Preservation of the live storage of reservoirs is a serious challenge for most of the countries that encounter drought. Flushing of deposited sediments through bottom outlets in a dam is one of the suitable means to transport sediments toward downstream of the dam. Due to complicated conditions of flow through bottom outlets, we are facing a lack of accurate information about the various phenomena that occurs in bottom outlets such as cavitation, corrosion, and abrasion. Cavitation is the most important problem that causes remarkable damages on the lining of these tunnels. A numerical model based on the finite volume method for fully three-dimensional (3D) open channel flow equations is incorporated into a modeling cavitation and aeration system along the dam outlets. In this study, complex two-phase turbulent flow is simulated using the model. The pressure and velocity distribution under different conditions of gate opening and reservoir water level are computed along the tunnel and validation of the work has been obtained by comparison with measurements of a laboratory model. For both nonaerated and aerated flow, cavitation index in flow direction have been estimated and compared with each other.