After a weekend that saw South African shores pummelled by massive waves and rough seas, the City of Cape Town has provided a detailed explanation regarding the dramatic changes to beaches along the Atlantic coastline over the last ten days.
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Yesterday, the CoCT explained that the sandy beaches from Lagoon Beach to Sunset Beach, including Milnerton, are known to be receding or eroding slowly over time, causing changes to sediment transport processes and sediment supply to the coastal system.
This sediment transport process, which is essentially beach sand loss in winter and sand replenishment during summer, has been interrupted by urban development, resulting in a gradual net loss 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 more severely.
This dropped beach levels by approximately 1.5 metres, which 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.
Breaking waves cause general coastal erosion and sediment transport by suspending sand from the seabed, beaches, and dunes as fierce turbulence washes over the sandy surfaces.
Once suspended, the currents caused by the wind and waves and larger circulations move the sand off the beach, making beaches steeper and thinner.
Open sandy coastlines are typically not straight or smooth but often have isolated small embayments and bumps, called cusps.
This stretch of coastline is known for its formation of cusps, which are important in understanding the sediment dynamics of the coastline and their contribution to increased erosion at these isolated cusp locations.
Waves and rip currents interact with minor irregularities in the coastline to naturally form cusps, which develop 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 are not fixed. They migrate along the coastline depending on the wave and current conditions 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. When these cusps align with infrastructure, the undermining of the infrastructure and the dune happens.
‘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 on around 1 September, followed by consistently large waves the following week and a storm event on 11 September,’ said the City’s deputy mayor and mayoral committee member for spatial planning and environment, Eddie Andrews.
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‘Furthermore, 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.’
‘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 Saver 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.’
In some locations, the low beach levels also exposed an underlying geological layer or old shell and pebble layers, which should not be disturbed. The dark, soft brown and red rock layers seen exposed are examples of this and should not be confused with oil spills or dumped material. The presence of the rock layer helps to limit beach erosion to some extent.
‘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. The coastal process driving these changes are of a large scale and influenced by multiple factors. This demonstrates the importance of wide coastal buffers as the best defence to coastal dynamics,’ added Andrews.
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Picture: Supplied / City of Cape Town