A probabilistic framework is developed to calculate the cross-dike failure probability by overtopping waves on grass-covered dikes. The cross-dike failure probability of dike profiles including transitions and damages can be computed to find the most likely location of failure and quantify the decrease in the failure probability when this location is strengthened. The erosion depth along the dike profile is calculated using probability distributions for the water level, wind speed and dike cover strength. Failure is defined as the exceedance of 20 cm erosion depth when the topsoil of the grass cover is eroded. The cross-dike failure probability shows that the landward toe is the most vulnerable location for wave overtopping. Herein, the quality of the grass cover significantly affects the failure probability up to a factor 1000. Next, the failure probability for different types of damages on the landward slope are calculated. In case of a damage where the grass cover is still intact and strong, the dike is most likely to fail at the landward toe due to high flow velocity and additional load due to the slope change. However, when the grass cover is also damaged, the probability of failure at the damage is between 4 and 125 times higher than for a regular dike profile.

Wave overtopping results in erosion of grass-covered flood defenses which can result in a dike breach. Damages to the dike cover decrease the cover strength and simultaneously increase the hydraulic load on the dike cover. Therefore, damages on the dike cover are vulnerable locations for wave overtopping failure. In this study, the failure probabilities of different types of damages are calculated and compared to the failure probability of a regular dike profile to determine the vulnerability of damages for wave overtopping. The method is applied to a river and lake dike where the model results show that the total failure probability is smaller on the crest and upper slope for the lake dike and higher on the lower slope and at the inner toe. The failure probability increases between a factor 4 and 14 for damaged location where the cover still contains grass vegetation. However, lake dikes are more vulnerable for damages compared to lake dikes in cases where only a bare clay cover is left. In that case, a dike with a damage is almost 3000 more likely to fail compared to a regular dike for a lake dike and around 120 times more likely to fail for a river dike.

No abstract