The weir-gate compound structure hydraulic characteristics and the hydraulic behavior of the pier mounted by a collar at the base are investigated experimentally. This investigation refers to the fluid-structure interference and, on the other hand, represent the interference among two different structures installed inside the channel. So both structures have a direct influence on the flow features between them. The main target of this work, reveal how the pier-collar system share in raise water depth and how share in change the hydraulic features of weir-gate. The study adopted different weir shapes with an ellipse gate, as well as different sizes of the pier and the collar, to perform the experiments. From the experiments, it was found that the pier size, collar size, and weir shape produced a nonlinear water surface profile at the compos-ite hydraulic structure downstream region. The drag force, static force, and moment that are grown at the pier base are more influenced by the alteration in flow velocity and flow depth that occurs owing to the presence of the pier and the collar. More specifically, the change in the flow velocity and the flow depth due to the presence of the pier and the collar will be reflected directly in the obtained values of the Froude number and Reynolds number. Overall, the collar has a moderate effect on the hydraulic characteristics as compared with the pier. The result shows a discharge coefficient has a direct proportional with actual discharge. The quantitative result which appears from this study the downstream water depth and actual flow rate obey and satisfy confident level equal to and greater than 95%.