Twenty-Second International Water Technology Conference, IWTC22 Ismailia, 12-13 September 2019 478 AN INVESTIGATION CONCERNING THE WATER ENERGY DISSIPATION AND FLOW AERATION OVER STEPPED SPILLWAYS Mohamed A. Ashour 1 , Tawab E. Aly 2 , Mohamed K. Ali 3 1 Professor, Civil Eng. Dept., Assiut University, Egypt, mashour475275@yahoo.com. 2 Associate Prof., Civil Eng. Dept., Assiut University, Egypt, tawab_aly@yahoo.com. 3 M.Sc. Student, Civil Eng. Dept., Assiut University, Egypt, mkhairy@aun.edu.eg. Abstract: Spillways usually used for escaping water from the U.S. having a high-water level, to D.S. having a low water level in most of the diversion head structures through water streams. The D.S. of such spillways usually suffers from the destructive impacts of the generated kinetic energy of the flowing water, having a very high speed, which may cause cavitation in such spillways body. In the present work, we are introducing some geometrical treatments on the back of the spillway body, for increasing its efficiency in dissipating the kinetic energy of the flowing water, having great potential energy, and improving the flowing water quality by increasing its dissolved oxygen content, through generating huge aeration at the flow in the back, in addition to prevent cavitation may occur, and generated on the back of the spillway body, and protecting its safety. Literature proved that the stepped back of the spillway body is one of the most practicable trails done for achieving the above-mentioned goals. In this paper, we present a review of previous authors' technical methods to obtain the best design of the spillway geometric that dissipating high values of the kinetic energy and improving the flow aeration. Key words: Stepped spillway, Energy dissipation, Dissolved oxygen content, Re-aeration 1. INTRODUCTION The design and construction of spillways or stepped weirs are very complicated due to difficulties and risks such as erosion, cavitation, scour and high flow kinetic energy. The water discharging over the spillway has to fall down from a higher elevation to lower. So, the flow has a high velocity and the potential energy turned into a kinetic energy at the toe of spillways, therefore, the surface of the spillways should be able to dissipate this energy which causes erosion and scour downstream spillways. The back-surface roughness of spillway can be generated by adding some energy-dissipating devices, such as baffles or steps. Steps on the back surface of the spillway are the most common energy-dissipating devices. The stepped spillway design enhances the rate of energy dissipation on the spillway, thus reducing the size and the cost of the downstream stilling structure. The geometry of steps such as number of steps, angle of the back surface of the spillway, and any accessories on the step affects the efficiency of the spillway in the dissipated energy and re-aeration of the flow. Many authors studied the effect of such geometric changes on the flow over the spillways, a lot of results indicated that; the energy dissipation increases with decreasing number of steps and decreasing the angle of the back surface of the spillway (Khalaf, 2014; Al-Husseini, 2015; Irzooki, 2016; Tabari and Tavakoli, 2016), while Rad and Teimouri, 2010 and Jahad et al. ,2017 concluded an inversely result with the number of steps (Ns), they found that; with increasing the number of steps, the energy dissipation increased. Owing to such a conflict in results, the present study will be conducted to clerify which of these results are true, and at the same time, to introduce a new geometrical treatments for the spillway back body, that insure better hydraulic performance, and more efficient degree in energy dissipation which increasing the flow dissolved oxygen content for improving the irrigation water quality. Such achievements will be through studying the most effective number of steps (Ns) and the most effective angle of the back surface of the spillway (Ɵ  ). Also, estimating the optimum value of the relative ratio between the step height to the spillway height (h/H). The geometrical arrangement and distribution of the