Research Paper Hydraulic characteristics of circular crested weir based on Dressler theory Amir H. Haghiabi* Department of Water Engineering, College of Agriculture, Lorestan University, Khorramabad 68149-84649, Iran article info Article history: Received 4 December 2011 Received in revised form 16 April 2012 Accepted 6 May 2012 Published online 9 June 2012 Circular crested weirs are used for flow measurement and water level control in irrigation systems and reservoirs. Using Dressler theory and applying a velocity correction factor at the weir crest section, analytical equations are obtained for discharge coefficient, and velocity and pressure profiles at the weir crest section of circular crested weirs. Existing experimental data are used for validation of the obtained equations. The results of the study indicated that obtained equations have good agreement with experimental data. ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved. 1. Introduction Weirs are one of the most important components of hydraulic structures. The most common types of weirs are the sharp crested weir, the ogee crest weir and circular crested weir. Circular crested weirs are used for flow measurement and water level control in irrigation systems and reservoirs. The advantages of the circular crested weir compared to the other weirs include simplicity in design, stable overflow pattern, large coefficient of discharge, and associated low cost. A simple sketch of flow past a circular crested weir is shown in Fig. 1. The unit discharge q over a circular crested weir can be related to the total energy head above the weir crest H 1 , and the discharge coefficient C d as (Bos, 1978) q ¼ 2 3 C d ffiffiffiffiffiffiffiffiffiffiffi 2 3 gH 3 1 r (1) where g is acceleration due to gravity. Using Dressler theory (1978), the normalised velocity profile over the circular crested weir can be expressed as (Ramamurthy & Vo, 1993a) u U 1 ¼ 1 ð1 þ y=RÞ (2) where y is vertical distance from the weir crest; u is horizontal velocity component at vertical distance y from the weir crest; U 1 is maximum velocity at the weir crest section; and R is radius of circular crest. Also, Heidarpour and Chamani (2006) used potential flow around a circular cylinder, and expressed u/U 1 as u U 1 ¼ 1 2  1 þ  1 ð1 þ yÞ=R  2  (3) The hydraulic characteristics of circular crested weirs have been studied by Hager (1985, 1995), Ramamurthy, Vo, and Vera (1992), Ramamurthy and Vo (1993a, 1993b), Chanson (2006), Heidarpour, Mohammadzadeh Habili, and Haghiabi (2008), Castro-Orgaz, Gira ´ ldez, and Ayuso (2008), Castro-Orgaz (2009), Tadayon and Ramamurthy (2009), Bagheri and Heidarpour (2010), and Mohammadzadeh Habili and Heidarpour (2010). Using potential flow around the upper half of the circular cylinder, Heidarpour et al. (2008) derived semi-empirical equations for the maximum crest velocity, the normalised crest pressure, and the pressure correction coefficient as the function of the average upstream velocity and a velocity correction factor. Also, Bagheri and Heidarpour (2010) used potential flow of free vortex and derived semi-empirical * Tel.: þ98916 161 0580. E-mail address: haghiabi@yahoo.com. Available online at www.sciencedirect.com journal homepage: www.elsevier.com/locate/issn/15375110 biosystems engineering 112 (2012) 328 e334 1537-5110/$ e see front matter ª 2012 IAgrE. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biosystemseng.2012.05.004