Understanding the flow of suspended sediments across reefs

A new study has developed a model that moves researchers a step closer to improved predictions of changes in shorelines adjacent to coral reefs and the transport of suspended sediments in reef systems.

This study at Tantabiddi, Ningaloo Reef in the northwest of Western Australia, deployed more than 20 instruments to measure how waves transform over the reef and in the lagoon, how currents develop and circulate throughout the reef system as well as the concentration of sediment in suspension in different zones of the reef.

The results published in Journal of Geophysical Research reveal that although variability in suspended sediment concentration occurs at tidal (or shorter) timescales in the different reef zones, the majority of the variability occurs over longer slowly varying (subtidal) timescales, which is related to the arrival of large swell waves at a reef location.

More than 20 instruments set up to measure suspended sediment flow at Tantabiddi, Ningaloo Reef (Image: Andrew Pomeroy)

The study, supported by Western Australia Marine Science Institute (WAMSI) Dredging Science Node, the Australian Research Council and the U.S. Geological Survey Coastal and Marine Geology Program captured low and high wave conditions, as well as a period of strong alongshore transport driven by wind. Wave generated currents flowed across the reef, separated in the lagoon and exited via the channels in the reef.

Lead author Dr Andrew Pomeroy from The University of Western Australia and the Australian Institute of Marine Science explained that up to 95% of the variability in the concentration of suspended sediments can be described by variability in waves and currents at tidal and longer timescales.

“This study shows that in coral reefs, suspended sediment transport varies at a number of different timescales – for example by waves, tides or time periods longer than tides such as storm systems, and by different processes – which is most important will depend on the question being asked,” Dr Pomeroy said.

Dr Pomeroy says the emphasis now needs to be placed on understanding and describing the physical processes that suspend sediment from the bed within coral reef canopies. Data within coral canopies as well as close to the bed is lacking because it is difficult to accurately obtain.

“This is important as it will enable changes in shorelines adjacent to reefs (by erosion or accretion) to be described as well as improve predictions of suspended sediment transport in coral reef environments for a range of conditions,” he said.

Pomeroy A, Lowe R, Ghisalberti M, Winter G, Storlazzi C, Cuttler M (2018) Spatial variability of sediment transport processes over intra‐ and subtidal timescales within a fringing coral reef system. Journal of Geophysical Research doi.org/10.1002/2017JF004468

The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.


Dredging Science