Flash flooding was once-in-a-100-year occurrence

10 November 2016 - 16:16 By TMG Digital
The devastation in Alexandra, Johnnesburg after flash floods struck the area on 9 November 2016.
The devastation in Alexandra, Johnnesburg after flash floods struck the area on 9 November 2016.
Image: Kingdom Mabuza

The storm water systems were overwhelmed by the severe rainfall that struck Johannesburg on Wednesday‚ the South African National Roads Agency acknowledged today. However‚ the flash flooding is unlikely to reoccur soon.

"Yesterday’s rainfall was so extreme that it is likely to be and is a once-in-a hundred-year occurrence. Unconfirmed reports indicated that the storm was indeed severe with between 90 mm and 150mm rain falling in a very short period‚" Sanral said.

The agency said a team of engineers will be inspecting its network in Gauteng urgently to check for damage following the flash flood‚ including checks for any signs of damage to roads and bridges to make sure they are safe for use.

 

In addition‚ the agency will appoint engineers with extensive experience in floods and storm water systems to investigate the extreme flooding‚ and to advise on any remedial actions that may be required. According to Edwin Kruger‚ Sanral's bridge network manager‚ these freeways would have in the past been designed to pass a 1:20 year flood or up to a 1:50 year flood in the case of larger rivers.

 "This means that in any year there is a 5% chance that a flood of this magnitude or greater will occur if the design recurrence period is 20 years. The design standards used in South Africa are very similar to the varying international norms."

Sanral noted that‚ frequently‚ floods are not isolated events but can occur within days of each other depending on the type of storm encountered.

It is not possible to guarantee that a road or bridge will never be damaged or overtopped. Unfortunately‚ due to the possible effects of global warming this means that ever increasing extreme events and some flooding of roads can be periodically expected.

After a severe storm‚ there is potential for soil erosion (commonly known as scour damage) or sinkholes opening up and these are closely monitored by the roads agency’s routine maintenance contractors. Should problems be observed‚ the applicable portion of highway is closed at short notice.

A further factor that influences floods is also development in the catchment areas which may increase run-off and consequent flows. "It is not possible to design for each and every extreme event."

The encroachment of squatter camps is a challenge. "The blocking of storm water pipes‚ culverts and bridges by homeless people is also increasingly becoming a problem‚" Sanral said.

"Although they are removed on a regular basis‚ the illegal structures blocking the storm water systems are often reconstructed within a day or two of them being removed."

"An example‚ very applicable to the area affected by the flash floods‚ occurred just last week.

The maintenance unit came across a homeless man who had used concrete blocks to build a shelter in a culvert at Gillooly’s – they cleared this but it is uncertain if the structure was rebuilt a day or two later‚" said Alex van Niekerk‚ a Sanral project manager.

The agency noted that the Gauteng freeway network was constructed 40 years ago. Since then the landscape changed from predominantly agricultural land to developed land.

Water absorption of agricultural land is higher than areas that are built-up as grass and soil absorb water whilst asphalt and concrete do not. In addition‚ when drainage systems in built-up areas fail – they are not built to manage floods – the run-off will increase and will build up in low-lying areas such as Gillooly’s.

With the large volume of rain that fell in a very short space of time and the accumulated run-off‚ much more water found its way onto the road due to the storm water systems being overburdened.

"There is an accumulative effect‚ if the storm water drainage in the surrounding area cannot handle the water run-off‚ it continuous to swell causing a flash flood‚" Van Niekerk said.

Sanral provided these tips for road users:

- If the speed of the flood water doubles‚ the force it exerts on you/your car is quadrupled.

- Just 15cm (6 inches) of fast flowing water can knock you off your feet and be enough for you to be unable to regain your footing.

- 60cm (2 ft) of standing water will float your car.

- Just 30cm (1ft) of flowing water could be enough to move the average family car.

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