A Critical Analysis and Validation of the Accuracy of Wave Overtopping Prediction Formulae for OWECs

Abstract

Abstract:The development of wave energy devices is growing in recent years. One type of device is the overtopping wave energy converter (OWEC), for which the knowledge of the wave overtopping rates is a basic and crucial aspect in their design. In particular, the most interesting range to study is for OWECs with steep slopes to vertical walls, and with very small freeboards and zero freeboards where the overtopping rate is maximized, and which can be generalized as steep low-crested structures. Recently, wave overtopping prediction formulae have been published for this type of structures, although their accuracy has not been fully assessed, as the overtopping data available in this range is scarce. We performed a critical analysis of the overtopping prediction formulae for steep low-crested structures and the validation of the accuracy of these formulae, based on new overtopping data for steep low-crested structures obtained at Ghent University. This paper summarizes the existing knowledge about average wave overtopping, describes the physical model tests performed, analyses the results and compares them to existing prediction formulae. The new dataset extends the wave overtopping data towards vertical walls and zero freeboard structures. In general, the new dataset validated the more recent overtopping formulae focused on steep slopes with small freeboards, although the formulae are underpredicting the average overtopping rates for very small and zero relative crest freeboards.

Average and Wave-by-Wave Overtopping Performance of Steep Low-Crested Structures

“…The experiments were performed in the wave flume of the Department of Civil Engineering at Ghent University (Belgium) which has a length of 30 m, a width of 1 m and a height of 1.2 m. The wave flume is equipped with a piston type wave paddle with a maximum stroke length of 1.5 m. The experimental setup of the UG13 dataset (Gallach-Sánchez et al, 2018) consists of a horizontal foreshore (Figure 2, top). For the UG14 dataset (Gallach-Sánchez et al, 2014) and the UG15 dataset (Gallach-Sánchez et al, 2016) it consists of a mild 1:100 foreshore slope over 15 m. The influence of the foreshore slope on the average overtopping rates is considered negligible for all the datasets as the foreshore does not influence the wave transformation processes taking place in the wave flume.…”

“…This close to linear shape was already observed in the data when comparing UG13, UG14 and UG15 with Eq. 2 (Gallach-Sánchez et al, 2014, 2018. The value selected for c in the update of the coefficients c = 1.1 yields a prediction trend close to a straight line in a log-linear plot, matching better the shape of the data.…”