in the open channels to provide a unique relationship between the upstream head and the discharge [3,5]. Weirs are among the major types of measuring structures which because of their low cost, easy installation and good accuracy have met with great interest. They are generally classified into three groups, namely, broad-crested, short-crested and sharp-crested weirs depending on the ratio of the weir upstream head to the weir crest length. For a broad crested weir, the ratio of crest length (L crest ) to upstream head over crest ðH  ZÞmust be typically greater than 3 [23,8]. L crest H  Z 41:5  3 ð1Þ Hydraulically, a broad-crested weir is a flat-crested structure with a crest length large compared to the flow thickness [22,34] for the streamlines to be parallel to the crest invert and the pressure distribution to be hydrostatic [6,23,35,8]. In this case, critical flow conditions occur on the crest [50,46,17]. If the up- and downstream faces of the broad crested weir be vertical (namely as Standard Broad Crested Weir), it will operate with a rather significant loss in head and also the sediment or solids may be deposited at the upstream side of the weir, which will adversely affect its accuracy and increase the operating costs. To deal with such problems, the broad-crested weir's structural design can present some flexibility to provide better hydraulic characteristics and discharge efficiency. For example, if the weir upstream entrance condition changes from square-edged to the round- nosed corner, the weir discharge coefficient will be increased [40]. A review on previous studies shows that, to prevent deposi- tion of sediment or solids in the upstream side of the weir as well as to prevent cavitation at the downstream corner of the weir, the upstream and downstream faces of broad crested weirs can be sloped [44]. This type of weirs can be classified as trapezoidal profile weirs [51], embankment-shaped weirs (Fritz and Hager [14]), ramped broad crested weirs [1], or broad-crested weirs with upstream and downstream side slopes [44]. In this paper, the term trapezoidal broad crested weir is used because the models of weirs which were used in this study are in the range of broad crested weirs (Eq. (1)) and also, they have trapezoidal shape in the longitudinal section. According to Fritz and Hager [14], the discharge equation under free flow condition for this type of weirs can be calculated by Q ¼ C D B ffiffiffiffiffiffiffiffiffiffiffi 2gH 3 q ð2Þ where H is total overflow head, g is the acceleration due to gravity, B is the weir width, and C D is the discharge coefficient and is obtained by C D ¼ 0:43 þ 0:06 sin½πðε  0:55Þ ð3Þ Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/flowmeasinst Flow Measurement and Instrumentation http://dx.doi.org/10.1016/j.flowmeasinst.2014.05.014 0955-5986/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. Tel.: þ98 9119231606; fax: þ98 3413222043. E-mail addresses: Mohamad_Reza_Madadi@yahoo.com (M.R. Madadi), ahdalir@tabrizu.ac.ir (A. Hosseinzadeh Dalir), farsadi@tabrizu.ac.ir (D. Farsadizadeh). 1 Tel.: þ98 9143166284. 2 Tel.: þ98 9143135801. Flow Measurement and Instrumentation 38 (2014) 139–148