Effects of Transitions and Objects on the Erodibility of Grass Revetments on Dikes

This study deals with the effects of transitions and/or objects such as roads and trees in grass revetments on dikes/levees under (wave) overtopping conditions. Three types of impact categories are defined, that is 1) little impact, 2) moderate impact and 3) large impact. Based on the grass erosion model (i.e., the cumulative overload method) a relation is deduced that describes the critical overtopping discharge as function of the load increase factor for both a sea and a river regime. This impact factor represents the load increase near transitions and objects which is here expressed by a decrease of the critical overtopping discharge. Lognormal distributions for the critical overtopping discharge are used to calculate the failure probability due to overtopping with and without transitions and objects. For several dike-sections in the Netherlands the differences are calculated with the model PC-Ring. The study shows that the differences are largest for dikes along the coast and lakes. For dikes in a river regime where the significant wave height is usually low the erosion differences are marginal.