Letchworth does have a real flood risk, and almost all of it comes from rain, with rivers playing almost no part. About a third of its postcodes rate high on at least one flood measure. The river-and-sea flood zones cover almost none of the town. Nothing has been recorded flooding. The risk here comes from rainwater that the drains can't carry away fast enough.
On the Environment Agency's river-and-sea mapping, under 1% of Letchworth's postcodes fall in flood zone 3, the highest-probability band, and under 1% fall in flood zone 2, the medium-probability fringe outside it. The two bands don't overlap, so together they still cover only a sliver of the town. Those zones are about rivers and the sea, and they measure a different thing from the surface-water indicators further down. If you are judging Letchworth on the flood zones alone, it looks almost entirely dry.
Surface water is where the risk is. When heavy rain falls faster than the ground and the drains can take it, it pools on streets and in low gardens, with no river involved. Raymond Unwin set out Broadway from three old oaks on the town's central plateau, and the river maps back his choice of high ground. The rainfall maps are less kind.
The Environment Agency's climate-adjusted flood zones, which project how the river-and-sea extents could spread, still take in less than 1% of Letchworth's postcodes. The town's flood risk in future is mostly the same question as today's: what happens when heavy rain falls on hard surfaces.
For insurance, a home outside the river-and-sea zones can still pick up a surface-water rating that an insurer will look at. Premiums can differ between neighbouring streets depending on where water collects. The useful check is to put the exact address into the Environment Agency's long-term flood-risk service and read the surface-water result separately from the river result. Then get an indicative insurance quote on that address while the sale is still at an early stage.
Across Letchworth's postcodes, about 34% rate high on at least one of several flood measures once surface water is counted, about 31% rate medium on at least one, and about 53% rate low on at least one. A postcode can appear in more than one of those groups, because each counts any of several measures.
Counting properties is a sharper test. The Environment Agency's rivers-and-sea mapping lists 213 properties in Letchworth, spread across the 10 postcodes that carry any river-or-sea risk at all. Of those, 23 are at high risk, 2 at medium, 17 at low and none at very low. Twenty-one of the 23 high-risk properties are homes. Every other postcode in town is left off that list, because it carries no river-or-sea risk to count.
Under 1% of postcodes fall inside the Environment Agency's modelled high-risk extent without a defence, and a similarly small share fall inside it with one. "Defended" only means the model includes a defence. It says nothing about whether that defence keeps the water out of a particular house. In a town where the main problem is surface water, a river defence has very little to do with the question anyway.
The nearest tide gauge to Letchworth is at Portsmouth, where sea level rose by about 2 millimetres a year between 1962 and 2023. That is the rate measured on the south coast over those years. A town this far inland has its nearest tide gauge in Portsmouth, which shows how small a part the sea plays in Letchworth's flood picture.
There is no recorded historic flooding in Letchworth. That is reassuring, but the record only includes floods that were logged and mapped, and it says nothing about the ones that weren't. The weather the town's history does mention is the tornado of November 1981.
Letchworth's flood risk is poor, and nearly all of it comes from rain landing on the town. The rivers are a minor part of it.