Hanwell has a small river flood risk and a much bigger one once surface water is counted. Only a sliver of the town is in the river-and-sea flood zones. About one postcode in five rates high on at least one flood measure. The last recorded river flood was decades ago. The map most buyers check first is the one that tells them the least.
About 2% of Hanwell's postcodes are in the Environment Agency's flood zone 3, the high-probability band for rivers and the sea, and another 3% are in zone 2, the medium-probability fringe beyond it. The Environment Agency has three flood events on record here. The latest, in December 1977, came from the main river when its channel ran out of capacity, with no raised defences in place. Hanwell's name probably comes from a spring or stream, and the river zones still leave most of the town alone.
Surface water is the risk those zones leave out: rain that cannot drain away fast enough and collects in streets, gardens and low ground. It has no river to follow, so it can appear on roads nowhere near the Brent. In Hanwell it is the measure that changes the picture.
The Environment Agency's climate-adjusted flood zone covers about 6% of Hanwell's postcodes. That is the modelled river-and-sea zone as it is expected to look under a changing climate, so it describes what the zone may grow into and puts no date on any flood.
For insurance, the gap between the two measures matters. Premiums are priced from the property's own risk, and an address that sits outside the river zones can still carry a surface-water rating that an insurer will notice. Put the exact address into the Environment Agency's long-term flood risk service, which shows surface water separately from rivers, and get an indicative insurance quote while you are still deciding.
On the postcode flood indicators, which count a postcode if it carries a rating on any one of several measures, surface water included, about 19% of Hanwell's postcodes rate high, 21% medium and 45% low. A postcode only needs one of those measures to qualify, which is why these shares are so much larger than the river-zone figures above.
The Environment Agency's rivers-and-sea mapping counts 1,283 properties in the Hanwell postcodes that carry any risk at all. Of those, 18 are at high risk, 150 at medium, 409 at low and one at very low. Postcodes with no river or sea risk are left out of the count entirely, so that total is the at-risk end of town, not every home in it.
In the Environment Agency's modelled flood extents, about 1% of postcodes fall in the high-risk area without defences, and the share is the same with defences included. "Defended" means only that a defence appears in the model. It says nothing about whether a particular house stays dry, and here it does not shrink the high-risk footprint at all.
The nearest tide gauge is at Portsmouth, where sea level rose by about 2 millimetres a year over its record from 1962 to 2023. That is the rate measured at the gauge over those years. With the nearest gauge that far away, the sea comes low on Hanwell's list of flood worries.
Flood risk is a weak spot for Hanwell, and most of it is surface water, which the river maps do not show.