Sand mobilised during extreme storms could protect beaches by offsetting some impacts of sea-level rise

Extreme storms can cause extensive damage to property, beaches, dunes and the surrounding infrastructure, but a new study led by Australian researchers has shown that they could also help protect beaches from the impact of sea-level rise.

For the first time, scientists have looked not just above the water – where the impacts of extreme storms are easy to see – but below the ocean’s surface to study the movement of sand in the wake of these events.

“What we found was that hundreds of thousands of cubic metres of sand were entering these beach systems during these events – that’s similar to the scale of what engineers use to nourish a beach artificially,” says lead author Dr Mitchell Harley from the University of New South Wales (UNSW) Water Research Laboratory.

“This could potentially be enough to offset some of the impacts of sea-level rises caused by climate change, such as retreating coastlines, and by several decades in the long-term. It’s a new way of looking at extreme storms.”

The study, published in Nature Communications, examined three coastlines across Australia, the United Kingdom and Mexico that had been subject to extreme storms or extended storm clusters, followed by a milder period of beach recovery.

In Australia, researchers studied Narrabeen Beach in Sydney in the wake of the 2016 extreme storm that eroded about 50 metres of beach and damaged waterfront properties.

“For the first time, we were able to mobilise specialised monitoring equipment to get really accurate measurements before and after a storm,” explains Harley. “We used a combination of a twin-engine aeroplane equipped with a lidar scanner, drones and jet skis going back and forth along the beach taking measurements below the surface right before and after the storm hit.

“This was how we were able to get an accurate picture of the volume of sand moving for each storm.”

They found that the gains in sediment measured were enough to theoretically offset decades of projected shoreline retreat.

Scientists now understand that, despite causing upper beach and dune erosion, extreme waves can potentially contribute positively to a coast’s overall sand budget – though whether this extra sand has come from offshore, from around the corner, or both, is still unknown.

Extreme storms should be considered in long-term projections

This is important because predicting how coastlines might change due to sea-level rise is a key question in the face of the escalating impacts of climate change.

In the past, this has been estimated using a simple approach known as the Bruun rule – for a given metre of sea-level rise, the coastline retreats between 20 and 100 metres depending on the steepness of the coast.

However, this rule doesn’t take into account many other complex contributing factors – such as the presence of sand stored in deeper water immediately off the coast and its potential to be mobilised during extreme storms.

These new findings highlight that extreme storms need to be considered in long-term projections of sediment movements on beaches.

But more research is still needed, according to Harley, because there are so few measurements of the seabed immediately off our coastlines that it’s hard to tell how much sand could potentially be mobilised in the future.

“We’re only scraping the surface here. We need to repeat these types of monitoring measurements for more storms and different types of coastal settings under various conditions,” he says. “Only then will we be able to get a clearer understanding of how much sand is stored off the coast that could potentially help buffer the impacts of sea-level rise – and a clearer picture of what our beaches could look like in the year 2100 and beyond.”