The doctoral disertation considers the analysis of the hydrodynamic conditions near the bottom-hinged flap gates on a side weir, which are used to regulate the amount of the lateral discharge over the side weir. The disertation is divided into two parts: the experimental and the numerical part. Within the experimental part, discharge and water surface measurements for 380 variants were carried out. A new equation for the discharge coefficient of a bottom-hinged flap gate on a side weir was proposed, which covers the influence of the position and the width of the flap gate on the discharge coefficient. In addition, measurements of water levels near the flap gate and also measurements of the velocity fields were carried out with the computer-aided visualization method. From these measurements, it was possible to show that the contraction of the water jet varies with the gate opening angle. It was also found in which position a flap gate has the most favorable hydrodynamic shape. Within the numerical work, a 3D numerical model of two-phase flow, which uses the so-called VOF method for determining the interface was established. For this purpuse, the software code OpenFOAM was used. Within the numerical modeling three different turbulence models were analysed, these are the k-ε and k-ω SST turbulence model and the LES turbulence model with a Smagorinsky subgrid scale model. With the help of a calibrated 3D numerical model, the influence of different geometric and operational parameters of the flap on the pressure distribution at the gate, caused by the flow of water over the side weir, was analysed. Furthermore, the resultants of the forces and torsion moments in the hinge of the gate were determined on the basis of the results of the numerical model. An update of the OpenFOAM software code was also applied. In this context, a new solver named interWaterFoam was established, which allows more stable and robust calculations with limiting the velocities in the phase of air.