Coastal erosion can be described as the removal of beach or dune sediment, or the weathering of rocks by wave action and currents, tidal currents or drainage. Coastal erosion and change is generally slow and constant over time, with the exception being large storm and associated swell events, which cause extensive coastal erosion.
Long period swell associated with winter storms in the Western Cape has the greatest erosive effect, due to the deeper orbital motion of water particles in waves, which cause increased wave set-up and run-up along beaches and increases the amount of sediment that can be held in suspension.
Causes of coastal erosion
Climate change will likely cause an increase in the intensity and frequency of coastal storms, and with associated sea-level rise cause an increase in coastal erosion, increasing the risk to human life and the natural and built environments. The extent of coastal erosion is generally dependent on four factors:
- Geology and geomorphology of the coastline
- Sediment type
- Wave climate (period, height and direction)
- Nearshore bathymetry (especially coastline gradient)
It usually has three, often simultaneous, results:
- Loss of coastal land and damage or destruction to any structure in the vicinity.
- Destruction of coastal dunes which act as natural sea defence mechanisms.
- Destruction of any artificial sea defences.
Coastal erosion occurs as a result of storms removing sediment from the normal swell beach profile, forming a storm beach profile. Storm beach profiles generally have gentler gradients as a result of eroded material often being removed from the berm and beach face and deposited in offshore bars. Following a storm event the beach profile is out of
equilibrium, and attempts to return to its original swell profile shape and position. However, reduced sand availability (often as a result of human development within the coastal zone) prevents beach building from happening, and even normal, constructive waves cause further coastal erosion.
A South African example of coastal erosion
The March 2007 storm event along the KwaZulu-Natal (KZN) coastline resulted in approximately 4 000 000 m³ of sand being eroded from the coastline, which is equitable to nine years worth of sediment (approximately 460 000 m³ per annum of sand is transported along the KZN coast by longshore drift).
Building too close to the coastline
Coastal erosion can cause significant problems if people build too close to the coast. Beaches move constantly, and naturally oscillate between periods of erosion and accretion (onshore collection of beach sediment). Sometimes these fluctuations reach extremes. Extreme fluctuations are only ever a problem when humans develop buildings or infrastructure too close to the coast.
Coastal hazards occur when dynamic coastal movements interact with static human resources that we have placed along the coast. Places where coastal sand-dunes occur are also under threat as human interference has disrupted their natural “migratory” cycles. Coastal cliff failure can occur in a number of ways from slumping to large block failures. The coastal cliffs along many parts of our coast are slowly eroding.
By drafting your own Family Emergency Plan and identifying all your risks, including those hazards affecting your own environment, you can make a positive contribution to preparedness.