Erosion at transitions in landward slopes of dikes due to wave overtopping

Destructive wave overtopping tests have been performed at more than 20 dike sections in the Netherlands using the Wave Overtopping Simulator to simulate the overtopping waves. The tests show the combined behaviour of turf and substrate up to mean wave overtopping discharges of 75 ?/s per m, meaning wave volumes up to 5,500 ?/m with flow velocities up to 8 m/s. Observed failure mechanisms are (1) erosion of the grass cover on the slope, (2) erosion at transitions to a horizontal berm or at the toe, and (3) erosion related to non-water retaining structures such as a concrete staircase in the slope, fences, and poles. Based on observations during the tests, preliminary conclusions are that transitions from slope to horizontal dike sections are the most vulnerable locations for damage. The dike slope itself covered with grass on clay never failed by erosion due to a mean overtopping discharge of 10 ?/s per m or less. Obviously, non-water retaining structures such as a staircase in the slope induces significant scour holes due to the concentrated flow adjacent of the staircase. The paper deals with the failure mechanism erosion at transitions. The overtopping wave acts as a jet impacting on the horizontal section and creates a scour hole. The scour hole increases during each overtopping wave with a certain ”amount of energy” above a threshold value determined by the characteristics of the grass-clay cover. Model development is going on based on both the analogy with jet erosion and the method according to the excess of the shear stress. Additional tests will be carried out from 2012 on sea and river dikes to further develop the erosion models.Hydraulic EngineeringCivil Engineering and Geoscience