Source and supply of sediment to a shoreline salient in a fringing reef environment

Cuttler, Michael; Hansen, Jeff E.; Lowe, Ryan J.; Trotter, Julie; McCulloch, Malcolm T.

doi:10.1002/esp.4516

Cited by 14 publications

(21 citation statements)

References 57 publications

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“…(2018) and Cuttler et al. (2019), and is only summarized here. The reef crest is located 2.0–2.5 km from the shoreline with a reef flat that is ∼0.6–1.5 m below mean sea level at still water and is ∼500 m in width.…”

Section: Methodsmentioning

confidence: 98%

The Contribution of Currents, Sea‐Swell Waves, and Infragravity Waves to Suspended‐Sediment Transport Across a Coral Reef‐Lagoon System

et al. 2021

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Tropical beaches protect and support communities of hundreds of millions of people globally (Hoegh-Guldberg et al., 2019). These beaches are primarily accumulations of sediment derived from the calcium carbonate skeletal remains of organisms such as coral, crustose coralline algae, and mollusks from adjacent coral reefs (Perry & Hepburn, 2008). When these skeletal remains break down into particle sizes that can Abstract Coral reefs generate substantial volumes of carbonate sediment, which is redistributed throughout the reef-lagoon system. However, there is little understanding of the specific processes that transport this sediment produced on the outer portions of coral reefs throughout a reef-lagoon system. Furthermore, the separate contributions of currents, sea-swell waves, and infragravity waves to transport, which are all strongly influenced by the presence of a reef, is not fully understood. Here, we show that in reef-lagoon systems most suspended sediment is transported close to the seabed and can, at times, be suspended higher in the water column during oscillatory flow transitions (i.e., near slack flow) at sea-swell wave frequencies, and during the peak onshore oscillatory velocity phase at infragravity wave frequencies. While these wave frequencies contribute to the transport of suspended sediment offshore and onshore, respectively, the net flux is small. Mean currents are the primary transport mechanism and responsible for almost 2 orders of magnitude more suspended-sediment flux than sea-swell and infragravity waves. Whilst waves may not be the primary mechanism for the transport of sediment, our results suggest they are an important driver of sediment suspension from the seabed, as well as contributing to the partitioning of sediment grain sizes from the reef to the shoreline. As the ocean wave climate changes, sea level rises, and the composition of reef benthic communities change, the relative importance of mean currents, seaswell waves, and infragravity waves is likely to change, and this will affect how sediment is redistributed throughout reef-lagoon systems. Plain Language Summary Most of the sandy sediment found on coral reef coastlines is produced by organisms living within the reef. This sediment is then transported by waves and currents across the reef and distributed throughout the lagoon. Little is known about these transport processes, including the relative importance of how currents or different types of waves drive transport. This study shows that most of the sediment in the water column (suspended sediment) is transported close to the seabed by mean currents. At times, this sediment can be suspended higher in the water column by short waves (5-25 s) when the flow transitions from being onshore directed to offshore directed, or when the velocities of longer period waves (25-250 s), called infragravity waves, are directed onshore. The timing of the suspension by these waves helps to sort the sediment into different sizes across the system, but the quantity of sediment transported is ...

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Section: Methodsmentioning

confidence: 98%

The Contribution of Currents, Sea‐Swell Waves, and Infragravity Waves to Suspended‐Sediment Transport Across a Coral Reef‐Lagoon System

et al. 2021

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“…Thus, the shoreward bedload transport over the reef flat is more than 3 times that of the lagoon. Modeled circulation patterns on the reef show strong return flow through the north and south channels via the lagoon (Cuttler et al, 2018b). This does not mean that all the sediment is being exported out of the reef system through the channels.…”

Section: Gradients In Accumulation and Bedload Transportmentioning

confidence: 99%

“…At least some of this excess sediment is most likely being spread out on either side of the salient, eventually being transported toward the shore by asymmetrical waves. Indeed, Cuttler et al (2018b) showed that mean wave direction north and south of the salient is normal to the shoreline, not the reef crest, and hence the refraction of waves affects the direction of bedload transport and bedform migration. Shoreline accretion rates of ∼1 m/year were estimated by Cuttler et al (2018a).…”

Section: Gradients In Accumulation and Bedload Transportmentioning

confidence: 99%

“…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, however, are still not well understood.…”

Section: Introductionmentioning

confidence: 99%

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Spectral Wave-Driven Bedload Transport Across a Coral Reef Flat/Lagoon Complex

et al. 2020

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Coral reefs are an important source of sediment for reef-lined coasts and help to maintain beaches by providing protection though dissipation of wave energy. Understanding the mechanisms that deliver sediment to the coast from coral reefs and quantifying the total volume of sediment generated at coral reefs 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 wave energy constituents 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 occurs on the reef flat, which is subsequently transported 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 correlated with an increase in 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.

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“…The site is characterized by a steep 1/20 fore reef slope, a relatively narrow (100s of m) approximately 1 m deep reef flat, and a 2 km wide shallow lagoon (<5 m deep). Inside the lagoon the seabed is predominately sandy, whereas the seabed around the reef crest is much rougher due to relic and living coral (Cuttler et al, 2019). The site is micro-tidal with an average tidal range of 0.8 m (Figure 1e).…”

Section: Field Observationsmentioning

confidence: 99%

A Numerical Study of Wave‐Driven Mean Flows and Setup Dynamics at a Coral Reef‐Lagoon System

et al. 2021

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Wave breaking on reefs, many of which have steep slopes, results in elevated mean water levels (or wave setup) across the reef, which can drive mean flows over the reef and inside adjacent lagoons (if present) (e.g., Lowe & Falter, 2015;Monismith, 2007). The mean flows and setup dynamics in such environments have been found to greatly depend on the reef properties at both large scales (reef geometry and bathymetry Abstract Two-dimensional mean wave-driven flow and setup dynamics were investigated at a reef-lagoon system at Ningaloo Reef, Western Australia, using the numerical wave-flow model, SWASH. Phase-resolved numerical simulations of the wave and flow fields, validated with highly detailed field observations (including >10 sensors through the energetic surf zone), were used to quantify the main mechanisms that govern the mean momentum balances and resulting mean current and setup patterns, with particular attention to the role of nonlinear wave shapes. Momentum balances from the phaseresolved model indicated that onshore flows near the reef crest were primarily driven by the wave force (dominated by radiation stress gradients) due to intense breaking, whereas the flow over the reef flat and inside the lagoon and channels was primarily driven by a pressure gradient. Wave setup inside the lagoon was primarily controlled by the wave force and bottom stress. The bottom stress reduced the setup on the reef flat and inside the lagoon. Excluding the bottom stress contribution in the setup balance resulted in an over prediction of the wave-setup inside the lagoon by up to 200-370%. The bottom stress was found to be caused by the combined presence of onshore directed wave-driven currents and (nonlinear) waves. Exclusion of the bottom stress contribution from nonlinear wave shapes led to an over prediction of the setup inside the lagoon by approximately 20-40%. The inclusion of the nonlinear wave shape contribution to the bottom stress term was found to be particularly relevant in reef regions that experience a net onshore mass flux over the reef crest.Plain Language Summary Coral reefs that are located in close proximity to a coastline are typically characterized by a steep slope and reef crest that is connected to the coast or front a shallow lagoon. At the reef crest, waves break and drive onshore-directed currents and elevate the mean (timeaveraged) water level in the lagoon. In this study, we combined measurements of waves, currents and water levels with simulations from an advanced computer model to understand the physical mechanisms that determine the current patterns and water level variations at a coral reef-lagoon system in Western Australia. Friction generated by the water moving over the rough reef structures was found to reduce the mean water levels inside the lagoon. This friction was explained by the combined presence of both waves and mean currents. Furthermore, near the reef crest, the waves peak and pitch forward before they break, and this nonlinear wave shape was found to enhance the friction from ...

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Source and supply of sediment to a shoreline salient in a fringing reef environment

Cited by 14 publications

References 57 publications

The Contribution of Currents, Sea‐Swell Waves, and Infragravity Waves to Suspended‐Sediment Transport Across a Coral Reef‐Lagoon System

The Contribution of Currents, Sea‐Swell Waves, and Infragravity Waves to Suspended‐Sediment Transport Across a Coral Reef‐Lagoon System

Spectral Wave-Driven Bedload Transport Across a Coral Reef Flat/Lagoon Complex

A Numerical Study of Wave‐Driven Mean Flows and Setup Dynamics at a Coral Reef‐Lagoon System

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