In this paper, 2D greenwater overtopping of a floating box is numerically investigated using CFD. The incident waves used are compact wave groups which correspond to the maximum relative wave elevation at the edge of the box according to linear theory. The motion of the box for which only heave and pitch modes are considered is calculated using linear potential flow theory and then prescribed in the CFD simulation. It is found that the normalized maximum horizontal momentum flux of greenwater overtopped onto the moving box is consistent with that for a fixed box when the incident wave length is much smaller than the box length. For longer incident wave lengths there is arguably more scatter in the momentum flux compared with that of a fixed box, although the result is still bounded by a value of 1.5 times that predicted by a classical dam break solution. This additional scatter is shown to be due to the effect of box motion.