Footpaths between the Olympic stadium and Stratford's two stations, plus paved areas around the Olympic facilities, would cover vast, shallow stormwater storage tanks designed to pick up rainwater running off the stadium and its paved approaches. Paved areas leading to the sports venues will be porous, allowing water to drain into these holding tanks, reducing the risk of flooding, and making water available for reuse by cleaners and in toilet cisterns.
The idea for these tanks or attenuating ponds developed as it became clear that existing stormwater drainage infrastructure would be unable to cope with increased runoff from the Olympic venues and surrounding paved areas.
At the moment the Olympic site's mix of overgrown waste dumps and railway sidings can hold back some stormwater, limiting the burden on Thames Water's sewers. Construction of Olympic venues and vast associated paved areas would reduce water held in the ground and increase the amount of run off, increasing flood risk as a result.
Stormwater from the area is currently pumped into an interceptor sewer comprising five 2.7m diameter pipes buried in an embankment known as the Greenway, which crosses the southern end of the Olympic Park site. This feeds into Thames Water's Beckton sewage treatment works where it is treated and then discharged into the Thames. But the interceptor sewer also handles a large chunk of east London's foul water and it is calculated that during a storm the combination of foul and stormwater would overload the Beckton works.
'In early discussions with Thames and the Environment Agency it was agreed that if we take out the surface water and separate the foul and surface water systems, the foul water capacity will be enough, ' says Buro Happold civil engineer Vanessa Quirk.
This resulted in the decision to create specially built attenuating ponds underneath the main concourses to hold back stormwater, filtering it through granular fill and microbe treatment sytems before it is discharged into the River Lea.
The ponds would be relatively shallow - just 500mm - but vast in size since they mirror the footprint of the paved areas.
These areas would be surfaced with porous asphalt or paving slabs to help drainage and prevent surface flooding.
'The key to the drainage system design is dealing with surface water, ' says Quirk.
As a result her team has incorporated plans to hold back run off in rainwater harvesting systems, and through the use of turfed or 'green' roofs.
Landscaping work around the River Lea will also include sustainable urban drainage systems (SUDS) which use vegetation to hold back run-off water.
After the Games it is planned to redistribute the attenuation ponds around the banks of the Lea. Lower concentrations of people would be expected to visit the area after the event.
As a result much of the paving and the attenuation ponds beneath would be ripped up.