Browsing by Author "De Bondt, Franco Hibbard"
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- ItemEffect of the nearshore profile on wave overtopping at a recurve seawall(Stellenbosch : Stellenbosch University, 2020-03) De Bondt, Franco Hibbard; Schoonees, J. S.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering.ENGLISH ABSTRACT: Infrastructure located close to the shoreline is generally highly valuable, even though it is often exposed to significant flood risk. With an increased wave attack caused by climate change and the consequent rise in sea levels, the risk of economic damage or loss of life due to violent overtopping is constantly increasing. Reduction of overtopping at coastal defensive structures is therefore crucial in terms of the design, management and adaptation of coastal structures, especially when existing coastal defensive structures are evaluated for suitability in future conditions. In places where space does not allow for the construction of dikes or rubble mound breakwaters, vertical seawalls are often built to decrease the risk of flooding and damage to landward infrastructure, assets and injury to people, especially pedestrians. As some of the existing vertical seawalls are now being exposed to increased wave attack, more sudden, severe overtopping events are taking place which is dangerous to both property and humans. In order to decrease overtopping volumes, often vertical defensive structures incorporate a recurve seawall. The recurve shaped overhang reduces overtopping by deflecting water that is forced upwards by the vertical face of the structure, back seaward. By adding a recurve section onto the crest of a vertical seawall, the crest height of the structure can be reduced. Although recurves are often incorporated into seawall design, literature offers limited guidance for the design of such structures. This study, therefore, investigates the effect that the nearshore profile has on the overtopping of an optimised recurve seawall. A secondary objective was to determine the effect that the nearshore profile has on the increase or reduction of the incoming wave height. In order to achieve the objectives, 2D physical modelling tests were performed in a glass wave flume equipped with a piston type wave paddle that is capable of active wave absorption. The tests were performed on four different nearshore profiles, three which were typical to the Southern African coastline, and the other an average beach profile used in preceding studies. The three typical Southern African profiles included a steep, flat and mild nearshore profile. Tests were performed with 3 different water levels and 5 different wave periods, while the seawall design, crest height and wave height were kept constant. Analysis of the findings indicated that the nearshore profile had a significant influence on the overtopping of a recurve seawall. Results show that wave overtopping increases with a decrease in the steepness of the nearshore profile. Consequently, the gentlest nearshore slope led to the highest overtopping volumes of the recurve seawall. However, the mild profile with an offshore berm also produced large overtopping rates, mostly due to colliding reflected and incident waves causing large individual overtopping events. It is recommended that further model tests be conducted on the effect that beach erosion or accretion has on the overtopping of recurve seawalls. Another possible study can be done on the effect of different wave heights on the overtopping of recurve seawalls. The wave forces caused by different wave conditions should also be investigated, as forces on a structure is a necessary component for the structural design of recurve seawalls.