5/2021 – Functional AND beautiful: retention pond delight

During the drought in 2017, the Cape Peninsula University of Technology (CPUT) Bellville Campus announced that all their existing green spaces were being irrigated with recycled water that is collected and pumped into an on-site lake. In August 2020 – to comply with City of Cape Town’s by-laws and regulations for storm-water management – this lake was extended and upgraded to increase the capacity of the existing retention pond as well as refurbish the existing outlet and inlet structures.

Expanding pond capacity and looks.


The capacity of this pond is now greater than ever and has been improved in terms of aesthetics and functionality

The capacity of this pond is now greater than ever and has been improved in terms of aesthetics and functionality

The capacity of this pond is now greater than ever and has been improved in terms of aesthetics and functionality, so that the campus can deal with flooding and drought alike. Says Douglas Curran, CPUT’s Chief Horticulturist: “It’s been nearly 30 years since the treated effluent water is used to irrigate the sport fields and their drainage system collects the water and sends it to the campus’ lake which pumps it back to the dam. The irrigation system ends up using the same water five times.”

What’s in a pond?


The existing pond was losing a lot of water, and that is why CPUT decided to build a 420m long retaining wall around its circumference

The existing pond was losing a lot of water, and that is why CPUT decided to build a 420m long retaining wall around its circumference

A retention pond (or wet pond) is a man-made reservoir – built in lower land areas that tend to accumulate excess water – that is designed to catch storm-water from higher elevation areas, to give that water a place to go, where it can be concentrated and contained.

This prevents erosion and foundational issues in the immediate surroundings, as well as reduce the risk of flooding. Retention ponds also aid in the removal of pollutants, such a fertilisers, oils, petroleum, and animal droppings from the storm-water runoff: the algae, bacteria, and other biological organisms in the water consume the pollutants, eventually purifying the water to a large extent.

Block lining to the rescue.


Constructing new storm-water inlets for the pond with Terraforce retaining blocks

Constructing new storm-water inlets for the pond with Terraforce retaining blocks

Westcoast Retaining Systems, Terraforce Recommended Contractor, Western Cape, was contracted by Ruwacon, a leading construction company in Southern Africa (51% black ownership and a 9GBPE rating from CIDB), to install the Terraforce L12 interlocking retaining blocks along the edges of the pond.

Says Hannes Mostert, Westcoast Retaining Systems: “The existing pond was losing a lot of water, and that is why CPUT decided to build a 420m long retaining wall around its circumference. The blocks were installed on a 750 x 300mm, 25 Mpa concrete foundation, with drainpipes running behind the entire length of the wall. All blocks were concrete filled for extra stability. In some places the wall had to be built higher to accommodate some big trees growing along the pond boundary, to protect their roots from too much water ingress.”

Project Scope:


• Demolishing the existing inlet and outlet structures.
• Earthworks & excavations to increase size & depth of retention pond. Terraforce structures to keep it in place.
• Installation of gabions and erosion protection.
• Constructing new storm-water inlets for the pond.
• Constructing a new outlet for the pond to control the storm-water outflow for different rainfall events.
• Reinstating grassed areas around the pond.

In some places the wall had to be built higher to accommodate some big trees growing along the pond boundaries, to protect their roots from too much water ingress.

In some places the wall had to be built higher to accommodate some big trees growing along the pond boundary, to protect their roots from too much water ingress

Terraforce L12 interlocking retaining blocks along the edges of the pond

Terraforce L12 interlocking retaining blocks along the edges of the pond

Constructing a new outlet for the pond to control the storm-water outflow from the pond for different rainfall events

Constructing a new outlet for the pond to control the storm-water outflow


Overcoming challenges.


Albert Botha, contracts manager, Ruwacon, says that it was difficult with rain, sludge, and ground water seepage to get the pond to the new design levels for installation: “The sludge within the pond and with the reducing of levels during construction made it difficult to install the concrete foundation upon which the Terraforce blocks were installed. We had to install a concrete stone pioneer layer before the concrete could be cast. The cast could also only be for short lengths. A pump had to be run a lot of the time during installation to keep the water at bay”.

Once these challenges were overcome, the verges of the pond were 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 can enjoy alike. On a functional level, leakage is curbed, and more water will be available for future water shortages.

Visually the pond now showcases a lush, green, and peaceful setting that students and animals can enjoy alike.

Visually the pond now showcases a green, and peaceful setting that students and animals can enjoy alike

Project Participants.


Client: Cape Peninsula University of Technology
Consultants: KFD Wilkinson Consulting Engineers
Main Contractor: Ruwacon
Sub-Contractor: Westcoast Retaining Systems
Terraforce Block Supplier: Klapmuts Concrete
Project Value: R6,400,000.00

 

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