An Experimental Study of Wave-Induced Set-Up Over a Horizontal Reef With an Idealized Ridge

Abstract

Wave action has been the primary focus of near-shore hydrodynamic studies over decades. The wave-induced set-up due to wave breaking is one of the important factors to consider in determining both water level and mass transport above the reef-top, which has ecological as well as engineering significance. Previous investigations into reef hydrodynamics (including laboratory experiments, field investigations, theoretical analysis and numerical simulations) focused mainly on wave-induced set-up on reefs with a horizontal reef-top (Gourlay, 1996. J. Coastal Eng. 27:161–193). It has been observed that a ridge (reef crest) may be present at the reef edge, but so far we are not aware of any published studies on the effects of this type of ridge on the wave-induced set-up over the reef-top. To understand the role of the ridge in wave-breaking mechanism and wave-induced set-up over the reef, a series of experiments were carried out in a wave flume of 36m long and 0.55m wide, with idealized reef-ridge models being installed at the reef edge to simulate fringing reefs with rectangle ridges. The surface elevations at four locations over the reef were measured with Ultralab sensors (General Acoustics), revealing the variation of wave-induced set-up along the reef. Experimental results are reported for two water depths and eighteen regular wave conditions. Also discussed are the effects of the ridge width on the wave-induced set-up. The focus of this paper will be given to the comparison of wave-induced set-ups with and without the rectangular ridges. Preliminary analysis shows that the ridge controls the water level above the reef-top in a way similar to that a broad crest weir controls the water level in open channel flows. Furthermore, the presence of the ridge is found to alter significantly the wave transformation process near the reef edge, especially the strength of the reflected waves and the locations of breakers. Experiments also show that the wave-induced set-up over reef-top with a ridge is generally much larger than that without a ridge. Finally, an attempt is made to introduce a new dimensionless parameter in order to take the ridge configuration into consideration and achieve a better agreement between experiments and predictions when ridges are present.