Building a new bridge, preserving the old one

Building, upgrading, maintaining roads is what SANRAL does – but it is always very careful to take the environment into account, as was done with the construction of a new bridge over the Vaal River near Warrenton in the Northern Cape and the consequent realignment of the N18 to Mafikeng and Botswana.

Roads are usually seen as being negative for environmental biodiversity by fragmenting natural habitats. Increasingly, though, roads are recognised as potential and crucial biodiversity corridors, linking islands of bio-diverse habitats formed as a result of agricultural and urban development.

“A comprehensive botanical specialist study is part of the essential planning. Frequently, innovation is required, which may include a realignment of the planned road upgrade.

That is what we do everywhere we build or upgrade a road or construct a bridge. We believe in a sustainable road network where there is a balance between the needs of the road user and the environmental and socio-economic interests.” – Vusi Mona, SANRAL’s communications manager.

This bridge over the Vaal is a prime example of this approach.

The original low-level bridge was frequently under water during the rainy season, posing a hazard to vehicles, pedestrians and cyclists crossing and resulting in costly diversion of traffic. It was decided that a new bridge is needed, which resulted in the re-alignment of the N18 with pedestrian sidewalks.

Many factors had to be taken into account. Where the bridge is constructed is also an important spawning area for several fish species and SANRAL ensured that the habitat is not significantly transformed by construction activities.

Also, soil erosion along the river banks was a possibility and measures to minimise or prevent this were necessary. Support pillars for the bridge were designed so that the potential for floating debris to be snagged is minimised while energy dissipaters and rock-packed mattresses were used to minimise any possible erosion. Silt fences were used on the riverbanks for the same purpose.

Odd as it may sound, says Mona, the rocks that were removed to accommodate the construction work have been retained and replaced with reference to the natural features during the rehabilitation phase. The aquatic habitat has been returned as close to its original state as possible to conserve aquatic fauna and flora.

Unnecessary clearing of vegetation was avoided and fishing and hunting of local fauna strictly prohibited. Some Aloe grandidentata were found on-site, relocated and transplanted during rehabilitation so that the ecosystem remains much as it was – this was done despite the fact this specific aloe is not on the endangered list.

As always, exotic vegetation is removed on an on-going basis. But what is unusual about the Warrenton project is that the old bridge was not demolished. It was opened in 1931 and has thus been a feature for over 80 years.

It could only be demolished with approval of the Heritage Resources Agency but during the assessment the community insisted on retaining it. SANRAL has preserved this landmark bridge following talks with the municipality.

Working on a road means land is transformed, water resources are affected, substances emitted, waste generated, there is interaction with local communities – and vegetation may be affected.

“We at the roads agency are very aware of all of this and we work hard at trying to reduce the impact on the natural environment and serve the local community better by providing improved roads.”