The effect of variations in water level on wave overtopping discharge over a dike: An experimental model study

The water level during a storm event is never constant but variable in nature. A variable water level leads to varying crest freeboards at the coastal defence structure. A change in crest freeboards is directly linked to a changing average overtopping discharge q over the coastal defence structure. The existing prediction formulae for q are developed for constant water level (CWL) and do not take into account its variability (VWL — variable water level) during a storm event. Hence, in this study the influence of the VWL on q was investigated experimentally. Model tests were conducted in the wave flume facility at Ghent University, where a range of VWL was modelled as representative for typical storm surges (total variation of the water level dh= 0.75–4.8 m) and a range of storm durations (time t= 2.7–22 h). During testing the q for these cases of VWL over a reference impermeable sloping (seaward slope of the structure cot(α) = 2) dike structure was measured. Initial validation of model results for the CWL situation showed that the model setup was capable of reproducing the predicted q values from Eurotop (2018). For the VWL situation it was noted that the existing prediction formulae (Eurotop, 2018) are overestimating the measured q value, especially for smaller relative crest freeboards (relative freeboards Rc/Hm0<1.5) when the smallest freeboard is considered. By using linear regression a new equivalent relative crest freeboard was derived to reduce the difference between q predicted by existing EurOtop (2018) and q measured during variable tests. The new equivalent freeboard is mainly dependent on the relative crest freeboard during the peak of the storm (Rcpeak/Hm0), as well as the total storm surge variation (dh). The existing formulation from EurOtop (2018) was kept by replacing Rc by this new equivalent crest freeboard. The adapted formula for q is able to accurately predict the overtopping discharge for both, CWL and VWL situation