Effect of the form of the overhang of a recurve seawall to reduce wave overtopping

[摘要] ENGLISH ABSTRACT: Coastal areas around the world are experiencing a rise in sea level due to the effects of globalwarming. As a large percentage of the world's population resides within the coastal zone, therise in sea level is placing evermore developments and people at risk, consequently increasingthe demand for more effective coastal defence structures.One of the most common types of coastal defence structures are seawalls, which reduce orprevent wave overtopping and flooding of the landward side of the structure. These structuresare often designed as vertical impermeable walls with high crests to ensure protection againstovertopping. However, these designs are not always favourable as the high crest levels oftenobstruct the view of the sea. Recurve seawalls provide the solution, as they reduce waveovertopping without excessively compromising the sea view. However, existing guidelines forthe design and research on the effectiveness of different recurve seawalls are very limited.This project aims to determine the effectiveness of different overhang forms of recurve seawallson reducing wave overtopping. In order to achieve the objective of this study, the performanceof different recurve forms in reducing wave overtopping was evaluated by conductingovertopping tests in a physical model. The model was constructed in a glass wave flumeequipped with a piston-type wave paddle. A total number of 147 tests with varying water levelsand wave periods were conducted for five different overhang forms, providing a comprehensiveset of results which were analysed to determine the most effective overhang form.Analysis of the findings clearly indicated that the shape of the overhang has a strong influenceon the overtopping reduction capabilities of a recurve seawall. It was found that the concaveshape with a squared overhanging edge offered the most reduction in overtopping, whencompared with the performances of the remaining forms. For the high relative freeboard cases(large difference between crest level and water level), small amounts of overtopping wereobserved as a result of colliding incident and reflected waves. These overtopping eventsconstituted mainly of white water spray which were negligibly small but should be treated withcaution as the presence of an onshore wind, which was not modelled in this study, could havea significant influence on the amount of overtopping.In addition, it was found that overtopping generally increases with an increase in the wavereturn angle of the recurve. When comparing the two different concave recurves tested, the recurve with the rounded overhanging edge produced up to fifty percent more overtopping, dueto the adhesion of water along the rounded edge. This finding led to the conclusion that apartfrom the overhang length, the shape of the overhanging edge also significantly influences thereduction of overtopping. As opposed to the findings of a previous study, increasing the waveperiod up to 14 seconds consistently led to an increase in overtopping.It is recommended that further model tests should be conducted on concaved recurves,including variations in the vertical dimension of the recurve. The effects of different beachslopes and wave heights on overtopping of recurve seawalls should also be investigated, asthese remained constant throughout this study.

[发布机构] Stellenbosch University