& DRAINAGE Sustainable attenuation pond designs in precast concrete

When UD Trucks in Kraaifontein, Western Cape, needed a solution for heavy stormwater flow in an area with a very high water table – during both summer and winter – an attenuation pond was proposed.

The system developed for this site employs 2 158 Terraforce L11 hollow core concrete retaining blocks purpose-designed for landscaping and erosion control, countering the many pressures working in and around the pond.

A sustainable urban drainage system (SUDS), attenuation ponds are commonly found around new construction projects, such as residential and commercial property developments, as well as new roadways.

“Essentially, the principle is to temporarily store stormwater for phased release back into a watercourse or main stormwater network. The stormwater is collected and routed into the stormwater system the normal way, but with the use of flow controls,” says Holger Rust, founder and head of Terraforce.

Design layout

Terraforce L11 blocks – concrete-filled for this application – were chosen for their weight per square metre to help deal with the surcharge of the parking area, as well as any internal or external water pressure on the pond walls. The pond was also installed with an impervious plastic lining of 500 micron that comes up 1.2 m behind the Terraforce retaining wall. Any water generated by

Engen Mbekweni Convenience Centre

Project Team

Main contractor:

Isipami Construction

Civils contractor:

Neil Zaaiman Civils

Subcontractor:

LFC Construction seepage is taken away immediately by an additional agricultural drain behind the blocks.

To finish off the base of the pond, grass blocks were placed on top of a 250 mm thick drainage layer consisting of a 35% sand and 35% sub-base mix, topped off with 30% clay grit. This special mix provides a solid, but porous base for the pond.

Permeable solution for Engen

Demonstrating the flexibility of Terraforce’s systems is another recently completed SUDS project for the Engen Mbekweni Convenience Centre in Wellington, Western Cape. For this project, Terraforce L12 blocks and Terracrete permeable pavers were used to create a naturally draining pond.

A single drainage system (pipe, bidim and sand layer) was installed behind the Terraforce retaining wall to catch water ingress and channel it into the main catchment. Gabions were used as stub walls facing the inlets to slow down the flow of stormwater at its peak. This reduces flow sufficiently to allow it to seep through the large holes of the Terracrete pavers, which are soil-filled to encourage plant growth, further contributing to a reduction in water velocity.

“These projects showcase the effectiveness of environmentally engineered systems to form a diverse range of stormwater interventions. They are vital for combatting erosion, as well as for flood mitigation,” Rust concludes.

Project Team

Consulting engineer: Consulting Engineers

Architect: FDA Architects

Main contractor:

Longworth & Faul

Civils contractor: Neil Zaaiman Civils

Subcontractor:

LFC Construction