Wave Overtopping at Seadikes

Wave overtopping has been responsible for many dike failures, dike breaches and severe floodings in the past. An example of dike failures due to wave overtopping is given. At first, it seems to be astonishing that dike failures are initiated on the landward slope since the incoming waves are hitting the seaward slope. Three aspects are responsible for this misinterpretation. First, wave overtopping has been often neglected for the design of seadikes because a no overtopping criteria was assumed. Second, landward slopes of seadikes are often steeper and the cover (mostly clay) of the landward slope is thinner than the cover of the seaward slope. Third, the landward slope has been designed based on experience and no physically based design methods are available at present. In the future, the necessity of a well designed landward slope is increasing due to the uncertainties concerning sea level rise and the increasing frequencies and intensities of storm surges worldwide. Present formulas for wave overtopping consider average overtopping rates. Average overtopping rates are required to design the drainage of the hinterland of seadikes but not for the stability of the landward slope. The stability of the landward slope depends, besides geotechnical aspects, on the instantaneous loads due to wave overtopping, which can be described by layer thicknesses and overtopping velocities.