Images after violent coastal storms usually focus only on the extensive damage to beaches, sand dunes, property and surrounding infrastructure However, a new international study shows that extreme weather events can help protect beaches from rising sea levels - by bringing new sand from deeper waters or from nearby beaches
Data map
The study, led by Dr Mitchell Harley of the water research laboratory at the University of New South Wales (UNSW), was published today in nature communications earth & amp; environment.
"We know that extreme storms can cause significant coastal erosion and damage to coastal property," Harley said, "For the first time, we studied not only the conditions on the water surface, that is, the effects of extreme storms, which are easy to see, but also the depths below the water surface. We found that in these events, hundreds of thousands of cubic meters of sand entered these beach systems - similar to the scale used by engineers to artificially nourish beaches. This may be enough to offset some of the effects of sea-level rise caused by climate change, such as the retreat of sea coastline and long-term growth In the long run, it is decades. This is a new way to look at extreme storms. "
Wave after wave
In collaboration with researchers from the University of Plymouth and Baja California Autonomous University, the study examined three coastlines in Australia, the UK and Mexico. Each coastline experienced a series of extreme storms or extended storm clusters, followed by a mild beach recovery period.
In Australia, researchers studied Narrabeen beach in Sydney. After a storm in 2016, a famous storm tore a swimming pool away from a property overlooking the coastline.
Using high-resolution measurements of beaches and seabed, the researchers found that the increase in sediment was enough to theoretically offset decades of expected coastline retreat.
"For the first time, we were able to mobilize specialized monitoring equipment to obtain truly accurate measurements before and after the storm. We used a combination of twin engine aircraft equipped with lidar scanners, drones and jet skis to measure the subsurface along the beach before and after the storm. That's why we can accurately understand the amount of sand movement in each storm," Harley said.
In the UK, researchers from the coastal process research team at the University of Plymouth have been studying perranporth beach in Cornwall since 2006. They used a combination of monthly beach topographic survey and quasi annual bathymetric survey.
Here, the impact of extreme winter in 2013 / 14 and 2015 / 16 led to a very large loss of sand in the intertidal beach and dune system. However, when looking at the total sand budget, the beach got 420000 cubic meters of sand.
"We're not quite sure whether the extra sand comes from offshore or from around, or even both, but we do now understand that, despite the erosion of the upper beaches and sand dunes, extreme waves are likely to make a positive contribution to the overall sand budget," said Professor GERD masselink, who leads the coastal process research team.
Brunn rule
How much the coastline will change as sea levels rise is a key issue for coastal managers as they plan for the escalating impact of climate change.
In the past, this was estimated by a simple method called Bunn rule. The rule states that for every one meter rise in a given sea level, the coastline is expected to retreat by about 20 to 100 meters, depending on the steepness of the coast.
By using the Brunn rule, the global sea level rise caused by climate change has been predicted to lead to the massive retreat or loss of almost half of the world's beaches by the end of this century.
Professor masselink said: "however, the Bruun rule has been criticized for its simplicity because it does not take into account many complex factors about how individual beaches respond to sea-level rise. This includes the presence of sand stored in deep water close to the coast - and its potential to be mobilized in extreme weather events."
Dr Harley said the findings highlighted the need to consider extreme storms when making long-term predictions of sediment movement on beaches.
See the world through the eyes of the storm
Dr. Harley pointed out that there are very few measurements of the seabed close to our coastline, so it is difficult to say how much sand may be mobilized in the future.