Fierce coastal storms are known to leave terrifying devastation in their wake, often pushing back the shoreline significantly. But in the long term, the extreme weather events could protect some sandy beaches from rising sea levels, according to a study: The researchers use three examples to show that the forces of nature can lead to a significant net gain in material by bringing sand from the depths . The scientists say that this effect should now be taken into account when assessing the development of the coasts in the context of climate change.
A creeping deluge is threatening the world: global warming is causing the earth’s ice masses to melt and is slowly driving up sea levels. As a result, many low-lying coastal regions are threatened by flooding and land loss. In addition to the fight against further climate change, measures are now also required to adapt to these developments. A key question is how much certain coastlines could change. In the past, this has been estimated using a simple approach that, depending on the steepness of the coast, predicts a certain retreat as a function of sea level rise.
Tracking down the effect of storms
This so-called Bruun rule predicted severe losses, especially for the sandy beaches of the world: According to this, the global sea level rise caused by climate change would probably lead to a sharp decline or loss of almost half of the world’s sandy beaches by the end of this century. “However, Bruun’s rule has been criticized for its simplicity, as it fails to account for the many complex factors with which certain beaches respond to sea level rise. This includes the presence of sand stored in deeper areas just offshore – and its potential to be mobilized during extreme weather events,” says Gerd Masselink of the University of Plymouth. He and his colleagues have now specifically investigated this possible effect.
To do this, they examined three sandy stretches of coast in Australia, the United Kingdom and Mexico. Each of these areas experienced a succession of extreme storms during the study period, followed by a milder recovery phase for the beaches. “We know that extreme storms cause major coastal erosion and damage to beachfront properties,” says lead author Mitchell Harley of the University of New South Wales in Sydney. “For the first time, we looked not only above the water, where the effects of extreme storms are easy to see, but also deep below the water,” says the researcher. The information was provided, among other things, by examinations by lidar scanners, which use laser measurements to show hidden structures in the water. In this way, the researchers were able to record and compare the changes in the material accumulations in the beach system before and after the storm events.
Net profits are emerging
As the team reports, the results showed that in all three cases the storms caused visible losses in the visible area of the beach and the dunes. But there was a disproportionately large compensation in the underwater area of the beach due to deposits of mobilized material: “We found that such events caused hundreds of thousands of cubic meters of sand to get into the beach systems,” says Harley. As the researchers explain, these additional amounts correspond to the material that is typically used in protective measures to artificially build up beaches.
According to Harley, this changes the view on the role of extreme storms. Because the sediment increases caused by them could be sufficient on some coasts to compensate for decades of predicted decline in the coastline. “Evidence shows that extreme waves can potentially contribute positively to the material’s overall sand budget, while causing erosion of the upper beach and dunes,” says Masselink. These effects of the strong storms should therefore now also be taken into account in long-term projections of sediment movements on beaches. “However, it is becoming clear that we need to study beach by beach to understand how sandy shorelines will change as global sea levels continue to rise,” says Harley.
As a result, there is now a need for further research: “So far, we have only scratched the surface with our results. We need to repeat these types of monitoring measurements for more storms and different types of coastal environments under different conditions,” says Harley. “Only then will we be able to better understand how much sand is stored offshore, which could potentially help offset the effects of sea level rise. This could ultimately give us a clearer picture of what our beaches might look like in 2100 and beyond.”
Source: University of New South Wales, professional article: Nature Communications, doi: 10.1038/s43247-022-00437-2