When UD Trucks, Kraaifontein, South Africa, needed a solution for heavy storm water flow in an area with a very high water table in summer and winter, an attenuation pond was proposed, lined with 2158 Terraforce L11 blocks to deal with the many pressures working in and around the pond.
What are attenuation ponds?
Attenuation ponds are a form of sustainable urban drainage systems (SuDS) commonly found around new construction projects, such as residential and commercial property developments, new roadways, etc.
Basically, attenuation means to temporarily store storm water for a while, to then release back into a watercourse or main storm water network. The storm water is collected and routed into the storm water system the normal way, but with the use of flow controls; this allows a controlled volume to flow through into the main system.
Taking off the pressure at UD Trucks
Terraforce L11 blocks – concrete filled – 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 1.2m behind the Terraforce retaining blocks. Any water generated by seepage is taken away immediately by an additional agricultural drain behind the blocks.
Base layer design
To finish off the base of the pond, grass blocks are to be placed on top of a 250mm thick drainage layer consisting of a 35% sand, 35% subbase mix, topped off with 30% clay grit, a special mix being tested as a solid, but porous base for attenuation ponds.
Permeable storm water management, Engen
At another site, the Engen Mbekweni Convenience Centre, it was necessary to manage storm water from the different elevations into the main catchment pond before being discharged into the main storm water system. The Terraforce L12 was chosen for its weight per square meter and the Terracrete permeable paver was chosen for permeability.
A single drainage system (pipe, biddem and sand layer) is installed behind the 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 storm water at its peak. This reduces flow enough to allow it to seep through the large holes of the Terracrete pavers, that are soil filled to encourage plant growth, slowing down the water even more.