Effect of the overhang length of a recurve seawall in reducing wave overtopping

[摘要] ENGLISH ABSTRACT: With the slow but steady rise in sea level, which is due to global warming, the pressure on the coastal zone across the world has greatly increased. In the past coastal developments have frequently encroached onto the shore, therefore progressively more of these developments have recently come under increasing wave and storm attack, with large portions of the world's coastlines needing protection.A solution to adequately protect the coastlines under threat would be to increase the crest height of existing seawalls. However, since this would often obstruct the sea view, such a solution would be unacceptable to seaside property owners. The construction of recurve seawalls to reduce overtopping provides a solution, while limiting the obstruction of the sea view.Historically, seawalls have been used to protect coastlines. Recurve seawalls, where the sea-facing side of the wall is shaped concavely forward to re-direct wave attack back out to sea, were first designed in the 1980s. However, very few guidelines for the design of recurve seawalls are currently available.Against this background, the current physical model study was conducted with the objectives of quantifying the reduction in wave overtopping in terms of the various geometrical properties of the recurve seawall, such as the overhang length and freeboard height (being the difference between the crest level and water level), and developing design curves for recurve seawalls. To achieve the objectives of this study, a 2D physical model was designed and built. More than 200 tests were undertaken in order to cover a wide range of water levels and wave periods for nine different recurve seawall designs.The study found that as the overhang length increases, the reduction in overtopping increases up to a certain point, after which a longer overhang length has no further significance. However, under certain conditions, the 0.3 m overhang length produced worse overtopping reduction results than the vertical wall. Further, a recurve seawall with a parapet angle greater than 50º will not improve the reduction in overtopping, when compared to the results for a vertical wall under similar conditions.It was concluded that the crest level, in combination with the freeboard level, are critical parameters in the determination of overtopping. It is recommended that the freeboard should be sufficient so that the incoming wave hits the vertical part of recurve wall. If sufficient freeboard is not available, the recurve wall will be drowned and will not provide any overtopping reduction. A combination of high freeboard and low water levels can produce up to 100% reduction in overtopping.The repeatability of the tests showed that the accuracy is very good, and significantly better than the data in the CLASH database. Comparison of the measured overtopping with the prediction of the most referenced database, the EurOtop dataset, showed that the EurOtop method should be used with caution.Additional tests should be conducted to investigate the influence that the beach slope, as well as the wave height, will have on the effectiveness of the recurve wall. Further overhang lengths in a critical area (for example the 0.2 m and 0.4 m overhang lengths), are also required to expand the usefulness of the design guidelines. The stability of the recurve seawalls and of their foundations should also be considered in greater detail.

[发布机构] Stellenbosch University