Abstract
Coral reefs are an important source of sediment for reef-lined coasts by helping to maintain beaches while also providing protection in the form of wave energy dissipation. Understanding the mechanisms by which sediment is delivered to the coast as well as better constraining the total volumes generated are critical for projecting future coastal change. A month-long hydrodynamics and sediment transport study on a fringing reef/lagoon complex in Western Australia indicates that lower frequency constituents of wave energy are important to the total bedload transport of sediment across the reef flat and lagoon to the shoreline. The reef flat and the lagoon are characterized by distinctly different transport regimes, resulting in an offset in the timing of bedform migration between the two. Short-term storage of sediment is noted on the reef flat, which is subsequently washed out into the lagoon when offshore wave heights increase and strong currents due to wave breaking at the reef crest develop. This sudden influx of sediment is a significant control on bedform migration rates in the lagoon. Infragravity wave energy on the reef flat and lagoon make an important contribution to the migration of bedforms and resultant bedload transport. Given the complexity of the hydrodynamics of fringing reefs, the transfer of energy to lower frequency bands, as well as accurate estimates of sources and sinks of sediment, must but considered in order to correctly model the transport of sediment from the reef to the coast.
Highlights
The importance of coral reefs in protecting and supplying sediment to reef-lined coasts is becoming widely recognized, as highlighted by many recent studies (Pomeroy et al, 2017; Cuttler et al, 2018b; Reguero et al, 2019; Storlazzi et al, 2019)
Given that grain density of carbonate sediment can be less than typical quartz beach sediment (Cuttler et al, 2017), we propose this to be an important factor in predicting bedform geometry on coral reefs
We found that the mechanisms responsible for bedform migration differ greatly from the reef flat to the lagoon, and that lower-frequency constituents of wave forcing on the reef need to be accounted for to properly predict timing and rates of sediment transport by bedforms
Summary
The importance of coral reefs in protecting and supplying sediment to reef-lined coasts is becoming widely recognized, as highlighted by many recent studies (Pomeroy et al, 2017; Cuttler et al, 2018b; Reguero et al, 2019; Storlazzi et al, 2019). The precise mechanisms by which sediment is transported across a reef to the shoreline, are still not well understood. Sediment is transported as both suspended load and bedload, and laboratory studies have suggested that in a reef environment, the bedload component dominates (Pomeroy et al, 2015a). There have been few studies on the bedload transport of sediment in reef environments. Cacchione et al (1999) considered bedform migration in a reef channel off Kailua, Oahu, and found that the net transport direction varied according to the seasonality of the trade winds. Becker et al (2007) investigated the migration of bedforms in Waimea Bay on the island of Oahu using imagery at low tide, when There have been few studies on the bedload transport of sediment in reef environments. Cacchione et al (1999) considered bedform migration in a reef channel off Kailua, Oahu, and found that the net transport direction varied according to the seasonality of the trade winds. Becker et al (2007) investigated the migration of bedforms in Waimea Bay on the island of Oahu using imagery at low tide, when
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