Mass flux transported by breaking wave rollers over submerged breakwaters

Wave induced shoreline currents and surges are major contributors to beach erosion. Various coastal structures (e.g. breakwaters, seawalls, dikes and revetments) have been constructed to solve or reduce the problems of beach erosion by altering/controlling surges and currents. All forms of shore protection methods have certain disadvantages. Offshore breakwaters can retard erosion on existing beaches in a more environmentally friendly manner by promoting natural sediment to form a new beach alignment which maintains a recreational usable wide beach during storms. Offshore breakwaters may be shoreline parallel or oblique and either emergent or submerged. Submerged offshore breakwaters have been recently used to reduce coastal erosion without spoiling coastal landscape. This being increasingly important in case of recreation and residential shore developments. In addition, submerged breakwaters are beneficial in providing artificial breeding and juvenile shelter areas for fish and benthic species. The impact of submerged breakwaters on currents, beach profile and shoreline alignment depends on wave transmission over and wave-induced current behind the structure. The functional design knowledge of submerged breakwaters and their impacts on shore profile is far from adequate. An research program has therefore been undertaken to develop reliable models and engineering design methods for predicting hydrodynamic behaviour and morphological response in the vicinity of submerged breakwaters. The effects of submergence ratio, exposure ratio, distance of structure to shoreline, crest width of breakwater, water level and wave conditions have been investigated. The gradient of wave-induced set-up behind submerged breakwaters causes water to flow behind such structures. Wave induced set-up and current are dependent on mass flux over the breakwater which is contributed by wave averaged flow generated by radiation stress gradients, leakage of water through the porous structure of the breakwater and mass transport by breaking wave rollers (Lesser et al. 2003). Most of the existing models apply radiation stresses formula provided by Longuet-Higgins and Stewart (1964). In this formula wave set-up depends on incident wave energy, wave direction, water depth and wave number in shallow water. However, mass transport due to breaking wave rollers has not been considered in any hydrodynamic models for submerged breakwaters. Gourlay (1993, 1996) conducted several experimental tests for measuring wave set-up and mass flux over reefs. He observed wave set-up on a horizontal coral reef increases with wave height and wave set-up does not occur in large water depth allowing waves to pass over the reef. He also showed that wave-induced flow across the reef increases with increasing incident wave height and period in a similar manner to wave set-up. In his research, reefs with long crest width only were tested due to research focus being on broad coral reefs. The impact of crest width (shorter), surface roughness and permeability of structure were ignored. Symonds et al.(1995) presented a theoretical model for wave-driven flow over shallow reefs. They investigated the wave set-up and cross-reef currents generated by waves breaking on the reef face as a function of incident wave height and still water depth over the reef. They provided a linear theoretical model demonstrating how the relative magnitudes of the currents and set-up depend on the geometry of the reef and the magnitude of the forces including radiation stress. The model verification is limited to the specific field measurement conditions on John Brewer Reef , Queensland, Australia (Hardy et al. 1990 and Hardy, 1993). The model has not been verified for the wider range of submerged breakwater geometries, water levels and wave conditions tested this new experimental study. The aim of this study is to expand the range and accuracy of experiments by Gourlay (1996) and provide some hydrodynamic empirical formulas to calculate mass flux over shallow breakwaters with different geometry, porosity, roughness and wave conditions. The data has been obtained from a variety of experiments conducted in 2D wave flume.