Sand’s end? New models will help manage impact of sea-level rise

carbonate sands

The shape of carbonate sands. (a) 3D scans of the 18 Heron particles using a Hirox RH2000 microscope equipped with MXB2016Z. (b) Estimation of the projected area of the particles by considering the particles as different geometric shapes (squares, circles, rectangles, or ellipses). Source: Scientific Reports

The method of accounting sand is important for understanding the way reefs, atolls and coastal regions will cope with the effects of climate crisis. Scientists have discovered models for measuring sand were giving incorrect information and have developed more accurate engineering models.

Sand is considered by many as an infinite resource. But the resource being used in many industries including manufacturing and construction is not infinite. From the glass in mobile phone to base for roads, sand is used across economy. Sand and gravel are the most extracted materials, exceeding that of fossil fuels.

Sand is mostly measured in a wrong way, which gives a wrong perception. The number of stars in the universe are more than grains of sand on world’s sea beaches. Some information on sand now being considered are misleading. Now scientists have discovered the correct way to keep track of this resource.

In some areas in the world, sand is in short supply. Some areas import sand while some areas export sand as a commodity. Science journal Nature last year reported that illegal sand mining is active in about 70 countries, and hundreds of people have been killed in clashes over control of sand in the past decade.

The new report said:

Sediment transport calculations are used globally in the numerical models that coastal managers, scientists and engineers use to assess and forecast coastal change. Most of the existing sediment transport equations were defined based on experimental results using siliciclastic sands. Yet these equations are applied to all types of sand, including carbonate sands that have different characteristics and therefore, settling behavior.

Published in the Nature journal Scientific Reports, (Amin Riazi, Ana Vila-Concejo, Tristan Salles, Umut Türker, Improved drag coefficient and settling velocity for carbonate sands, Scientific Reports, 2020; 10: 1, article number 9465, (2020)) Associate Professor Vila-Concejo’s team has shown that standard engineering models also overestimate transport of carbonate sands on the seafloor by more than 20 percent and underestimate suspended transport of this sand by at least 10 percent.

“This means we are not accounting for sand correctly,” she said. “While this has impact on construction and manufacturing, it could also have a big effect on the management of coastal areas impacted by climate change.”

“Not all sand is the same,” said Associate Professor Ana Vila-Concejo from the University of Sydney, School of Geosciences. “Yet the models for assessing sand and how it moves mostly rely on one type. This means we have an inaccurate picture of what is happening, especially in coastal areas that are vulnerable to climate change.”

Dr Amin Riazi from Eastern Mediterranean University worked with Associate Professor Vila-Concejo during a short stay at the University of Sydney to develop new engineering models that account for the different shapes of sand grains. Standard models assume sand grains are spherical, which is fine for common sands made up of ground-down silica and quartz rocks.

The report said:

Coral reefs are three-dimensional structures that comprise fine veneers of living coral colonies and other organisms overlying vast sequences of dead coral, as such they are mainly composed of sedimentary deposits – carbonate sand and rubble. These deposits are highly mobile, and their changing morphologies can influence the characteristics of the living regions of coral reefs and other biota. An example of these sedimentary deposits are the vulnerable coral-reef islands whose fate directly related to the effects of climate change (including sea-level rise, ocean warming and acidification and changes in wave climate), has been the subject of recent investigation and debate.

Carbonate sands derived from shells, corals and the skeletons of marine animals tend to be elliptical, less dense and have more holes and edges. The new research has considered this with astounding results, finding that existing models underestimate the surface area of carbonate sands by 35 percent.

Associate Professor Vila-Concejo said: “While sand wars are not happening in Australia, we do have areas with chronic coastal erosion and sand loss such as at Jimmys Beach in Port Stephens.”

The scientists took carbonate sand from near Heron Island on the Great Barrier Reef and observed how it responded under experimental conditions. Based on these observations, they developed new mathematical equations that much better predict how carbonate sands move.

The team confirmed this by applying their equations to existing data on carbonate sand movement accumulated over six years from observations off the north coast of Oahu, Hawaii.

“Keeping track of carbonate sand will become increasingly important,” said Dr Tristan Salles, also from the School of Geosciences in the Faculty of Science.

“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.”

He said these new equations are likely to be used to update all sediment transport models. “This will include evaluating beach and atoll responses to ocean hydrodynamics in carbonate-sand-rich regions, some of which are most vulnerable to the impacts of climate change,” Dr Salles said.

At present, coastal engineering uses models based on siliciclastic sands. Associate Professor Vila-Concejo hopes that the models her team has developed can be used to improve management of coastal areas.

“This means we can develop a far more accurate picture of how changing oceans will affect marine ecosystems where carbonate sands are dominant,” Associate Professor Vila-Concejo said.

“Understanding how, why and when sediments move is crucial to managing and predicting the effects of climate change and our new work will help in the development of mitigation and adaptation strategies.”

On implications for sediment transport, the report said:

“In practical applications to bioclastic environments, settling velocities are used to describe multiple reef zones and to predict sediment transport mode. These applications are critical to understand and predict sediment transport pathways through these systems but also to evaluate the damage to corals exposed to sedimentation. In a world where sand and gravel are being extracted faster that they can be replaced, it is crucial that we refine existing sediment transport calculations to minimize waste of sand.”

The scientists in this study said that they have only evaluated “the transport of loose sands and therefore do not estimate the impact of reef rugosity on frictional dissipation in the spectral wave model neither the effect of hard coral disintegration in loose particles.”




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