The maximum wind effect on wave overtopping at dikes with crest elements

Abstract

Climate change affects sea level rise and the safety of the people living behind seadikes. In order to prepare for this, several adaptation measures are available to reduce the overtopping discharge. Wave overtopping can be reduced by including roughness elements or adding a berm. Another possibility is to employ crest elements such as a vertical crest wall, promenade or parapet on the crest wall to reduce wave overtopping discharges.

The overtopping discharge is key when defining the dike dimensions. Moreover, the effect of wind on the overtopping discharge is not included in existing guidelines, whereas wind is complex due to its dynamic behaviour. Also, the knowledge niche regarding the position and height of the vertical crest wall is the reason for performing more research. Physical model tests are conducted to gain more knowledge about the maximum wind effect to obtain a better understanding of how it affects the overtopping discharge and the loading on seadikes.

The following research question is covered in this master thesis: What is the maximum wind effect on wave overtopping at dikes with crest elements? The aim of the present research is to examine this wind effect based on physical model tests, which are performed at Deltares in Delft, the Netherlands. Experiments on a small-scale model of a seadike with a smooth outer slope at an angle of tan⁡(α)=1:3 are conducted. In total, four dike configurations were tested, consisting of a crest wall, which is placed at the seaside of the dike crest in one case and on the land side in another. A paddle wheel is used to simulate the maximum effect of wind, based on the idea that all vertical spray exceeding the dike crest is transported over the crest by onshore wind. The maximum wind effect is determined by comparing the tests with and without the use of the paddle wheel.

The results of this investigation show an optimisation of the existing guidelines, such as TAW (2002), for calculating the overtopping discharge. For non-breaking waves, the wave steepness is not included in the TAW (2002) overtopping formula, but the data show a clear dependency on this parameter.

One of the most significant findings from this study is a quantification of the maximum wind effect. It provides an amplification factor on the overtopping discharge. This maximum wind effect is defined as the ratio q_wind/q, which indicates the overtopping discharge with maximum wind effect due to onshore blowing winds (q_wind) over the overtopping discharges without wind effects (q).