Variability of depth-limited waves in coral reef surf zones. (31st October 2018)

• Record Type: Journal Article
• Authors: Harris, Daniel L.; Power, Hannah E.; Kinsela, Michael A.; Webster, Jody M.; Vila-Concejo, Ana
• Is Part Of: Estuarine, coastal and shelf science. Volume 211(2018)
• Page Start: 36
• Page End: 44
• Publication Date: 2018-10-31
• Subjects: Estuarine oceanography -- Periodicals; Coasts -- Periodicals; Estuarine biology -- Periodicals; Seashore biology -- Periodicals; Coasts; Estuarine biology; Estuarine oceanography; Seashore biology; Periodicals; 551.461805
• DOI: 10.1016/j.ecss.2018.06.010
• Languages: English
• ISSNs: 0272-7714

Abstract

Abstract: Wave breaking and transformation on coral reef flats is an important process protecting tropical coastlines and regulating the energy regimes of coral reefs. However, the high hydraulic roughness, shallow water, and steep bathymetries of coral reefs may confound common surf zone assumptions, such as a depth-limited and saturated surf zone with a constant wave height to water depth ratio ( γ ). Here, we examine wave transformation across a coral reef flat, during three separate swell events, on both a time-averaged and a wave-by-wave basis. We use the relationship between significant wave height and water depth ( γ s ) to examine the change in surf saturation across the reef flat and compare the measured wave height decay to results of modelled wave energy dissipation in the surf zone. Our results show that γ s was not cross-reef constant and varied according to location on the reef flat and local water depth. On average, γ s was greatest at the outer reef flat, near the reef crest, and progressively reduced towards the inner reef flat, near the reef lagoon. This was most pronounced in shallow water with large γ s values ( γ s > 0.85) at the outer reef flat and small γ s values ( γ s < 0.1) at the inner reef flat. This indicates that there is an increase in wave energy dissipation in shallow water, most likely due to increased breaker and bed frictional dissipation. The measured wave energy dissipation across the entire reef flat could, on average, be modelled accurately; however, this required location specific calibration of the free parameters, the wave friction factor ( f w ) and γ, and further suggests that there is no value for either parameter that is universally applicable to coral reef flats. Despite model calibration inaccuracies were still observed, primarily at the outer reef flat. These inaccuracies reflected the observed cross-reef variation of γ on the reef flat and potentially the limitations of random wave breaker dissipation models in complex surf zones. Our results have implications for the use of wave energy dissipation models in predicting breaker dissipation and subsequent benthic community change on coral reef flats, and suggest that careful consideration of the free parameters in such models (such as f w and γ ) is required. Highlights: Saturated surf zone waves were measured and compared to a common wave energy dissipation model. Surf zone saturation and depth limited wave conditions varied depending on water depth and location on coral reef flats. Errors observed in the dissipation model correlated with variation in wave height to water depth ratios in the surf zone. Some errors were corrected by spatially varying bed frictional dissipation on the reef flat. Model calibration from surf zone data was required to accurately describe wave height decay on coral reef flats.