Extreme weather in South Africa can lead to problems for those who own homes. Here's how to prepare for it.
News reports have, over the past six months, highlighted some “extreme weather conditions”. We consulted with Kevin Rae, Chief Forecaster: Disaster Risk Reduction (DRR) of the South African Weather Service (SAWS), who describes weather phenomena and provides advice to homeowners on how protect themselves.
There have been at least three weather conditions over the past half year that have alarmed South Africans: heavy thunderstorms, tornadoes; and flash floods. Some have earned the headlines of being ‘freak’ but says Rae, meteorologists prefer to avoid using this term as it implies that such phenomenon is somehow unnatural or beyond the realms of possibility.
“’Extreme’ however is far more acceptable although there are many nuances in terms of the exact scientific or mathematical definition of what describes such a weather condition. “Universally, and within the industry, we consider ‘extreme’ to be represented by the upper 5%, or even 2% of the distribution of the phenomenon. Moreover, we consider how frequently you would reasonably expect such an event to occur, before suggesting it to be ‘uncommon’ or ‘rare’.”
Flash floods for example. These can occur at any time that the rate of rainfall exceeds the infiltration of the ground to absorb the rain, resulting in overland runoff, so they are not entirely uncommon in SA environments.
Urban and flash flooding
This is exactly what happened between 8-11 December last year in Gauteng. With accumulative rainfall of some 200-300 mm over northern Gauteng, the excessive runoff entered the Sesmylspruit River that runs through Irene and Centurion, causing it to burst over its banks and flood the urban environment. Rae says that “urban flooding” is somewhat of a recent phenomenon, and Centurion is a case in point.
“The Centurion CBD is considered relatively new given it remains under construction since the mid-1980s. Pre-development, the CBD was characterised by pristine highveld grassland and smallholdings on its slopes. Now modified and built up, the area is interspersed with tarred roads, highways, and flat concrete or paved surfaces, all of which tend to restrict or prevent infiltration of rainwater resulting in rapid runoff into the nearby river system. Hence the occasional flooding incidents that residents have become familiar with.
“However, such runoff has morphed from minor, short-lived and localised disruptions into more major, widespread events with a distinct potential to cause damage or to be life-threatening,” warns Rae.
And it’s not just an urban issue either. Think back to the flooding disaster in the small Western Cape town of Laingsburg in 1981, where some 100 people lost their lives, and 184 houses destroyed. What made this an ‘extreme’ event was that the region is arid and rocky, but when the Buffalo River burst its banks, the force of the water was so immense that some of the survivors needed to be rescued up to 21 km away.
Safety during flash floods
SAWS is very concerned about the loss of lives during flash flooding, especially those in urban areas, which Rae believes can often be avoided. “SAWS invariably warns radio and television stations if a spell of sustained or set-in heavy weather conditions are indicated, so residents are advised to monitor weather bulletins on a regular basis.
“Similarly we advise of the potential of a flash flood, which we determine through the use of a flash flood guidance system. This system maps individual drainage basins across the country, and those are compared to radar and satellite-derived rainfall in near real-time.
“Rainfall generated by computerised weather predication models is also taken into account. Together these analyses allow us to determine the rate and/or amount of rainfall that will exceed the infiltration capacity of the ground, and thereby cause an overland runoff that will enter streams and rivers, which in turn prompts the issuing of a flash flood warning,” says Rae.
“It is then that pedestrians and motorists are strongly urged NOT to attempt to cross such water features, even when there is a familiar bridge or roadway nearby,” says Rae. “Even a large motor vehicle like an SUV can be swept away when a water level reaches a depth of only 30cm (approximately axle-level).” This obviously depends on the lateral force of floodwater, making such conditions unpredictable and treacherous.
Rural settings are the most dangerous, for both motorists and pedestrians. A river may appear innocuous but strong currents may be present beneath the surface of the water. “At SAWS we are particularly concerned for persons on foot, such as scholars, who convince themselves that they can traverse a river in flood because they need to make their way to an important test or exam. Rather arrive at your destination via a longer, safer route, or miss the event and live to tell the tale,” says Rae.
Tornadoes in South Africa are a relatively ‘uncommon’ phenomenon, nevertheless we can expect to experience some 10-20 a year. However these are comparatively weak and far less numerous when compared to the Tornado Belt in the United States. Usually associated with supercell type thunderstorms, the more long-lived, violent examples are manifested by the presence of strong wind shear between the earth’s surface and the cloud base.
Tornadoes are measured on the Enhanced Fujita (EF) scale from 0-5. The strongest tornado on record in South Africa was the EF4 Mount Ayliff event in the Eastern Cape on 18 January 1999. SAWS has yet to formally document an EF5, although confirms Rae, it’s just a matter of time before it has the opportunity to do so. “Whilst southern African tornadoes are not particularly common, we could still potentially experience the full range of tornado intensity. Most domestic tornado events appear to favour an intensity between EF0 – EF3.”
The Eastern Cape, the Transkei, and KwaZulu-Natal are known for occurrences of tornadoes, especially during hot humid summer months, specifically October and November, however they are not uncommon in the flat, high-altitude rolling grasslands of the Highveld region, the eastern and north-eastern Free State, the southern and central parts of Gauteng, and the eastern highveld escarpment of Mpumalanga.
While tornadoes of any strength are a highly destructive extreme weather phenomenon, it might be surprising to learn that wind damage, resulting from non-tornadic storms, often surpasses that of a tornadic storm. Rae says that under certain conditions, even relatively weak thunderstorms can create havoc exceeding that of a tornado.
“SAWS classifies a storm to be severe if one of more of the following are realised: a) if there is large hail – having a diameter of 19mm or more; b) significant urban flooding; or c) large amounts of small hail or the occurrence of a tornado (of any strength).”
There is a general expectation that as the subcontinent warms as a whole, likely as a result of global warming, the eastern third of southern Africa will become hotter and moister in the years to come, while the western parts will become more arid. “It is entirely possible that the frequency and intensity of extreme weather, including thunderstorms, may increase with the passage of time,” says Rae.
SA homes resilient###
The general building style of homes in South Africa, is that of well-constructed brick residences with good foundations, a combination that makes them remarkably resilient towards extreme weather like thunderstorms and strong winds. It is however the roofs, be those thatch, corrugated iron or tile, that have a tendency to come loose or leak during heavy downpours or strong wind conditions.
Rae points out that even moderately strong winds, of the order of 60-80km/h, may loosen roofing material, which when lofted into the air by strong winds, have the potential to main or kill. Corrugated iron sheeting in particular, can be deadly. Even a fairly weak EF2 tornado typically generates winds in the region of 200 km/h, and non-tornadic thunderstorms can cause surface wind gusts approaching 100 km/h for short periods.
Weatherproofing your residence
RAE suggests that during the dry season, eg winter months, homeowners should invest in ensuring one’s house roof is in good repair and well-waterproofed. “Similarly gutters and downpipes should be inspected frequently to remove the accumulation of leaves and other debris.
“With water resources threatened as urban populations increase, homeowners would also be well-advised to consider investing in plastic or fiberglass water storage tanks to collect rainwater, particularly that from the roof.”
Indigenous gardens featuring drought-resistant shrubs and trees are becoming an economically popular choice in South Africa, as is the planting of frost-resistant species.
“Climate change effects are resulting in warmer summers and milder winters, thus shortening the frost season over many parts of the interior located at higher altitude, the Highveld for example. But you don’t have to be a weather analyst to realise this, for it does seem that Spring is arriving earlier and earlier every year, doesn’t it?” concludes Rae.