The South African National Roads Agency Limited (SANRAL) has sent out a team of engineers to urgently inspect its network in Gauteng after the recent floods.
In addition, the agency will appoint independent, reputable registered professional engineers with extensive experience in floods and storm water systems to investigate the extreme flooding, and to advise on any remedial actions.
SANRAL also expressed its condolences to the families of those who lost their lives during the flash floods.
SANRAL’s assistance units – on road services – worked closely with emergency services from the metropiltans and municipalities to help stranded motorists, those needing medical care and evacuation.
Alex van Niekerk, project manager at SANRAL, said: “Given the severe storm warning for today by the weather services, we caution road users to take care on the roads.”
The road network and flood resistance
SANRAL engineers conducting the first infrastructure survey will look for obvious signs of damage to roads and bridges to make sure they are safe for use.
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 varying international norms.
The rainfall on 9 November 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 90mm and 150mm rain falling in a very short period.
Frequent 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 topped over. 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 sink holes opening up. These are closely monitored by SANRAL’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 blocking of storm water pipes, culverts and bridges by homeless people is also becoming a problem. 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 Van Niekerk.
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 (not on the road network) 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,” explains Van Niekerk.