Major works by PUB to improve flood protection in Orchard Road are about halfway done, the national water agency said yesterday.
The Stamford Diversion Canal and Stamford Detention Tank will reduce the load of the Stamford Canal - which flanks Orchard Road on both sides - by 30 per cent, a spokesman said.
The diversion canal, which will relieve Stamford Canal of a portion of water, is close to 50 per cent complete.
The detention tank, which will hold water temporarily so that less water flows into Stamford Canal during heavy rain, is more than 50 per cent complete.
The 2km diversion canal will divert rainwater from 240ha of the total 630ha catchment area into the Singapore River - through two underground tunnels and drains that are 6m to 14m wide, said Mr Ridzuan Ismail, director of PUB's catchment and waterways department.
He was speaking during a media briefing at the Environment Building in Scotts Road.
The construction of the diversion canal is expected to be completed by the first quarter of 2018. It will stretch from Tanglin Road to Grange Road and off Kim Seng Road.
For 1km of the stretch, works for two tunnels under Grange Road - measuring 4.5m in internal diameter - will start next month.
Stamford Canal, which stretches 4.7km under the Orchard Road shopping belt with malls such as Ion Orchard, Wisma Atria and Lucky Plaza, could not cope with heavy rain in several instances in 2010 and 2011, leading to floods in the area.
"Through these projects, the flood risk for the main Orchard Road area will be reduced, because we are diverting the flows from the upstream areas," Mr Ridzuan said.
He also pointed to data covering 35 years that shows that rainfall has become more intense, and heavy rainfall more frequent.
The detention tank, the second in Singapore after another in Opera Estate, can store as much water as 15 Olympic-size pools, or 38,000 cubic metres. It will sit 28m under the Singapore Botanic Gardens coach carpark.
Water flowing towards Stamford Canal from Holland Road will overflow from a drain along the road into a chamber where two pipes - measuring 2.5m in diameter internally - will channel the water into the tank.
Water sensors will alert the automated system to release the collected water when rainfall has subsided. The water will flow both ways, pulled by gravity.
Minister for the Environment and Water Resources Masagos Zulkifli visited the tank's construction site yesterday.
"The Stamford Detention Tank is both to help out and mitigate possible flooding in the Orchard Road area and, more importantly, in the longer term, we hope that it will also help to mitigate the variable weather pattern we are going to see," he told reporters.
The tank is expected to be ready by the first quarter of next year.
He also added that by law, developers are now required to include detention tanks in buildings with land sizes over 0.2ha, and that more than 30 buildings are now thus equipped.
PUB attributed a delay in the completion of the detention tank to hard rock that needed time to break down. As for the delay in completion of the diversion canal, a spokesman said: "We did a detailed investigation of the services, for example, cables and pipes alignment, to ensure that the tunnelling depth will not affect the services above."
Giving an update on the drainage improvement programme, Mr Ridzuan said that projects at 256 locations have been completed since 2013, projects at 92 locations are ongoing and those at 24 more are planned to start this year.