It is a popular fact that mechanical processes are usually accompanied by the dissipation phenomenon. Therefore, it is not possible to retain the initial energy over a temporal period or a local distance without spending additional energy. Because these motions are accompanied by conversion into heat. In other words, due to some factors such as friction, cross-section changes, etc., there are some energy losses that the process is irreversible, and some of the system's energy is mainly dissipated as heat. In the most cases of the open-channel transitions, the energy losses of turbulent flows are so complex that it is not easy to define specific relationships for them, and the energy losses are usually determined empirically or experimentally. Examining how the mechanical energy loss of three-dimensional turbulent flows can be estimated without measurements is one of the important subjects of this paper. In this study, in order to better understand the mechanism of the energy losses, during a journey from hydrodynamics to hydraulics, the process of the energy loss of turbulent flows and its relationship with the turbulence parameters are examined through theoretical analysis and 3-D numerical simulations. The relationship between the mechanical energy loss and the roughness coefficient is further obtained and investigated. The results are presented in the form of the application of the new analytical relationships in open-channel expansions and determining the contribution of the effective parameters of turbulent flows to the energy loss.