HOW TO MAKE RUN-OFF STORM-WATER DISAPPEAR

02 February 2008

 

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Storm Water Control

APID urbanization is putting increasing strain on municipalities’ drainage systems, as run-off storm-water overloads reticulation and disposal systems, contaminating streams and rivers and inflicting environmental damage. Now, engineers and town planners have found an answer to the problem: Aquaflow, a permeable paving block drainage system which limits run-off and flooding by collecting and storing storm-water.

 

ONE of the best current examples in the construction industry where the ingenious permeable paving block has proved itself is the Kensington Boulevard shopping mall complex in Durban North, where excess run-off from the car park and building roofs is “harvested” and stored in a holding tank and gradually released into the municipal drainage system after the storm has passed.

 

The secret is in the design of the permeable Aqua Trojan concrete paving block which has slots which allow water from heavy downpours to flow into a compacted sub-base of stones, and then be gradually released into sewers or water-courses. Water leaving the Aquaflow system is cleaned naturally by filtration and microbial action.

 

Known as a Sustainable Urban Drainage System (SUDS) and originally developed and tested in the UK, the specially designed paving block – made under licence in South Africa by concrete paving specialists Concor Technicrete - has been specially designed for large catchment areas such as shopping centre car parks, pedestrian malls, housing complexes and industrial estates.

 

Increasing numbers of local authorities are insisting that engineers and developers make allowance for run-off from hard surfaces such as these, saysTaco Voogt, Technicrete commercial product manager. “With Aquaflow, water can be attenuated in the sub-base by using a choice of retention methods, including plastic or specialized geo-membrane liners, where the water can be held for a few days before being released at a measured rate, pumped out or harvested.”

 

A number of sites in the UK are harvesting and re-using the ‘grey’ water from the system for flushing toilets and irrigating landscape gardens, Voogt says. “And I’m told it has been found that the water from the Aquaflow system, is kinder to plants than tap water.”

 

During dry weather, heavy metals, hydro-carbons, silt, rubber dust and other detritus are deposited on impermeable surfaces, Taco Voogt says. “The problem is compounded when it rains, as these materials are scoured off and carried into municipal water systems and rivers where they can cause severe environmental harm. Aquaflow is one way of preventing these pollutants from entering the environment.”

 

Main contractor for the specialised paving operation – covering an area of about 480 m² of the Technicrete Aqua Trojan blocks - was Model Contracting. The system called for a bottom layer of compacted 63/10mm stone, on which was laid a mesh lining. Above that was a 100mm deep layer of 20/4mm compacted stone, and above that a geo-fabric liner, covered by a 50mm deep layer of compacted 6mm clean stone. The slotted paving blocks were laid on top.

 

Model Contracting MD Tony Bezuidenhout says almost as soon as the paving project was completed it was put to the test by exceptionally heavy and prolonged rain storms in the Durban area. “It’s working like a charm,” he says.

 

Wayne Jordaan, of Key Projects, the project managers, said the local municipality had imposed flow restrictions from the site into the municipal storm-water network and specified attenuated site storage. “However, there was very little area available on the site for a storage tank of sufficient capacity, but that problem was solved by the engineers - the sub-base beneath the paving blocks became a storage reservoir.”

 

He also reports on the success of the project and estimates that in a heavy prolonged rain at least 60% of run-off surface water is captured.

 

Storm Water Control

But the benefits of the Aquaflow system at Kensington Boulevard extend well beyond the parking lot: the sub-base reservoir also receives run-off from other parts of the site which enters through the side of the storage system and is distributed over the length of the reservoir via a perforated pipe.

Detailed environmental impact research went into the project. “Many former residential areas, such as this particular site, have been rezoned for commercial and industrial purposes, with the result that there is now much more storm-water run-off into municipal drainage systems,” says Alistair Avern-Taplin, a director of consulting engineers Arup.

 

“We liaised with eThekwini storm-water and coastal management from the beginning of the project. They warned that the site was historically within a risk area due to the sloping topography and dense residential pattern. Previous storms had resulted in major damage to properties in the area.

Hardening of gardens and expansion to properties were also listed as reasons for increased run–off.”

 

Avern-Taplin added that after exhaustive investigation, it was established that the only practical place to discharge the stored storm-water was into Kensington Drive, west of the site, and which is at the elevated end of the site. To achieve the correct falls – and due to space constraints – the 60m³ attenuation storm water reservoir was constructed as a reinforced concrete tank suspended over the basement parking. Storm-water from the roof reaches the tank through a pipe network which discharges into the tank, while run-off from roads and paved areas enters the tanks through the pavers.