The Maximum Effect of Wind on Wave Overtopping at Rubble Mound Breakwaters with the Influence of the Slope and a Crest Element

Abstract

The impact of climate change, particularly the rise in sea levels, obstructs the effectiveness of existing coastal structures. Additionally, climate change can also have an amplifying effect on wind speeds (Takagi & Esteban, 2013). Without proper control, the accumulation of changing environmental boundary conditions could lead to disastrous events.
Currently, the design of rubble mound breakwaters is based on the EurOtop (2018) guideline. However, these guidelines may overlook essential influencing factors. Recent studies by Van Gent et al. (2022) and Irias Mata & van Gent (2023) have contributed to new insights that address these limitations. They aimed to identify numerous influencing factors that enhance the accuracy of wave overtopping expressions for rubble mound breakwaters. However, for several factors a more thorough understanding is still required.
Furthermore, recent insights on the influence of wind have proven to be significant. Previous studies indicate that the mean overtopping discharge has the potential to amplify its quantity several times beyond its initial value (de Waal et al., 1996; Wolters & van Gent, 2007; Van Gent et al., 2023 and Dijkstra, 2023). These investigations included tests on vertical sea walls and dikes.

This research focuses on investigating the maximum effect of wind on wave overtopping for a rubble mound breakwater, considering varying slope angles and crest element designs. The key parameter of interest is the maximum wind effect factor, described as a function of the non-dimensional overtopping discharge. Small scale models of rubble mound breakwaters with mild (1:6) slopes, steep (1:2) slopes, and various crest walls are examined to find their influence on the maximum effect of wind.
An extensive experimental programme was carried out within the Pacific Basin at Deltares. In this basin, a flume was constructed in which the breakwater models were built. Various hydraulic conditions, incorporating the water level and the wave characteristics, were systematically tested. All conditions were repeated, both with and without including the maximum wind effect.

A new overtopping expression is developed, taking the form of an exponential function that includes all relevant parameters. The influences corresponding to the crest wall are thoroughly investigated, while the other factors (roughness, obliqueness, and a berm) are only taken into account. The exponents and constant factors were obtained iteratively and were found to accurately predict the overtopping discharges obtained during the experiments.
The relationship between the maximum wind effect and non-dimensional overtopping, as observed in previous research, was confirmed for these measurements. This observation suggests that a maximum effect of wind increases when the non-dimensional overtopping discharge decreases. Only the configuration with a mild slope and the tallest crest wall stood out by showing a notable influence on the maximum wind effect. Moreover, the wind effect is more pronounced for an increasing water level when similar amounts of overtopping discharges are taken into consideration. Based on these findings, new factors for wind were derived.

In conclusion, this research has contributed to a deeper understanding of the influence of wind on wave overtopping. The presented wind factor expressions significantly improved the accuracy of the expression for each measurement, offering broader applicability across different structure types. Moreover, the amplification factors constructed in previous studies and in this study, have proven to be applicable across various structure types. This enhances overtopping predictions initially obtained for this study.