Tolerable wave overtopping during construction of a breakwater

Abstract

Wave overtopping is an important aspect for the design of rubble mound breakwaters. Acceptable overtopping limits are usually based on the mean overtopping discharge or the maximum overtopping volume (EurOtop, 2018). For temporary situations, such as breakwaters under construction, overtopping limits are not clearly defined. This complicates the choice for a working crest level, where the safety of staff and equipment on the partially constructed breakwater is the main concern.
In this study, OpenFOAM in combination with the waves2foam toolbox (Jacobsen et al., 2012) is used to model wave overtopping over a breakwater under construction to identify the flow characteristics (i.e. the flow depth and velocity) caused by overtopping waves. The aim of the study is to provide a method to define a safe working level for rubble mound breakwaters under construction, based on the tolerable flow depth and velocity. The safety of the staff and equipment is expressed in limit functions. The method of Sandoval (2016) is applied to define the limit function for workers safety on the breakwater under construction (based on combinations of flow depth and velocity). The safety of the hydraulic excavators is mainly based on the overtopping flow depth, e.g. the flow depth should not exceed the cabin-level.
2D physical model test results on a partially constructed breakwater are processed in order to obtain benchmark data to validate the numerical model. Once the numerical model was validated, a numerical test program was set up that varied the wave height, wave steepness and the relative freeboard, in order to investigate the dependence of the overtopped flow depth and velocity on the varied conditions. A JONSWAP spectrum was used to simulate 8 different wave fields of 250 waves. On the crest of the numerical breakwater, the flow depth and velocity are measured by means of equally spaced gauges and probes in order to describe the flow behavior.
Overtopping events due to the irregular wave train are statistically analysed in terms of flow depth and velocity. The empirical 2% exceedance values are considered as design parameters as these can be compared to existing literature on impermeable and smooth dikes, to highlight the effects of the partially finished and permeable structure.
Empirical relations between the investigated parameters and the overtopping flow depth and velocity on the core crest edge of the breakwater are presented. It is found that the maximum flow depth reduces exponentially over the crest. The reduction appears less strong when saturation of the crest material starts to play a role. The flow velocity increases over the crest for the cases with larger initial flow depths and/or flow detachment from the breakwater crest edge, in contrast to the formulas for dike cross sections. This latter phenomenon was explained by the acceleration caused by the redirection of the flow on the breakwater crest. From the numerical results, two distinct areas of the unconstructed breakwater crest can be identified: a “transition zone” where the flow is detached from the breakwater crest and a “flow zone” with attached flow where the limit functions are valid.
Based on the numerical findings, empirical guidance is given for the land-based workability of breakwaters under construction. It is shown how the newly developed empirical formulas can be applied in practice to determine a safe working level. It is noted that the maximum overtopping flow depth and flow velocity do not necessarily occur at the same moment. A method is introduced that shows how the safety can be assessed using a combinational number for the flow depth and flow velocity (based on numerical modelling) and the stability limit for people. It is shown how a safe working level can be found based on exceedance values of safety limits.