The dynamics of infragravity wave transformation over a fringing reef

Abstract

[1] A 3 week field study was conducted to investigate the dynamics of low-frequency (infragravity) wave motions over a fringing reef at Ningaloo Reef, Western Australia. Short-period wave motions (0.04-0.2 Hz) were observed to dissipate on the reef crest beyond which infragravity wave motions (0.004-0.04 Hz) gradually dominated toward the lagoon. However, both the short waves and the infragravity waves were relatively small (both <0.3 m) on the reef flat owing to the shallow water depth (<2 m). The results revealed that the surf zone generation of free infragravity wave motions on the steep ($1:20) fore-reef slope was dominated by breakpoint forcing (as opposed to shoaling bound waves), which was also supported by detailed numerical simulations of the generation process. This is consistent with theory suggesting the efficiency of the breakpoint forcing mechanism should be high in this steep-slope regime. Shoreward propagating infragravity waves traveled across the reef but were damped by bottom friction dissipation; however, this was at a rate much smaller than experienced by the residual short waves. With these rates of frictional dissipation also strongly dependent on the water depth over the reef, the infragravity wave heights increased at higher water levels and hence were strongly modulated by the tide. Due to the strong dissipation of infragravity waves over this wide and shallow reef that is hydraulically rough, any seaward propagating infragravity waves that reflected at the shoreline were small, leading to the dominance of progressive (shoreward propagating) infragravity wave motions throughout the reef and lagoon.

Response of a fringing reef coastline to the direct impact of a tropical cyclone

“…Stationary SWAN simulations were run at 60 min intervals using updated currents, water levels, and wind from Delft3D-FLOW; wave simulation results were then passed back to Delft3D-FLOW. Although SWAN does not model IG energy, which has been shown to be important to lagoonal processes under non-TC conditions (Pomeroy et al 2012), it does account for wind-wave growth, which we show below to be a more critical factor in determining the coastal response from TC Olwyn. Model performance was evaluated using the Murphy (1988) Skill Score (Supporting Information Text S2).…”

“…1b) and resulted in incident waves on the forereef reaching 5.8 m (H sig,SS ). To place the TC Olwyn conditions in the context of typical wave conditions (H sig,SS 1-2 m, Pomeroy et al 2012), the observed waves and water levels during the cyclone were compared to a "typical" swell event (H sig,SS 5 1.8 m) that occurred prior to TC Olwyn (Fig. 2).…”

Reef Flat Wave Processes and Excavation Pits: Observations and Implications for Majuro Atoll, Marshall Islands

“…Previous observational and modeling studies of SS waves at study sites similar to Majuro Atoll (Péquignet et al, 2011;Pomeroy et al, 2012;Van Dongeren et al, 2013) strongly suggest that wave breaking is important in a narrow surf zone near the reef edge and that frictional dissipation over the shallow, rough substrate of the reef flat is the primary dissipation mechanism for SS waves as they propagate toward shore. Hence, the water depth and bed roughness are two key considerations for assessing frictional dissipation.…”

“…Péquignet (2012) and Pomeroy et al, (2012) directly estimated the nonlinear energy transfer between SS and IG waves at fringing reefs similar to our study site and found that the rates were negligible compared with dissipation rates caused by bottom friction. Van Dongeren et al (2013) used models to show that bottom friction over the reef flat strongly dissipated both SS and IG waves and concluded that nonlinear energy losses were not important.…”

“…Van Dongeren et al (2013) used models to show that bottom friction over the reef flat strongly dissipated both SS and IG waves and concluded that nonlinear energy losses were not important. As pointed out by Pomeroy et al (2012), laboratory and model studies of fringing reefs that have suggested the importance of nonlinear energy transfer on the reef flat (Demirbilek, Nwogu, and Ward, 2007;Nwogu and Demirbilek, 2010;Sheremet et al, 2011) were based on idealized smooth substrates with considerably less bed roughness than found at field sites such as Majuro Atoll.…”