Sand is such a sought-after resource that by volume, the amount we use is second only to water. Sand is crucial for manufacturing cement, glass, and asphalt and even everyday items like paper and toothpaste. And an increasing portion of the 40–50 billion metric tons of sand mined each year comes from the world’s beaches, which are in turn threatened by rising sea levels due to climate change.
The smooth, round grains of quartz in desert sand are useless for many of our sand needs—we require rougher stuff, like the crushed corals and shells that make up the carbonate sands of tropical beaches. That means managing and accounting for this valuable sand will become more critical in the coming years, but according to a recent study published in Scientific Reports, we have been measuring carbonate sand all wrong. The study proposes a new, updated method that takes carbonate sand’s sought-after variety in shape and size into account.
“Our new method will help to estimate more accurately the sediment transport of carbonate sands in tropical environments,” said Ana Vila-Concejo, an associate professor at the University of Sydney School of Geosciences and coauthor of the paper. “This can directly translate into more accurate coastal management of eroding sandy coasts.”
A Fortuitous Collaboration
From her research on sand hydrodynamics, Vila-Concejo long suspected that carbonate sand transport models were inaccurate, but she could never convince her students to tackle the problem of making better-suited models. Then, Amin Riazi, at the time an early-career research assistant with Eastern Mediterranean University, sent her an email out of the blue. “He had exactly the right skills and was interested in pursuing this research,” Vila-Concejo said.
“As I was discussing the topic with Prof. Vila-Concejo, I realized that surprisingly, and contrary to plenty of studies on other sand types, there is a lack of research related to the settling velocity and the drag coefficient of carbonate sands,” said Riazi, now an assistant professor at Cyprus International University. He traveled to the University of Sydney for a short research stay, and the two got to work on the carbonate sand problem.
Carbonate Sand Is a Drag
Most equations for sediment transport are based on experimental results from smooth silicate sands, making them a poor fit for beaches, where most of the sand is made up of small bits of shell and coral. Because carbonate sands have more irregular shapes, they have larger drag coefficients than desert sand and move more erratically through the water. This drag, in turn, makes individual grains’ settling velocity much more variable, effectively spreading out the sand grains over a wider distance when they are suspended in water.
“If the settling velocity is not correctly accounted for, sediment transport models will give wrong predictions of beach erosion or accretion,” said Kwok Cheung, a professor in the Department of Ocean and Resources Engineering at the University of Hawai‘i at Mānoa. Cheung provided the raw data used to help calculate the new drag coefficient but was not involved in the study itself. “This will have severe consequences to beach management or nourishment projects.”
Riazi and Vila-Concejo and their coauthors found that existing models typically underestimate the surface area of carbonate sands by over 30%, overestimate the transport of these rougher grains over the seafloor, and underestimate transport of sands when they are suspended. The team hopes that their model will be implemented by coastal engineers and others who need to evaluate the way sandy beaches respond to ocean currents, waves, and the long-term impacts of sea level rise.
“The new model provides a more accurate description of the settling velocity, which is a primary input parameter for sediment transport calculations,” said Cheung. “The better the input, the better the output.”
Managing a Crucial Resource
Accurate accounting of the sand that makes up tropical beaches will become more important in the coming years. As sea levels rise, beach managers and coastal engineers will need to decide how best to preserve beaches as both a crucial coastal habitat and a natural coastal barrier. “When preparing for the impacts of climate change, the accuracy of our equations will mean that we don’t need to overengineer and use more sand than strictly necessary,” said Vila-Concejo. “This can directly translate [into] more accurate coastal management of eroding sandy coasts.”
Better accounting is also important in light of sand mining’s increasing intensity and the threat it poses to coastal and marine ecosystems. A construction boom in Asia and Africa has driven up the demand for sand in the past 2 decades, and the resource is expected to become much more sought after—and scarce—in the coming years.
“We need to better monitor changes in sediment transport, as we’re seeing increasing human impact on natural systems,” said Mette Bendixen, a research fellow at the Institute of Arctic and Alpine Research at the University of Colorado Boulder who commented on the phenomenon in Nature last year. “Despite the central importance of sand, we don’t possess any clear global overview, or statistics, of the sand resources available or those being mined. If we don’t have this overview of what we actually have available as a resource, we’re putting sustainability of the environment, and people’s livelihoods, at risk.”
—Rachel Fritts (@rachel_fritts), Science Writer
9 July 2020: This article has been updated to correct the affiliation of Amin Riazi.