Browsing by Author "Kretschmer, Marcus Malan"

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    Effect of the form of the overhang of a recurve seawall to reduce wave overtopping
    (Stellenbosch : Stellenbosch University, 2017-12) Kretschmer, Marcus Malan; Schoonees, J. S.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.
    ENGLISH ABSTRACT: Coastal areas around the world are experiencing a rise in sea level due to the effects of global warming. As a large percentage of the world’s population resides within the coastal zone, the rise in sea level is placing evermore developments and people at risk, consequently increasing the demand for more effective coastal defence structures. One of the most common types of coastal defence structures are seawalls, which reduce or prevent wave overtopping and flooding of the landward side of the structure. These structures are often designed as vertical impermeable walls with high crests to ensure protection against overtopping. However, these designs are not always favourable as the high crest levels often obstruct the view of the sea. Recurve seawalls provide the solution, as they reduce wave overtopping without excessively compromising the sea view. However, existing guidelines for the 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 seawalls on reducing wave overtopping. In order to achieve the objective of this study, the performance of different recurve forms in reducing wave overtopping was evaluated by conducting overtopping tests in a physical model. The model was constructed in a glass wave flume equipped with a piston-type wave paddle. A total number of 147 tests with varying water levels and wave periods were conducted for five different overhang forms, providing a comprehensive set 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 influence on the overtopping reduction capabilities of a recurve seawall. It was found that the concave shape with a squared overhanging edge offered the most reduction in overtopping, when compared 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 were observed as a result of colliding incident and reflected waves. These overtopping events constituted mainly of white water spray which were negligibly small but should be treated with caution as the presence of an onshore wind, which was not modelled in this study, could have a significant influence on the amount of overtopping. In addition, it was found that overtopping generally increases with an increase in the wave return 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, due to the adhesion of water along the rounded edge. This finding led to the conclusion that apart from the overhang length, the shape of the overhanging edge also significantly influences the reduction of overtopping. As opposed to the findings of a previous study, increasing the wave period 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 beach slopes and wave heights on overtopping of recurve seawalls should also be investigated, as these remained constant throughout this study.