As sea levels rise, many of South Africa’s beaches will migrate slowly inland to survive and thrive. That’s because beaches are dynamic, and far more than people realise — but not if their inland path is blocked.
Professor Andrew Green from the department of marine geology and sedimentology at the University of KwaZulu-Natal (UKZN), his UKZN colleague Professor Andrew Cooper and an international team of scientists, have refuted a paper published in Nature Climate Change in March, warning how half of the world’s sandy beaches could face near extinction by the end of this century from climate change-induced coastal erosion and sea-level rise.
Their rebuttal has been published in the same journal. By re-examining the data and methodology supporting the original study, the authors, led by Cooper, argue there is potential for beaches to migrate landwards as sea level rises and shorelines retreat.
There is no global information on the number of beaches falling into either the extinct or surviving category, and it’s impossible to quantify what proportion of the world’s beaches will disappear between now and 2100.
“In the short term, beaches can erode or accrete. In the longer term, they oscillate on a geological footprint that is controlled by longer-term changes to storms and swells (usually at the 10- to 100-year ranges) and they can migrate, usually controlled by sea-level, but [are] strongly influenced by large and infrequent storms.”
They “borrow” sediment from the dunes landward of the beach and sediment out past the breakers.
Green explains that there is a constant exchange of sediment between these zones, and with rising sea levels, this causes beaches to maintain their size and shape, while slowly moving landward. However, beach migration cannot occur when a hard structure blocks the path.
“What happens is that the sediment borrowing between the various parts of the beach is disrupted, and once backed against a hard structure, the seaward edge of the beach is eventually cannibalised by the waves, as the beach is fixed in place.”
This is called coastal squeeze, and can occur naturally with rising sea levels in areas of rocky coasts such as the Eastern Cape, where small beaches front cliffs or steep areas.
“Places where roads, houses, buildings and walkways are built, will all result in a coastal squeeze to some extent, and eventually in the loss of that sandy beach, as it is stopped from migrating.”
Strongly engineered coasts can’t cope with rising sea levels, says Green, who cites the Durban beachfront.
“It’s evident that during the windy season, large quantities of sand are blown into the back beach 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.”
There are many similar places in the country where beaches are at risk of being eroded and cannot migrate as they should.
“Where we see a loss of sand — this includes the beach, the dunes and even the sand that extends out to maybe 30m below the breakers and beyond — in this case, we’ll see the loss of infrastructure and the reclamation of these areas by the ocean.”
The implications are serious.
“We’ll either try more engineering schemes to combat this or leave the coast and retreat in a more fatalistic response. It’s not all doom and gloom though. This is not for everywhere, and is really where we have interfered in the natural ability for beaches to respond to changing sea levels and climate.”
In many places, there is ample space for the beach to move back into, known as accommodation space.
“In these areas, for example, Cape Vidal of KwaZulu-Natal, or Mpekweni beach in the Eastern Cape, beaches will survive and thrive due to the local situation still being healthy behind the beaches.”
As sea level rises, shoreline retreat must and will happen, but beaches will survive, says Cooper.
“The biggest threat to the continued existence of beaches is coastal defence structures that limit their ability to migrate.”
He argues that areas of the country’s coast that are heavily engineered are relatively small when compared with the Netherlands, for example. This makes 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.”
The removal of coastal seawalls and hardback beach structures, together with nature-based solutions such as Durban’s beach nourishment, may be the only methods to safeguard the future of urban beaches.
“It is likely we’ll 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.”