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Joburg flash floods down to various causes – authorities

Following heavy rains that led to flashflooding on key roads around Johannesburg on Wednesday afternoon, the South African National Roads Agency Limited(Sanral) has assembled a team of engineers to inspect its Gauteng network for damage.

Sanral will also appoint engineers to investigate the extreme flooding and to advise on any remedial actions.

The Johannesburg Road Agency (JRA) noted that while it was also assessing the damage, the Witkoppen road and the Linksfield road off- and on-ramps to the N3 had suffered significant damage.

The flood at Linksfield had also resulted in the loss of lives.

Further, the JRA said the R55 bridge in Kyalami collapsed, the John Nhlanhla bridge in Alexandra and the Buccleuch bridge washed away, and all low-lying bridges between Kyalami and Dainfern flooded.

It added that a wall had collapsed in Houghton, and another at the Johannesburg Zoo, while Siyabusa, near Midrand had flooded owing to insufficient stormwater capacity.

Several domestic and international flights had also been delayed, owing to the OR Tambo International Airport’s causeways and parking lots being flooded.

Addressing concerns that the city’s highways were inadequately designed and, therefore, unable to handle the large volumes of water, Sanral bridge network manager Edwin Kruger said the freeways would have, in the past, been designed to pass a one-in-twenty-year flood or up to a one-in-fifty-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 designrecurrence period is 20 years. “The design standards used in South Africa are very similar to the varying international norms,” he noted.

Further, Kruger said that Wednesday’s rainfall was so extreme and was a “once-in-a-hundred-year occurrence”.

Unconfirmed reports indicated that the storm had unleashed between 90 mm and 150 mm rain falling in the matter of an hour.

Meanwhile, Sanral said the blocking of stormwater pipes, culverts and bridges by homeless people was also increasingly becoming a problem. Although they are removed on a regular basis, the illegal structures blocking the stormwater systems are often reconstructed within a day or two of them being removed.

“An example, very applicable to the area affected by the flashfloods, 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 Sanral project manager Alex van Niekerk.

Further, Sanral stated that the Gauteng freeway network was built 40 years ago, when most of the surrounding area was agricultural land. However, with the landscape changing to developed land, it also meant the soil’s water absorption capacity was not available.

In addition, when drainage systems in built-up areas fail, the run-off will increase and will build up in low-lying areas such as Gillooly’s.

“Given the ongoing severe storm warning by the weather services, we caution road users to take care on the roads,” said Van Niekerk.

Sanral expressed its condolences to the families of those who lost their lives during the flash floods.

JRA added that the heavy rainfall was also beyond the capacity of its stormwater systems.

JRA MD Dr Sean Phillips said the agency had duly inspected recurring blockages in the areas termed flooding hotspots and it was monitoring low-level bridges prone to overtopping during heavy downpour.

He emphasised that maintenance of stormwater drains was being carried out by JRA on a regular basis. Repairs to damaged stormwater drains and blocked stormwater drains are also prioritised.


Source: Engineering News