In recent years, dike resilience against wave overtopping has become a more popular research topic due to the expected effect of future climate changes. Rising sea water levels and increased storminess will lead to a more common and more severe wave overtopping of existing dike defences. Wave overtopping can especially damage the rear slope of the dikes and might in worst cases lead to a complete failure of the dikes due to soil instability.

Recent 2D small scale experiments have evaluated the impact of overtopping flow depths and flow velocities on dikes. These experiments let to the development of formulae which can be used to estimate different flow parameters across a dike. The overtopping flow parameters have been coupled to the actual erosion of the landward dike slope from full-scale 2D tests using the so-called “Overtopping Simulator”.

Between November 2010 and January 2011, the team of Aalborg University (Denmark) performed 91 small-scale tests to evaluate the so-far unknown influence from oblique and short-crested waves on the flow velocity and flow depths at the crest and the landward slope of a sea-dike. This was done to extend the existing 2D formulae. Several incident wave directions and directional wave spreading were evaluated. Moreover, different water levels were considered to evaluate the influence from sea level rise. The dike model was smooth and with seaward- and landward slopes of 1:4 and 1:2.5, respectively. Overtopping flow depths were measured by 11 depth gauges mounted directly on the dike surface and an innovative “least square fit” of flow velocity magnitudes and flow directions was made from the time intervals between wave fronts passing the different gauges.

Based on the test results, correction formulae for the wave obliquity and directional wave spreading were proposed for use in the existing 2D formulae. The wave obliquity was seen to have a great influence on flow velocity, and therefore to significantly reduce the risk of erosion of the sea dikes.

The findings from the physical small-scale studies and the updated formulae can, in collaboration with the findings by other THESEUS partners, be used to perform more realistic estimates of dike resilience against increasing wave overtopping from future climate changes.

By Jørgen Harck Nørgaard