Arab J Sci Eng DOI 10.1007/s13369-016-2129-z RESEARCH ARTICLE - CIVIL ENGINEERING Effect of Bed and Side Slopes on Flow Measurements in Trapezoidal Free Overfall Channels Bahzad M. A. Noori 1 · Safa S. Ibrahim 2 Received: 19 September 2014 / Accepted: 20 March 2016 © King Fahd University of Petroleum & Minerals 2016 Abstract The aim of the present investigation is to study the effect of both side and bed slopes on the flow over free overfalls in trapezoidal channels, three models of trapezoidal free overfall channels have been built and tested in a labo- ratory flume. These models had 0.1 m bed width and 3.7 m length with different values of side slope (m = 0.268, 0.577, and 1). Each model had four different bed slopes ( S = 0, 0.0033, 0.01, and 0.02). A total of twelve series of the ex- periments were tested for different ranges of Froude number. Experimental results of all models showed that the variation of ( Qm 1.5 /  gb 5 ) with (my b /b) for different values of side slope and different bed slopes is a simple power equation. It was observed that the values of ( Qm 1.5 /  gb 5 ) increase with the increase in (my b /b) values. The correlation between (my b /b) with (my c /b) is found to be linear for different bed slopes. The ratio ( y b / y c ) decreases with the increase of slope ( S) with an average value of 0.729 for subcritical and super- critical flow on smooth beds of different slopes. An empirical expression is obtained for the variation of ( Qm 1.5 /  gb 5 ) with (my b /b), and ( S). The results of the present study have been compared with theoretical and experimental results ob- tained by other investigators showing well agreements. Keywords Bed slope · Side slope · Flow measurements · Trapezoidal channel · Free overfalls B Safa S. Ibrahim safa.subhi@uod.ac 1 Civil Engineering Department, Faculty of Engineering, University of Duhok, Duhok, Iraq 2 Water Recourses Engineering Department, Faculty of Engineering, University of Duhok, Duhok, Iraq 1 Introduction Drop structures are the most common hydraulic structures used in irrigation channel systems, water distribution, and waste collection networks, and in recent years, in stepped spillways. They are used as grade control structures needed to control channel degradation [1]. The free overfall has a distinct important in hydraulic engineering; it forms the start- ing point in computations of water surface in a gradually valid flow (such as discharge spills into a reservoir at the downstream end). Also the study of free overfall is impor- tant because it can be used as a simple discharge measuring device. Numerous theoretical and experimental studies have been carried out on the channel free overfall in order to estab- lish a relationship between the brink depth and the upstream flow depth. This relationship allows the use of the free over- fall as a discharge measuring device. The problem of free overfall as a flow rate measuring structure has attracted a considerable interest in the past [2]. Pioneering experimen- tal research was carried out by Rouse [3], Rouse was the first to suggest the possibility of using the free overfall as a simple flow measuring device which does not require any calibration. Rouse conducted experiments in a horizontal confined rectangular channel and concluded that the brink depth is a constant percentage of the computed critical depth of the parallel flow. He found that the end-depth ratio [EDR = end depth ( y b )/critical depth ( y c )] equals to 0.715 when Froude number equals to one. Rouse also obtained the EDR for variable upstream Froude number, F r . Keller and Fong [4] experimentally and theoretically studied the trapezoidal free overfall. The theoretical analysis involved the applica- tion of the momentum equation to a control volume bounded upstream by the critical depth section and downstream by the free overfall section. Ferro [5] studied experimentally the problem of free overfall in rectangular channels and found 123