Over the past week dangerous high tides have hit the province’s shores with the possibility of more rough weather with high winds and heavy seas in store, the South African Weather Service (SAWS) has warned.
Small Bay, Bloubergstrand Beach, Sunset Beach and Lagoon Beach were among some of the beaches where damage was reported due to the New Moon Spring Tide. The weather service issued another level 1 warning for damaging waves along the Cape coastline for this coming weekend.
As of Monday 18 September, the National Sea Rescue Institute has confirmed the deaths of two people. In Wilderness, a 93-year-old woman died after she was swept off her feet by a wave while she was watching the surf from a parking lot. The Kouga Municipality also confirmed that the body of an unknown man, estimated to be 68 years old, was found at Aston Bay near Jeffreys Bay in the Eastern Cape.
The NSRI has since urged people to be cautious along coastlines.
“We are appealing to the public to monitor the South African Weather Service alerts and be cautious. Don’t take unnecessary risks or put yourself in unnecessary danger,” says NSRI spokesperson Craig Lambinon.
One of the residents in Blouberg says he knew something “big” was coming when he noticed foam-like water coming from the ocean.
“We actually got off fairly easy compared to other parts of the province. The weather service said things might get worse. I just hope we will remain safe,” he says.
Coastal cusps
Despite widespread fears on social media about the ‘end times’, the City confirmed that this is a natural process.
The New Moon Spring Tide and other natural processes have had a significant, albeit sometimes lesser-known impact on our shores.
Due to the New Moon Spring Tide, significant coastal “cusps” have formed along the coastline between Lagoon Beach and Sunset Beach, resulting in large drops in the beach level in the area of these cusps and the resulting erosion of the adjacent dunes.
Cusps are natural coastal features, which take place in an already receding coastline. This results in visually dramatic changes to the shoreline.
These effects of general erosion and coastal processes are typically stronger and more pronounced during the stormy winter season in Cape Town.
The sandy beaches along Cape Town’s Atlantic coastline, from Lagoon Beach to Sunset Beach, including Milnerton, are known to be receding or eroding slowly over time.
Changes in sediment transport processes and sediment supply to the coastal system that have resulted from urban development contribute to this effect. This sediment transport process, which is essentially beach sand loss in winter and replenishment of sand during summer, has been interrupted by urban development, reducing the summer sand replenishment of beaches, resulting in a net loss over time and therefore coastal erosion.
More recently, the sand level at isolated points along the Milnerton and Sunset beaches, where cusps are currently located, has also dropped severely. This dropped beach level of approximately 1,5 m, has resulted in the undermining of the bottom (toe) of the dunes at these locations, leading to slumping of dune faces and unstable slopes of up to three to four metres high.
Cusps are formed naturally by waves and rip currents interacting with minor unevenness or irregularities in the coastline, which then grow into small mobile bay features.
Over time, it reduces beach width landward in the middle of the cusp and builds up on the downdrift sides as the cusp develops.
Cusps migrate along the coastline depending on the wave and current conditions and are not fixed and may also disappear during calmer conditions.
“As a cusp develops, the water depth locally increases in the small embayment it creates, resulting in wave energy and current velocities focusing on these cusps. This wave energy is now close to the adjacent dune and attacks the dune slope resulting in rapid localised sand loss,” the City says in a recent statement.
Erosion
The recent large focused beach and dune erosion comes after consecutive erosion events on top of the general coastal erosion and generally lower winter beach levels, caused by very high spring tides around 1 September, followed by consistently large waves the following week and a storm event on 11 September.
“At certain locations along the beach in front of Woodbridge Island Estate and the Milnerton Golf Club, beach cusps were present on the coastline at the same time. The location of the beach cusps at these locations, coinciding with the conditions we have mentioned, resulted in focused, localised loss of sand and has led to the undermining of dunes and structural damage,” says the City’s Deputy Mayor and Mayco member for spatial planning and environment, Eddie Andrews.
Similar events
A similar erosion event was recorded in July 2018, which also caused similar large focused beach erosion and structural damage at the Milnerton Surf Life Saving Club, the Milnerton Golf Club and restaurant, as well as other localised locations along the coast.
“In between these winter storm events, the beach will slowly recover and new cusps could form. This is a natural and normal coastal process. This normal process is happening in addition to slow erosion, and with it meeting fixed infrastructure results in the dramatic pictures we are seeing along parts of this coastline,” Andrews says.
He adds the effect of future sea level rise will contribute to this effect and presents a long-term challenge along this stretch of shoreline.
“The City is monitoring the process and its effects on these beaches. We expect to see a slow recovery of beaches and with a slow net loss of dune width over time.”
