This report addresses the preparation and performance of laboratory tests on a the erodibility of clay cover with and without grass, subjected to impact pressures generated by controlled falling water mass Three types of clay, representing different erosion resistances as well as grass cover of moderate quality have been used in order to gain information on the processes of surface erosion and shear failure within a crack in a dike cover subject to impact pressures. A computer-controlled system was applied to generate the impact pressures in the range 12-25 kPa by using a mass of water which is suddenly dropped from a given height. The conceptual model of Führböter (1966) on the shear failure of a crack in a clay revetment is compared to the experimental results. The shear failure itself did indeed occur. However, the failure mechanism significantly differs from the predicted one. The tests with the clay samples subject to a series of impact pressure events provided results that fit relatively well with the model of Torri et al. (1987). The coefficients describing the parameters of the soil and the damping effect of the water layer were calibrated using the experiments. A new empirical approach to calculate the influence of the root network on the erosion of grass due to impact pressures and contribution to existing theories are discussed. The results are expected to considerably contribute to a better understanding of the mechanisms and to the prediction of breach initiation of sea dikes.