Hanoi floods fast in the rainy season: drains back up, and streets can hold twenty centimetres of water within minutes. Water is one of the few road conditions where the correct decision is often not to ride at all — and where the wrong decision can destroy an engine.
Judge before you enter
Never ride into water you cannot see the bottom of. Watch a car or a local rider cross first: how deep is it for them? If water reaches a car’s door line, do not attempt it on a bike. Where the surface is visible — mid-calf depth at most on a scooter — a careful crossing is possible.
How to cross
Enter slowly, keep a steady low speed, and do not rev hard — a scooter’s air intake sits low, and one gulp of water can hydrolock the engine. Avoid the deepest line at the curb; the crown of the road is usually shallowest. Feet up: flood water hides wire, glass and open manholes. If you must put a foot down, expect it to disappear into a hole.
What water hides
Potholes (potholes-bad-roads-motorbike-vietnam), missing manhole covers, and live electrical faults near fallen cables. After crossing, dry your brakes by dragging them gently for a few seconds — wet brakes do nothing until they dry.
When to stop and wait
Strong current, hidden surface, or water over the exhaust outlet: park on the high side of the street and wait. Floods in Hanoi usually drain within hours, and an engine rebuild costs more than any wait. If you are on a rental, a drowned engine is a real problem — outside-repair-shops-rental-motorbike and your rental agreement cover that territory.
The one-line version
If you cannot see the bottom, do not go in; if you can, crawl through on the shallow crown — and never gamble an engine against a schedule.
Electric two-wheeler riders have extra water considerations (IP ratings, wet connectors) — see riding-electric-motorbike-hanoi-rain.