Roshanka Ranasinghe, AXA Chair in Climate Change Impacts and Coastal Risk was part of a research team that has found that the Great Barrier Reef, as a whole, is a remarkably effective wave absorber, despite large gaps between the reefs. This means that landward of the reefs, waves are mostly related to local winds rather than offshore wave conditions.
As waves break and reduce in height over reefs, this drives currents that are very important for transport of nutrients and larvae. This reduction in wave height also has implications for shoreline stability. The aim was to see how wave height reduction is influenced by the porosity of the reef matrix, sea level and wind speed.
The Great Barrier Reef in Australia is the largest coral reef system in the world, extending 2,300 km alongshore. The reef matrix is a porous structure consisting of thousands of individual reefs, with gaps in between. The porosity varies in that it is much lower in the north where the continental shelf is narrow and there are extensive reef flats; and is greater in the south where the shelf reaches up to 300 km width and there are extensive lagoons.
Previously, there have been several studies investigating how individual reefs in the Great Barrier Reef influence ocean waves. However, this was the first, comprehensive, large-scale study of the influence of an entire offshore reef system on ocean wave transmission. The researchers used a state of the art 16 year-record of satellite altimeter measurements of wave heights.
The study was undertaken by Dr Shari Gallop, Research Fellow in Geology and Geophysics at the University of Southampton, Dr Ivan Haigh, also from the University of Southampton; Professor Ian Young, Vice-Chancellor of the Australian National University (ANU); Professor Roshanka Ranasinghe, Professor of Climate Change Impacts and Coastal Risk (UNESCO-IHE, Deltares, ANU), and Dr Tom Durrant (Bureau of Meteorology, Australia).
This new research, published in the journal 'Coral Reefs', has important implications for wave modelling near reef systems. This is because models that consider individual reefs only may underestimate the wave attenuation potential of a full reef matrix.
Professor Ranasinghe comments: “Plans are under-way to investigate the wave attenuation characteristics over the reef in more detail, using sophisticated numerical modelling. It is of critical importance to know the potential impacts of climate change effects such as sea level rise, and variations in wave conditions, on wave attenuation and current circulation on the Great Barrier Reef. This will aid the sustainable management of this natural wonder, and the surrounding national marine park.”
As a next step, in collaboration with Dano Roelvink, Professor of Coastal Engineering and Port Development and Deltares, Prof. Ranasinghe intends to engage an UNESCO-IHE MSc student to undertake a detailed wave modelling of the Great Barrier Reef to better characterize the level of wave attenuation over the reef, both under contemporary wave conditions and future climate change modified wave conditions.