Retention pond upgraded with Terraforce blocks

Retention pond upgraded with

TERRAFORCE BLOCKS

During the drought of 2017, the Cape Peninsula University of Technology (CPUT) Bellville campus began irrigating its green spaces with recycled water from an on-site attenuation pond.

The pond was subsequently found to be losing water and, in compliance with the City of Cape Town’s by-laws for stormwater management, it was upgraded and extended in August 2020.

Westcoast Retaining Systems, a Terraforce-recommended contractor, was appointed by main contractor Ruwacon to build a 420m retaining wall around the pond’s circumference using the Terraforce L12 interlocking retaining wall blocks.

The project was begun by demolishing the existing inlet and outlet structures and the size and depth of the pond were extended. In addition to Terraforce retaining structures, gabions were built for erosion protection. New stormwater inlets were installed and a new outlet was built to control stormwater outflows.

Hannes Mostert of Westcoast Retaining Systems says the blocks were installed on a 750 x 300mm, 25MPa concrete foundation, with drainpipes running behind the entire length of the wall.

“All the blocks were concrete-filled for extra stability and in some places, the wall had to be built higher to protect some tree roots on the edge of the pond,” he says.

With an enlarged capacity, the pond is now better equipped to deal with flooding and droughts.

(Above and right): The revamped attenuation pond lined with Terraforce L12 interlocking retaining wall blocks.

(Below): The new outlet.

“With an enlarged capacity, the pond is now better equipped to deal with flooding and droughts.”

Albert Botha, Ruwacon contracts manager, says rain, sludge and groundwater seepage were some of the challenges during construction.

“The sludge made it difficult to install the concrete foundation of the retaining wall and we had to install a concrete stone pioneer layer before the foundation could be poured, a process which had to take place over several short durations. What’s more, we had to pump out water during the casting,” says Botha.

Once these challenges were overcome, the verges of the pond were re-grassed and a family of ducks introduced to the area. Visually, the pond now showcases a lush, green and peaceful setting that students and animals alike can enjoy. On a functional level, leakage has been curbed and more water will be available during future water shortages.

Attentuation ponds (or wet ponds) are man-made reservoirs built in low-lying areas that control stormwater flows and prevent flooding. They trap stormwater from higher elevations and release it in a controlled manner either into stormwater drainage, or – as in this case – as “grey” water into green spaces for irrigation purposes.

They also prevent erosion and foundational issues in the immediate surroundings and aid in the removal of pollutants such as fertilisers, oils, petroleum and animal droppings. Algae, bacteria and other biological organisms consume the pollutants, largely helping to purify the water.

Project participants

Consultants: KFD Wilkinson Consulting Engineers

Terraforce block supplier:

Klapmuts Concrete Project value: R6 400 000