Wave overtopping a sea dike may pose a threat to people and property. However, knowledge of the overtopping features, in particular overtopping flow loads, are not well understood. The aim of this study was to understand the overtopping process on a dike crest and to develop an empirical formula for the resulting overtopping flow impact loads on a wall as a function of the property of the incoming waves and dike geometry characteristics. In this paper, we propose a new descriptor (the overtopping momentum flux) in order to predict the impact loads. To validate the proposed empirical function, a series of physical scale model tests was conducted. In these experiments, we measured the overtopping flow loads on a vertical wall at different locations on a dike, which were induced by broken waves. A correction coefficient for the wall effect on the initial flow depth, and an empirical initial flow depth coefficient for a broken wave were determined. These empirical coefficients allowed for an interpretation of the overtopping process of a broken wave from dike toe up to the front of the wall on the dike.