Longshore drift: life or death of a beach
Our suggested option of the vegetated dune does not even make the list
IN 2014 the then uMhlathuze ward councillor said that Richards Bay will soon lose its beach if government does not act fast to confront the effects of climate change on the local beaches (Razack, 2014).
How did we allow this to happen?
In this article we will address this issue by looking at the natural process of sand movement along our beaches and how, when disturbed, we lose our natural coastal defence.
Gilbert and Vellinga (1990) identified five alternative ways of reacting to a loss of the natural coastal defence:
a) accommodation
b) protection
c) beach nourishment
d) retreat
e) do nothing alternative
Such options can be further divided into two major categories of ‘hard’ and ‘soft’ engineering solutions, something we discussed last week.
Our suggested option of the vegetated dune does not even make the list.
Historical factors
Historically, Richards Bay beaches have followed a similar history to Durban’s loss of natural beach protection, which largely revolved around efforts to effectively operate a port.
Beaches were stable for at least 100 years prior to the initiation of dredging for the port. Dredging operations marked the onset of beach erosion.
It dramatically cut off the longshore drift, resulting in too little sand reaching the beaches, thereby starving the beach of sand.
Sand is supplied by our rivers flowing into the sea. Longshore (Littoral) drift is then responsible to move this sand along the coast line.
It is caused when waves approach a beach at a slight angle. After the wave has broken on the beach the swash runs back downslope at a right angle to the sea.
This results in sand removal downdrift from the beach. Sand is therefore transported predominantly in one direction. On our east coast beaches the direction is mainly from the south to north. It is the major factor responsible for the beaches’ configuration.
Disrupting this natural drift changes the beach configuration forever, requiring permanent man-made coastal defences to stabilise the situation.
This also happened at Richards Bay after the harbour wharf construction and the initiation of dredging operations.
Examination
A historical examination of our shore will show that this natural process of longshore drift is still occurring at beaches unaffected by human intervention.
In contrast to Richards Bay beach, the situation at Mtunzini beach has continued without any such disruption.
Taking a cross section through the beach zone at Mtunzini shows the development of a number of environmental zones.
These zones are strongly reflected in the type of vegetation occurring which further promotes the fitness and health of the beach.
In this article we have discussed two conditions – one promotes fitness and health (syntropic) and the other results in misfit and death (entropic).
All natural systems move between these two states, finding the fittest available environment and adapting to it, thereby making it more fitting.
Have we through our impact at Richards Bay caused an entropic misfit, a beach system unable to adapt or to obtain the fittest environment?
Has this resulted in the end of our beach as our Councillor seemed to think back in 2014? Can we still allow nature back in?
Do not miss next week’s discussion on global warming and its future consequences on our beaches!
• Simonis is a professor at the University of Zululand’s hydrology department
References
Corbella, S and Stretch, DD., 2012. Coastal defences on the KwaZulu-Natal coast of South Africa: a review with particular reference to geotextiles. JOURNAL OF THE SOUTH AFRICAN INSTITUTION OF CIVIL ENGINEERING, Vol 54, No 2, Pages 55–64
Mcharg, IL., 1992. Design with nature. Wiley & Sons, NY.
Razack, R., 2014. Bay beach erosion fears debated; February 6, 2014, ZO.
