Water column separation (WCS), the breaking of continuous water flow by a vapor cavity, is extremely dangerous because the rejoining of the separated water columns would lead to tremendous pressure rises. Therefore, the turbine draft-tube pressure after the load rejection in hydropower stations should be controlled to prevent such an unbearable load of turbine units. To understand the forming, growing and collapsing processes of WCS in pump-turbines during the load rejection transients with guide-vane closing, a model pumped-storage system was simulated by the one-dimensional and three-dimensional coupled computational fluid dynamics model, in which the water conveyance system and pump-turbine were treated as one-dimensional and three-dimensional domains, respectively. The entire processes of the breaking and rejoining of water flow in the pump-turbine, as well as the variations in the pressure field and hydraulic forces on the runner, were analyzed. The main finding is that the collision of the separated water liquids and the collapses of the cavities occur separately with a time difference, which induces two high-amplitude mono-pulses in both the axial and radial forces on the runner. These two successive hydraulic impacts are extremely destructive and should be considered with greater emphasis when evaluates the risk of WCS. (C) 2020 Elsevier Ltd. All rights reserved.