- Sixteen Mile Beach in Yzerfontein on the West Coast is 52m narrower, on average, than it was in 1937.
- 100m of the beach has vanished beneath the Atlantic Ocean at the worst-affected spot, according to an analysis of aerial photos and satellite images.
- By 2040, say Wits scientists, the ocean could be lapping at the foot of the dunes that separate the beach from Langebaan lagoon.
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A huge slice of South Africa’s longest beach has disappeared under the Atlantic Ocean over the last few decades.
Sixteen Mile Beach in the West Coast National Park is about 52m narrower, on average, than it was when aerial photographs were taken in 1937.
At the most eroded part of the beach, about 6km north of Yzerfontein, the shoreline is 100m closer to the dunes than it was 86 years ago, and scientists say by 2040 the ocean could be lapping at the foot of the dunes.
The stretch of coast analysed by scientists from Wits and the University of the Free State. (Remote Sensing)
About 80% of SA’s 3,000km coastline consists of sandy beaches, and the team from Wits and Free State universities which analysed Sixteen Mile Beach says: “Much of the country’s coastal developments and ecosystems could be at increased risk of destruction from coastal erosion.”
The “severe and rapid erosion” 80km north of Cape Town accelerated between 2015 and 2020, say the scientists, who published their findings on Thursday in the peer-reviewed journal Remote Sensing.
They do not speculate why Sixteen Mile Beach is vanishing, saying only that coastlines change dynamically under the influence of factors such as storms, sediment movements, infrastructure development and sand mining.
“While coastal erosion is exacerbated by climate-driven sea level rise and anthropogenic forces, the extent and severity vary from region to region,” they say.
A transect of the beach showing shoreline positions at 11 points between 1937 and 2020. (Remote Sensing)
The shoreline change they analyse “could potentially be attributed to broader changes in the overall coastal and inland dune systems that have occurred over the decades, or changes in regional ocean and climate systems”.
Sixteen Mile Beach and the neighbouring Yzerfontein main and Pearl Bay beaches stretch 30km along a windswept coastline popular with kitesurfers – swells routinely reach 5m-6m – and whale-watchers.
In Yzerfontein, properties and a road border the beach, and a caravan park is separated from the sand by a boardwalk over a dune. At its northern end, the beach is just over the dunes from Langebaan lagoon, a marine protected area and a wetland of international importance.
Shoreline forecasts for the next 10 years and 20 years at four locations in the study area. (Remote Sensing)
Tourists from Cape Town and further afield flock to the West Coast National Park in spring for its wild flowers and in summer for the lagoon’s warm, clear waters. In winter it is often lashed by violent frontal storms, but the long-term evolution of SA’s longest beach has never been studied, and the scientists set out to fill the knowledge gap.
They marked the shoreline on 20 aerial photographs from 1937, 1960 and 1977 and satellite images taken at five-year intervals between 1985 and 2020, then used specialised technology to create maps, tables and heatmaps depicting the erosion of the beach.
Overall, 95% of the beach had experienced erosion – about half of it “significant” – with an average loss of 38m. A 100m slice of the beach at its southern end lost 99.29m – 1.19m a year – making it an “erosion hotspot”, while a similar slice in the north gained 13,8m. “Sixteen Mile Beach experienced the greatest average change of 52,13m,” say the scientists.
The ‘erosion hotspot’ identified by the scientists about 6km north of Yzerfontein. In this image, the beach is about 27m wide at its narrowest point. (Google Earth)
“The most drastic changes are those between 2015 and 2020, whereby most of the shoreline shows an increased landward movement. All of Pearl Bay, Yzerfontein main beach and the southern end of Sixteen Mile Beach show the largest amount of change within this period.
The beach is a “logarithmic spiral”, meaning it has developed its gentle curve in the “shadow zone” created by the rocky headland on which most of Yzerfontein is built.
“A possible reason for these differences in erosion trends could be how the wave energy is dissipated along this extensive log-spiral beach system,” say the scientists.
When it comes to predicting future shoreline changes, the team – Jennifer Murray, Elhadi Adam, Stephan Woodborne and Mary Evans from Wits, and Duncan Miller and Sifiso Xulu from UFS – say their model does not show “substantial movement” from the current high-water line.
“The forecasting model is only based on statistical trends in the data and must therefore be used with caution since it cannot take into account other parameters such as underlying geology, beach profile or wave conditions,” they say. “Such forecasts cannot be the main tool for coastal management and planning.”