Coping with sea level rise and storm surges

Damage to coastal properties during the March 2007 extreme storm event. Flooding of a beach-side villa at Salt Rock, KwaZulu-Natal. Destruction of coastal infrastructure. The National Storm Surges Workshop, Knysna, February 2011 (Image: W.S. Goschen)

Analyses of a century’s worth of temperature records point to a rise in the average surface air temperature of the Earth over the last century. One consequence of this warming is a global rise in sea level.

The main causes of the sea-level rise are attributed to:

• an increase in volume of the oceans caused by thermal expansion of the seas due to warming
• melt-water entering the ocean from glaciers, ice caps and other ice at low altitude
• melt-water from polar ice-sheets, especially from Greenland and West Antarctica

These trends have been substantiated by measurements made in southern Africa, where analyses of about 50 years of data show that the sea level of the west coast of South Africa is rising by about 1.87 mm/yr, the south coast by about 1.47 mm/yr and the east coast by about 2.74 mm/yr, in agreement with current estimates of global trends.

These regional differences were attributed to:

• Different rates of vertical crust movements. The land mass off southern Africa appears to be tilting along an approximately north/south axis, with the east coast rising faster than the west coast.
• The long-term trends in barometric pressure. The west coast of South Africa has seen a drop in barometric pressure over the past few decades while the south coast has remained fairly static and the east coast has seen a rising barometric pressure. An increase in barometric pressure, as off the east coast, suppresses sea level rise.
• The differences between large-scale oceanographic processes off the east coast (Agulhas Current) and west coast (Benguela Current). The discovery that the temperature of the Agulhas Current has been increasing since the 1980’s, caused by an increase in wind stress curl in the South Indian Ocean due to the shift of westerly winds, and could contribute to the thermal expansion of the local waters.

Owing to these regional changes, the eustatic  rise in sea level was found to be lower along the west coast (about 0.42 mm/yr) but higher along the south coast (about 1.57 mm/yr) and east coast (about 3.55 mm/yr).

Increase in storminess

The energy of storms at sea is partly fuelled by the water over which they pass and the warmer the water, the more energy is available for the storms to feed on. So it is predicted that, with a rise in sea temperatures, there will be an increase in the frequency and strength of storms at sea, i.e. an increase in storminess.

On short time scales (order of hours to days), there are other processes that contribute to a sudden rise in sea level. These are:

• Tides: tides raise and lower the sea level by between one and two metres.
• Weather: variations in wind and atmospheric pressure can raise sea levels by up to forty centimetres and coastal trapped waves, propagating in resonance with weather systems, can add up to 1 metre to sea level height.
• Waves: during extreme storms waves can increase sea level by several metres.

Storm surges

A storm surge refers to an abnormally high sea level, which is often accompanied by severe meteorological conditions and high tides. The effects of a storm surge are felt differently at different coastal environments, depending on many factors such as bathymetry, geology, shoreline shape, shoreline elevation, nature of the coastline defences and the degree of exposure to waves.

With a rise in sea-level and increase in frequency and intensity of sea storms, accompanied by an increase in wave heights, the South African coastline is expected to experience:

• greater risk of damage by storm surges
• increased exposure to more intense and more frequent extreme events
• increased saltwater intrusion and raised groundwater tables
• greater tidal influence
• increased flooding, with greater extent and frequency
• increased coastal erosion
• more frequent destruction of coastal property and infrastructure
• periodic destruction or negative disruption of the coastal biosphere and environment

On 19 and 20 March 2007 an extreme storm, coinciding with abnormal string tides, produced swells in the range of 8.5 metres that devastated the KwaZulu-Natal coastline (pictures 1, 2 and 3). The aftermath was significant coastal erosion and an estimated one billion rand in damage to coastal property and infrastructure. The effect on the coastal biosphere and environment went largely unmeasured. From past records it is evident that waves of around 7 metres in height are experienced every year along the South African coast, while big waves of over 10 metres are recorded every 20 years.

National Storm Surges Workshop

These concerns led the Department of Environmental Affairs, through its Oceans and Coasts Branch, to organise the first National Storm Surges Workshop (picture 4). The workshop was held in Knysna in February 2011 and was well attended by coastal municipalities, regional and national government departments (including the South African Weather Service), coastal engineers, disaster managers, insurers and researchers. SAEON was represented by Dr Wayne Goschen.

The main objective of the National Workshop was to bring interested parties together in order to share knowledge and best practice in coping with storm surges along the coast of South Africa.

The outcomes were:

• A better appreciation of the risk posed by storm surges: the details of the hazard now and into the future, the identification of locations along the coast which are vulnerable to environmental, economic and social impacts, and an assessment of the capacity needed to cope with the threats.
• The development of a research agenda to address the risks posed by storm surges along the coast of South Africa, including consideration of the advantages of a Storm Surge Early Warning System for South Africa.

Sourced from articles and publications by Professor Geoff Brundrit, Breetzke et al. (2008), Mather et al. (2009), Rouault et al. (2009), Smith et al. (2007), eThekwini Municipality (Durban), Department of Environmental Affairs (Oceans and Coasts), CSIR, and the KwaZulu-Natal Department of Agriculture and Environmental Affairs (The Coastal and Biodiversity Management Unit).

¹ Dr Wayne Goschen co-authored a chapter on sea-level rise and its trends, impacts and adaptation for South Africa for the "South African Second National Communication under the United Nations Framework Convention on Climate Change".

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