The East London Museum has been documenting the changes in coastal profiles north and south of the city (including those changes to beaches within the city limits) for almost two decades.
The first formal recording of the coast focussed on the Chintsa Bay area with a photographic survey undertaken in 1997. This study was elicited, in part, as a result of observations of a changing coastline while the museum was debating the off-road vehicle issue (recreational vehicles accessing some beaches for fishing and other activities). A storm surge event (June 1997) impacted heavily on many of the embryonic dunes, removing some of them and causing dune slumping on primary dune faces. The classic cycle of reimbursement of sand back onto the beaches, allowing dune development with the concomitant sand stabilisation by pioneering plant species, was broken.
The coast was in effect left more vulnerable to irregular storm surges and weather events which could be exacerbated by heavy rains and large tidal variations.
Temperature records over the past century indicate an average rise in surface air temperatures. This has resulted in a global rise in sea level.
Measurements of sea level rise in southern Africa show a trend (50 years of data) that the sea level of the east coast is rising by about 2.74 mm/year (which agrees with estimates of global trends). The rise in sea level, on average, is less as one moves westwards towards Cape Town – 1.47 mm/year for the south coast and 1.87 mm/year for the west coast.
Three main causes of sea-level rise are:
- Melt-water produced by glaciers, ice caps and other ice entering the oceans at low altitude
- Melt-water entering the oceans from polar ice-sheets, especially from West Antarctica and Greenland
- Thermal expansion of the seas due to warming causes an increase in the volume of the oceans
The regional differences in sea level rise from the east to the west can be attributed to:
- Long-term trends in barometric pressure – dropping along the west of SA while the south coast has remained fairly even and the east coast has seen a rise
- Vertical crust movements – the land mass off southern Africa is tilting along a north/south axis with the west coast rising less than the east coast.
- There are differences between the oceanographic processes related to the Agulhas current (east coast) and Benguela current (west coast) – the temperature of the Agulhas current has been rising since the 1980’s. This has been caused by an increase in wind stress curl in the South Indian Ocean due to the shift of westerly winds. This could cause thermal expansion of the waters off the east coast.
On shorter time scales (hour to days) there are other processes that can contribute to a sudden rise in sea level. Included here are the following:
- Waves – extreme storms can increase sea level by a number of meters
- Tides – the sea level can be raised and lowered by 1-2 meters by tidal action
- Weather – variations in atmospheric pressure and wind can raise sea levels by up to 40 cm and wave height will be summated by those waves (propagating in resonance) trapped by the topography of the coast can build up to an additional 1 m.
Increased storm surge events along the east coast
Large storm surge events were not regularly documented along the east coast, but after the 1997 storm surge a dramatic change of the coastal profile was observed. Dune headlands were eroded back and eventually removed to beach level – the most classic early example being the area west of Chintsa West, to the right of the access from Khamanga Bay. These processes have persisted and recent photographic evidence shows the regression of most of the dune headlands at estuary mouths, either being eroded or removed.
Rainbow Valley Bay has changed dramatically over the past years with massive dune slumping resulting from the erosional action of trapped waves propagating in resonance in a confined space (between two rocky headlands).
A good recent example of a dramatic change in dune presentation and beach profile has been at Glen Eden. Within six weeks (5th June 2015 – 31st July 2015) the parallel dune system had been cut back by a number of meters and the beach has been eroded to bedrock in places.
Storms at sea are partly fuelled by the water over which they pass. In the case of the east coast the water is warmer and there is thus more energy available for the storms to feed on. There has been an increase in the strength and frequency of storm surge events in our region during the past decade. The most notable being in 2008, 2011 and 2015. Predictions that a rise in sea temperatures will result in an increase in storminess have been realised.
Storm surges can be defined as abnormally high sea levels, accompanied by severe weather conditions and high tides. Depending on the coastal environment these storm surges will translate differently depending on the following:
- shoreline shape
- shoreline elevation
- degree of exposure to waves
The South African coastline is expected to experience a greater risk of damage by storms with a rise in sea level (accompanied by an increase in wave height).
Indications in our region are that there will be an increased exposure to more intense and frequent storm surge events resulting in the following:
- increased saltwater intrusions
- increased coastal erosion
- periodic destruction or negative disruption of the coastal biosphere and environment
- greater tidal influence
An interesting study would be to quantify sand budgets for our coast considering the increase in coastal erosion. The higher energy waves generated by conditions described above move large quantities of sand as illustrated below (photographed at Kidd’s Beach).
Articles and publications by Dr Wayne Goschen, Data Manager and Marine Scientist, SAEON Egagasini Node, Mather et al. (2009), Professor Geoff Brundrit, Breetzke et al. (2008), Rouault et al. (2009) and Department of Environmental Affairs (Oceans and Coasts), CSIR, Smith et al. (2007)
Natural History ELM