A BLOCKED culvert - a drain that diverts water - caused the flood that submerged Orchard Road in knee-high water two days ago, the PUB said yesterday.
The culvert, which is 2.7m in width and depth, diverts water from drains in Nassim Road and Cuscaden Road into two sections of Stamford Canal, which runs along Orchard Road.
But Wednesday's storm swept so much debris - such as leaves, branches and litter - into the culvert that part of it became clogged.
As a result, water could not flow into Stamford Canal's smaller section, which stretches from Delfi Orchard to Lucky Plaza.
Instead, it flowed into the canal's other section stretching from Orchard Parade Hotel to Liat Towers and Wisma Atria. It was unable to handle the full load despite being larger.
Water surged out from this channel onto the junction of Orchard Road and Scotts Road - the worst-hit area - and disrupted traffic and business.
Mr Yap Kheng Guan, PUB's director of 3P networks, said yesterday: "Capacity wasn't a factor at all, as there were similar storms in the past and the canal handled such storms."
About 100mm of rain fell in the central parts of Singapore between 8am and 11am on Wednesday. This was more than 60 per cent of the average monthly rainfall for June.
Mr Yap said that the same amount of rainfall was recorded in central Singapore during downpours in January 2008 and November last year.
This time, however, flooding occurred because of the pattern of rainfall: An intense amount of rain in short bursts at about 9.30am and at 10.30am.
The first burst of rain washed the debris into the culvert, but there was insufficient rain in the ensuing hour to flush it out.
The next burst of rain then washed more debris into the culvert, aggravating the initial blockage.
When PUB officers combed the entire canal to clear debris from the drains, starting from Wednesday afternoon after the flood subsided, they found a third of the culvert clogged at 10pm.
To tackle the problem, the PUB will install additional debris traps in the Stamford Canal, as well as in upstream drains leading to it.
Its officers will also inspect the canal more frequently - monthly, instead of quarterly.
In the future, it plans to install more sensors in the canal, on top of the existing two sensors, to better monitor the water level in it.
The PUB is still investigating why two other areas - Bukit Timah and Veerasamy Road - became flooded.
Yesterday, Orchard Road was almost back to normal. National Environment Agency (NEA) officers had cleaned affected pedestrian underpasses by 10pm on Wednesday.
It was business as usual for most shopping malls and hotels in the area. These included Wheelock Place, Wisma Atria, ION Orchard, Shaw Centre, Tangs, and Marriott Hotel, which were not damaged despite being near the junction of Orchard and Scotts roads.
Staff of McDonald's at Lucky Plaza cleaned up the outlet overnight, and business also resumed yesterday, following a check by NEA officers.
PUB officers and contractors finished pumping water out of the basements of Lucky Plaza, Liat Towers, Tong Building and Delfi Orchard at 3.30pm yesterday.
However, shops in the Liat Towers basement, which was almost submerged by flood waters yesterday, were still cleaning up the mess left by the flood. Fast-food restaurant Wendy's threeday- old outlet there will be closed for the next four to six weeks, so that costs can be calculated and damaged repaired.
Kopitiam Group, which runs the chain here, said the flood damaged equipment and infrastructure worth more than $500,000, on top of perishable items.
Its managing director, Mr Alden Tan, said the outlet's staff would be deployed to other outlets in Lau Pa Sat and Jurong Point, which will open round the clock from today to July 12.
Starbucks Coffee Singapore said it would probably remodel its Liat Towers store, as the damage was extensive.
Miss Madeleine Ho, the marketing director for retailer Hermes Singapore, said: "While our products escaped unscathed, the walls, window decorations, furniture and furnishings were badly damaged. Restoration will take some time, as a substantial portion of the fittings have to be imported."
Another retailer, Massimo Dutti, said it was cleaning up its store and assessing the damage.
All three stores were unable to say when they would reopen.