Wave overtopping at berm breakwaters: Development of prediction formula and a study on the impact of sea level rise on the overtopping rate

Abstract

Ports and harbours are often protected from violent waves by rubble mound breakwaters. Among the diverse types of rubble mound breakwater structures, berm breakwaters are relatively new, having been introduced in the early 1980s. In general, their construction ensures maximum quarry utilization and they can be built using smaller construction equipment. This reduces the overall cost and opens opportunities for local contractors. The wave induced overtopping rate is often considered a measure of safety of coastal structures with sea level rise, caused by global climate change, increasing the frequency and intensity of wave overtopping of these structures. Hence, the mean wave overtopping rate, and how it may change over time, is a key factor to be considered in the design of berm breakwaters. As a result of this, the wave overtopping rate has been extensively studied in the past decade. However, few studies focus on the overtopping behaviour of hardly/partly reshaping berm breakwaters. Additionally, a simpler and more accurate overtopping prediction tool, which includes the influence of the governing variables on the overtopping rate, is required for berm breakwaters. Given the above, the primary aim of this research work was to assess the influence of the structural and hydraulic variables on the overtopping rate at hardly/partly reshaping berm breakwaters through a series of laboratory tests. As a first step, the available prediction methods were reviewed in detail and a sensitivity analysis was performed to understand the influence of the governing variables in estimating the overtopping rate at berm breakwaters. In addition, a detailed analysis of data used to develop the models were conducted to understand their applicability. Then, the existing small-scale physical model test data were collected from previous research and was analysed comprehensively. The resultant database represented mostly a fully reshaping condition with limited tests on hardly/partly reshaping berm breakwaters. Hence, physical model tests were conducted (as part of this research) to remove the paucity of data in the existing databases. The new data represented overtopping from a wider range of tested wave steepness, berm width, berm level and crest level for the hardly/partly reshaping structures. The existing and new data were combined to develop a comprehensive overtopping database with 701 data. The second phase of the research focussed on the development of the empirical prediction formula to estimate the wave overtopping rate of berm breakwaters. The sensitivity analysis showed that there were significant inconsistencies among the available models in determining the influence of the variables on the estimated overtopping rate. To overcome these deficiencies/inconsistencies, and with a view to develop a universal prediction model for berm breakwaters, a new empirical formula was proposed using the comprehensive database (created as part of this research). In order to establish the new formula, the model tree approach of data mining was utilized and dimensionless parameters were used to develop the model to generalise the results. Then, the performance of the developed model was compared with other existing, and more complex, prediction models. The accuracy measures such as RMSE and Bias showed that the new formula was more accurate than the existing prediction methods. In short, the developed formula provides physically sound influences of the governing parameters on the overtopping rate and therefore it can be used as a robust tool for engineers in the design of berm breakwaters. Another issue that now must be considered in the design of coastal structures is the effect of climate change. Coastal structures, including berm breakwaters, are increasingly at risk of excessive overtopping due to climate change effects such as sea level rise (SLR). SLR needs to be considered in the safety assessment of existing and design of newly constructed berm breakwaters. Most of the existing literature is concentrated on the stability, increase in the run-up and overtopping failure of conventional rubble mound structures. Hence, as a capstone to this research, the influence of sea level rise on the overtopping rate of berm breakwaters was investigated. The newly developed formula was utilized for this investigation since it considers the influence of water depth better than the existing prediction models. The effect of SLR on the overtopping rate and the required upgradation of the structure were represented as functions of the relative change in water level. The results showed that overtopping increased exponentially in the shallow zone compared to that of the deep zone. The increase in the crest freeboard, required to maintain the design overtopping rate was estimated to be less than the increase needed compared to berm width for the different sea level rise scenarios considered. Furthermore, the required crest freeboard was influenced less by the initial configuration of the berm width. Finally, in the last part of the research, the focus was extended on the probability of failure (in terms of the overtopping rate) and the optimum upgradation interval of berm breakwaters considering the influence of sea level rise during their service life. Optimum upgradation intervals were determined by minimising the cost of upgradation and failure. The results were further exemplified using the design parameters of the Sirevag berm breakwater in Norway. The outcome of the analysis could be used as a preliminary assessment of the upgradation measures to be adopted and requires detailed cost and feasibility studies. The outlined method can be used to quantitatively estimate the influence of the SLR on the overtopping rate and could also be included in the design philosophy of berm breakwaters.