Watford's rivers are a small problem; its rain is a bigger one. Very few postcodes fall in the river flood zones. About one in seven rates high on at least one flood measure once surface water is counted. A few dozen homes carry a high rating from the rivers. The town was built to stay dry, and mostly it does, until the water arrives from above.
In the Environment Agency's river-and-sea flood zones, about 1% of Watford's postcodes fall in zone 2 and under half a percent in zone 3. The two are separate bands: zone 3 is the higher-probability core, and zone 2 is the medium-probability fringe beyond it, so the fringe being the bigger slice is normal. The town was put on the first dry ground above the marshy edges of the Colne, and one reading of the name itself is "full of water". The founders read the flood map before anyone drew one.
Surface water is where most of the remaining risk lives. Rain that can't drain off roofs and roads collects in low streets and underpasses wherever it likes, and it ignores the river zones entirely, which is why the postcode-level picture below comes out much larger than the zone shares.
With the Environment Agency's climate-change allowance added, the share of postcodes in the flood zones grows to about 2%. That is the agency's modelled look ahead, worked out on the river and sea map, and it stays small.
For insurance, the river picture is unlikely to trouble most Watford policies, though a property with a surface-water history or a high rating on any measure can expect more questions and a higher premium. Insurers remember 2003 even if the brochure doesn't. The useful check is the Environment Agency's long-term flood map for surface water on the exact street, done early, because a house can be dry by river and wet by rain on the same afternoon.
Counted by postcode, on at least one of several flood measures including surface water, about 15% of Watford's postcodes rate high, 17% medium and 34% low. Any one measure is enough to put a postcode in its band, so the high share is the one to take seriously, and it is a long way above the river-zone figures because it is counting more kinds of water.
In homes, the Environment Agency's rivers-and-sea mapping lists 1,399 properties in the 66 postcodes that carry any risk at all, with 62 at high risk, 19 at medium and 291 at low. Of the high-risk properties, 29 are homes. Postcodes with no river risk at all don't appear in that list, so those numbers describe the exposed corners of town, and the rest is simply absent.
Defences make only a small difference to the picture. About 0.4% of postcodes fall inside the high-probability extent with no defences modelled, against about 0.2% with them counted. A defence in the model means one is present on paper; whether it holds when the Colne is full is a separate question, and the agency's record shows 21 recorded floods in the area, the latest in January 2003, when a main river overtopped a channel with no raised defences at all.
The nearest tide gauge is at Portsmouth, where the sea has risen about 2 millimetres a year across the record from 1962 to 2023. That is the rate measured at the coast, a long way from a Hertfordshire river town, and it matters to Watford roughly as much as the Solent ferry timetable.
Flood is one of Watford's weaker points: the rivers threaten only a few dozen homes, but surface water puts a high rating on far too many postcodes to wave the question through.