The group of scientists, from the United Kingdom, France, South Africa, Australia, New Zealand and the United States, were responding to a claim made by European researchers in a paper published earlier this year in Nature Climate Change. Professors Andrew Green and Andrew Cooper of the Discipline of Geological Sciences at the University of KwaZulu-Natal (UKZN) are among the authors of the rebuttal paper, titled "Sandy beaches can survive sea-level rise", published in the same journal.
A critical factor pointed out in the new research paper, led by Cooper - an honorary research professor at UKZN, is the fact that there is potential for beaches to migrate landwards as sea-level rises and shorelines retreat. The key notion behind this is that if beaches have space to move into under the influence of rising sea levels – referred to as accommodation space – they will retain their overall shape and form but in a more landward position. Beaches, ultimately, will survive given no impediment to their movement.
The researchers say that beaches backed by hard coastal cliffs and engineering structures, such as seawalls, are indeed likely to disappear in the future due to sea-level rise as these beaches are unable to migrate landward. They will first experience ‘coastal squeeze’, resulting in a decrease in width, and will eventually drown or be eroded.
"Strongly engineered coasts cannot cope with rising sea level. When working in tandem with wave action, sediment is stripped from beaches, but moved onshore into areas where beaches are not engineered to be," said Green. "The current Durban beachfront is a good case in point. It is evident that during the windy season, large quantities of sand are blown into the backbeach environments, yet there is no place for this to settle as these areas are now converted into roads and hard infrastructure. This is a small subset of what will continue to happen when sea levels rise and coastal squeeze becomes more pressing."
The research points out, however, that beaches backed by low-lying coastal plains, shallow lagoons, salt marshes and dunes will migrate landward as a result of rising sea level. In these cases, the shoreline will retreat, but the beaches are still likely to remain, albeit a little raised in elevation and located landward, rather than going ‘extinct’. Cooper added, "As sea level rises, shoreline retreat must, and will, happen but beaches will survive. The biggest threat to the continued existence of beaches is coastal defence structures that limit their ability to migrate.’
"The upshot is that most of South Africa is not heavily urbanised and as such the areas of coast that are heavily engineered are relatively small when compared with places such as the Netherlands, making us far less vulnerable to the impacts of rising sea level. Cities, well, they may not be as lucky given their strongly defined hard shorelines behind the beaches," said Green.
The new paper says there is currently no information available globally on the number of beaches which fall into either the extinct or surviving category and, as such, it is impossible to quantify what proportion of the world’s beaches will disappear between now and 2100.
"Numerical equations and models employed by coastal and civil engineers are often unsuitable without a strong link to geological and morphological observations, and applying these at broad scales can be very damaging to coastal management plans," added Green. "Coastal management needs a broader and more holistic range of inputs to ensure decision-making is more resilient."
The authors of the paper concluded that the removal of coastal seawalls and hard backbeach structures (termed managed re-alignment), together with nature-based solutions, such as those experienced in Durban, like beach nourishment, may be the only methods to safeguard the future of urban beaches.
As Green summed it up, "It is likely we will need to start looking for sand resources for coastal nourishment as the onshore resources such as river sands and coastal dunes slowly dwindle in volume. One key place is the offshore environment, beyond the beach where many countries from around the world have begun to turn their attention for the future."