Velocity of overflowing water on the levee was measured using electromagnetic velocity meters (Model 802,Valeport Limited, Totnes, Devon, UK) at the upper side of the levee slope and was derived at lower part of the levee slope from Particle Tracking Velocimetry using a camera system. Both velocities provide an upper and lower bound for the velocity at the levee slope. The duration is defined as the time duration of the overflow test until erosion of the grass cover layer started. For each test executed the velocity - duration relationship is plotted on top of the graph from Whitehead (1976) together with the state of the levee slope at the end of the test (failed or intact).