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Our topic for this week further deals with sea level rise.
Sea levels are currently monitored by means of ‘tide gauges’ at many locations along our coastline, especially near to centres of high density populations.
Measurements are made in the deep ocean by bottom pressures obtained from sea bed devices and by satellite radar altimetry.
Sea level rise is caused primarily by the thermal expansion of the ocean responding to global surface temperature increases and the melting of polar icecaps.
This happens at a geological time scale of thousands of years in contrast to tides and storms which happen on a hydrodynamic scale of days to weeks.
Sea level changes can vary from a few metres up to 150m over periods ranging from 500 to 100 000 years.
Global temperatures similar to those expected in the latter part of the 21st century occurred during the last inter-glacial, about 125 000 years ago.
At that time, paleo data indicates rates of sea-level rise of about six to nine metres – 1000y-1 with sea level reaching 6–9 metres above present day values and with polar temperatures about 3°C to 5°C higher than today.
These warmer conditions may serve as a useful analogue for the 21st century and beyond.
Models predict that our northern region will have more coastal retreat owing to dissipative beaches characteristically with fine sand that is easily eroded because of its shallow slopes.
These two characteristics make them more vulnerable to erosion and inundation respectively, and hence they display the greatest retreat.
Currently, global rates of sea level rise are accelerating, with the latest predictions a one meter rise.
Specifically for Durban, and thus for KZN, the present rate of sea level rise is 2.7mm yr-1.
Appropriate adaptation can significantly reduce the impact of sea level rise.
Planned adaptation will range from retreat from rising sea levels through planning and zoning of vulnerable coastal regions, accommodation through modification of coastal infrastructure and the construction of facilities such as cyclone protection centres and protection of highly valued coastal regions.
Planned adaptation is more cost effective and less disruptive than forced adaptation in response to the impacts of extreme events and needs to be undertaken in the context of the many pressures on coastal regions as a result of rapid coastal development.
Furthermore, coastal vegetation is unique in that it is resilient to the harsh salt spray, wind and unstable sand conditions of the coastal zone.
However, this resilience allows vegetation to serve as a protective barrier to coastal vulnerability. Different vegetation types can be identified.
There are colonisers that stabilise the shifting frontal dune, which in turn allows other plants to establish over time.
Coastal forests represent the final stage of such succession, comprising climax forest often found cladding the very high sand dunes in the iSimangaliso Wetland Park.
Clearly, protecting coastal vegetation has positive biodiversity implications and helps to protect our shorelines and sensitive dune systems against sea level rise.
• Jean Simonis is a professor in the hydrology department at the University of Zululand
European Commission Coastal Erosion, 2004. Evaluation of the need for action: Living with coastal erosion in Europe, Sediment and Space for
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Harris, LR, 2008. The ecological implications of sea level rise and storms for sandy beaches in Kwazulu-Natal, MSc, December 2008, UKZN.
Mather, AA, Bundy, SC, Haigh, ID, 2011. KwaZulu-Natal coastal erosion events of 2006/2007 and 2011: A predictive tool? South African Journal of Science; Volume 109, Number ¾, March/April 2013
Palmer, B, Van der Elst, R and Parak, O, 2011. Understanding our Coast: A synopsis of KZN’s coastal zone. KwaZulu-Natal Department of Agriculture, Environmental Affairs and Rural Development, Cedara, Pietermaritzburg.
UNESCO, 2010. Sea level Rise and Variability: A summary for policy makers: Intergovernmental Oceanographic.
Zwamborn, JA, 1970. Coastal Erosion and Beach Restoration Measures, Die SivielJean Simonis