Relating grass failure on the landside slope to wave overtopping induced excess normal stresses

A high quality safety assessment of levee systems requires a good prediction of when the grass cover of levees fail. Current methods relate the onset of failure to the peak in momentum or energy of the flow, instead of the peak in momentum transfer or energy transfer to the grass cover. The critical velocity necessary in the current methods is thereby difficult to quantify. In line with determining the peak in momentum transfer to the grass, here is shown that the onset of damage of the grass cover can be related to the peak normal stresses acting on the grass cover during wave overtopping. The peak in momentum transfer is thereby assumed to be located at the point of reattachment of the flow with the landside slope. The method is validated against the results of two wave overtopping experiments and benchmarked against the cumulative overload method. An advantage of this method is thereby that both the time and location of the onset of damage can be predicted.