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

Abstract

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 defense structure. A change in crest freeboards is directly linked to a changing average overtopping discharge q over the coastal defense structure. The existing prediction formulae for q are developed for constant water level ( C W L ) and do not take into account its variability ( V W L — variable water level) during a storm event. Hence, in this study the influence of the V W L on q was investigated experimentally. Model tests were conducted in the wave flume facility at Ghent University, where a range of V W L was modelled as representative for typical storm surges (total variation of the water level d h = 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 V W L over a reference impermeable sloping (seaward slope of the structure cot( α ) = 2) dike structure was measured. Initial validation of model results for the C W L situation showed that the model setup was capable of reproducing the predicted q values from Eurotop (2018). For the V W L 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 R c / H m 0 < 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 dependant on the relative crest freeboard during the peak of the storm ( R c p e a k / H m 0 ), as well as the total storm surge variation ( d h). The existing formulation from EurOtop (2018) was kept by replacing R c by this new equivalent crest freeboard. The adapted formula for q is able to accurately predict the overtopping discharge for both, C W L and V W L situation. • 2D Physical model tests on wave overtopping performance of a smooth dike. • Influence of variable water level conditions on the average wave overtopping discharge. • New equivalent relative freeboard is determined to include this effect in the EurOtop formula for non-breaking waves. • Predictions are significantly improved with the new equivalent relative freeboard for the new data.