Beaches to 'slowly march inland'
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Durban - With rising sea levels, beaches are likely to move inland in the future, rather than become extinct by the end of the century.
This could, of course, see the steps of the City Hall covered in water in years to come were it not for the feeding of sand on to Durban’s beaches, according to Andy Green of the UKZN’s department of geological sciences.
He, another UKZN professor, Andrew Cooper and academics from all over the world believe that given the future predicted rises in sea level, beaches will slowly march inland, keeping much the same shape and size as they are at present.
However, this doesn’t help the beaches of Durban where infrastructure such as the beachfront promenade and buildings will block the movement of inland-bound beaches and see them swallowed up by the rising oceans.
“Durban’s central business district is approximately located at sea level. With a moderate rise in sea level, this area would likely be transformed into marshland,” Green said, pointing out that the Mgeni River once flowed through it on its way into the Durban Bay.
The present system of nourishing Durban’s beaches with sand would need to carry on, Green told The Independent on Saturday. However, sand could become harder to come by.
“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,” said Green.
“One key place is the offshore environment, beyond the beach where many countries from around the world have begun to turn their attention for their future sand requirements.”
While sand will be at a premium, it’s not as if half the world’s beaches could be extinct by the end of the century as the authors of a paper published in Nature Climate Change reportedly said.
Green, Cooper and their team have delivered a paper in Nature Climate Change refuting these suggestions.
“By re-examining the data and methodology underpinning the original study, the authors of the new paper, led by Cooper, disagreed with the study’s conclusion, publishing their rebuttal and concluding that with the global data and numerical methods available today, it is impossible to make such global and wide-reaching predictions,” read a UKZN statement.
“A critical factor pointed out in the new research 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.”
It added that the researchers said that beaches backed by hard coastal cliffs and engineering structures, such as seawalls, were 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.”
Green said 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.
“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.”
Their 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.
Cooper added: “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 Independent on Saturday