Flash floods in Singapore

Two flavours

• Convective flash flood: a stalled thunderstorm dumps 40-80 mm in less than an hour on a drain that cannot take the peak. Common in the inter-monsoon.
• Monsoon-surge flood: hours of widespread rain saturate the island. December 2006 is the textbook case.

Why a rich city still floods

Concrete and canalised drains move water fast, which is good until the peak rainfall rate exceeds the design. Underpasses, low points of arterial roads, and older estates sit in those peaks. Drainage upgrades have cut the number of chronic spots, but they cannot erase a 512 mm day or a 70 mm/hour cell.

If water is on the road

• Turn around. Depth is not obvious, and a kerb or open drain can be under the surface.
• Do not walk through fast-moving water to "see if it is shallow".
• Once home, PUB and town councils handle debris and pumps. Report a burst or overflowing drain through the official channels, not only social media.

Questions