Nick Carne
Sand is far from ignored in our discussion of climate change and its impact, but it perhaps needs to be a little more front of mind.
A new Australian study has found that the way we monitor sand has given us misleading information about how much there is and where it is. In many instances, the researchers say, we have simply been measuring it the wrong way.
On a more hopeful note, a separate study suggests that sand and gravel may help some coral reef islands to naturally adapt and survive the impact of rising sea levels.
Ana Vila-Concejo from the University of Sydney, the lead author of the first paper, published in the journal Scientific Reports, says current models for assessing sand and how it moves tend to assume that all sand grains are the same – spherical.
That is fine for common sands made up of ground-down silica and quartz rocks, she says, but not for carbonate sands derived from shells, corals and the skeletons of marine animals, which tend to be elliptical and less dense, and to have more holes and edges.
When she and colleagues, including Amin Riazi from Eastern Mediterranean University, Cyprus, developed new engineering models that account for the different shapes, they found that the existing models underestimate the surface area of carbonate sands by 35%.
They also overestimate transport of carbonate sands on the seafloor by more than 20% and underestimate suspended transport of this sand by at least 10%.
“This means we are not accounting for sand correctly,” Vila-Concejo says, which could have a major impact on the management of coastal areas affected by climate change.
For their study, the researchers took carbonate sand from near Heron Island on Australia’s Great Barrier Reef and observed how it responded under experimental conditions.
Based on this analysis, they developed mathematical equations that better predict how carbonate sands move and tested them using data on carbonate sand movement accumulated over six years from observations off the north coast of Oahu, Hawaii.
Satisfied with the results, they now hope the equations will be used to update all sediment transport models.
“Keeping track of carbonate sand will become increasingly important,” says co-author Tristan Salles.
“If islands and atolls are at risk from erosion caused by sea-level rise, it will be vital to understand how the sands protecting them will respond to the ocean currents, waves and high-energy sea swells battering them.”
And that is a perfect segue to the second paper, published in the journal Science Advances.
A study led by the University of Plymouth, UK, with researchers from the University of Auckland, New Zealand, and Simon Fraser University, Canada, suggests “island drowning” may not be inevitable where sandy or gravel islands sit on top of coral reef platforms.
Numerical modelling of island morphology combined with physical model experiments suggest, they say, that such islands can evolve in the face of overtopping waves, with sediment from the beach face being transferred to the island’s surface.
In such a scenario, the island’s crest is being raised as sea level rises, says research leader Gerd Masselink, potentially allowing it to “support near-term habitability” – admittedly with “additional management challenges, possibly involving sediment nourishment, mobile infrastructure and flood-proof housing”.
“It is important to realise that these coral reef islands have developed over hundreds to thousands of years as a result of energetic wave conditions removing material from the reef structure and depositing the material towards the back of reef platforms, thereby creating islands,” Masselink says.
“The height of their surface is actually determined by the most energetic wave conditions, therefore overtopping, flooding and island inundation are necessary, albeit inconvenient and sometimes hazardous, processes required for island maintenance.”
For their study, the researchers created a scale model of Fatato Island, part of the Funafuti Atoll in Tuvalu, and subjected it to a series of experiments designed to simulate predicted sea level rises.
Three numerical modelling scenarios were then used to assess how the island adjusted to a sea level rise of 0.75 metres, the global average increase predicted for 2100 by the Intergovernmental Panel on Climate Change. The island crest rose by just under 0.7 metres.