Barnes is at real risk of flooding on paper, because it was built inside a loop of the Thames. Eight in ten postcodes are in the highest river flood zone. Nearly all of them fall inside the zone once climate change is allowed for. Very few individual homes are rated at high risk, because the river has been walled in. The danger here comes down to the defences and how long they keep working.
The Environment Agency's river-and-sea mapping puts about 81% of Barnes postcodes in flood zone 3, the highest-probability band. Another 8% are in flood zone 2, the medium-probability fringe beyond it. The two bands don't overlap, so together they cover almost the whole district. Barnes is centred in a bend of the River Thames, and the Championship Course for the Boat Race follows that loop for a reason: the water goes three quarters of the way round. A place the river wraps itself around was always going to be mostly zone 3.
Look ahead and the map gets worse. The Environment Agency's climate-adjusted zones take in about 90% of Barnes postcodes. That's a modelled view of a future river, built on assumptions about how much higher the water will get, but it pushes the same way as the present-day map.
An address in zone 3 is something insurers will ask about, and premiums and excesses can reflect it even where the house has never had a drop inside. Get an indicative buildings-insurance quote for the exact address early, and look up its own entry on the Environment Agency's long-term flood risk checker. That one check tells you more than any number on this page.
The postcode-level picture folds in surface water from heavy rain alongside rivers and sea, and it counts a postcode at its worst rating on any of those measures. On that basis about 21% of Barnes postcodes are rated high on at least one, 26% medium and 53% low. Because this mixes several kinds of risk, it measures something different from the zones above, and the two sets of numbers shouldn't be added together. Rain doesn't care where the river walls are.
Counted by property, the river looks much smaller. The Environment Agency's rivers-and-sea risk assessment lists 5,841 properties in Barnes, and only postcodes with some level of risk appear in it at all. Of those, 3 are rated high, none medium, 450 low and 3,872 very low. Two of the three high-risk properties are homes. A district that's mostly zone 3 on one map has three properties at high risk on the next, and the gap between the two is the defences.
That gap shows up in the modelled flood extents. About 11% of Barnes postcodes are inside the high-risk extent when the river is modelled without defences, against about 2% once defences are included. "Defended" only means the model has a defence in place. It says nothing about whether that defence will hold in a given storm, or what happens if it's breached or overtopped. The defence keeps the water back, and nobody has promised for how long.
The nearest tide gauge in the national record is at Portsmouth, where sea level rose about 2 millimetres a year between 1962 and 2023. That's the rate measured on the south coast over those years. Barnes sits a long way inland on a tidal river, and the gauge describes a different stretch of water.
The Environment Agency's recorded historic flood outlines don't cover any of Barnes. That's genuinely reassuring as far as it goes: no flood big enough to be mapped has been put on record here. It doesn't prove the streets have always stayed dry, and gaps in a record are a different thing from proof.
Flood risk is where Barnes does worst. The river surrounds it on three sides, and the defences carry most of the weight.