Yes, Hitchin carries a real flood risk, and most of it falls from the sky. The river itself barely registers on the official maps. Close to a third of postcodes rate high on at least one measure once rain is counted. The modelled defences make almost no difference to the high-risk picture. The water to watch is the kind that never needed the Hiz.
The River Hiz runs through the middle of town, past the east end of St Mary's, and on the Environment Agency's river-and-sea mapping it causes very little trouble. About 1% of Hitchin's postcodes sit in flood zone 3, the highest-probability band, and a further 2% fall in flood zone 2, the medium-probability fringe beyond it. The Hiz is a small river, and the maps treat it as one.
Surface water is a different matter. Heavy rain that cannot drain away fast enough pools on roads and in low gardens, and it pays no attention to where the river is. The river-and-sea zones leave it out entirely, which is why a house can sit well outside any flood zone and still be on a street that floods in a cloudburst.
Add the Environment Agency's climate allowance to the river-and-sea zones and the share of postcodes inside them rises to about 6%. That is the Agency's look ahead at a wetter future, modelled on the zones as they are drawn today.
For insurance, the gap between the two measures is the one that matters. An insurer pricing a Hitchin policy will look at surface water as well as the river, so a home outside the flood zones can still come back with a higher premium or an excess that reflects the street. Put the exact address into the Environment Agency's long-term flood risk service and read the surface water result alongside the river one, since that is the half the zone maps never mention.
Count every measure together and the picture changes. On the Environment Agency's postcode-level indicators, which flag a postcode if it rates on any one of several measures including surface water, about 31% of Hitchin's postcodes are at high risk, 30% at medium and 48% at low. A postcode only has to fail one test to count, so these shares say how widely the risk is spread across the town, and the river-and-sea zones say how much of it comes from the Hiz.
Counted as buildings, the river-and-sea mapping lists 1,845 properties across the postcodes that carry any risk at all, with postcodes that carry none left off. Of those properties, 104 are at high risk, 118 at medium and 401 at low, and 88 of the high-risk ones are homes. Postcodes with no river risk do not appear in that list at all, so these are the counts for the streets the Agency already has its eye on.
Defences do not change much here. About 1% of postcodes fall inside the high-risk flood extent with no defences modelled, and about 1% with defences included, while about 3% sit inside the low-risk extent without them. "Defended" only means a defence appears in the Agency's model. It says nothing about whether that defence will hold, and in Hitchin the model barely moves either way.
The nearest tide gauge to Hitchin is at Portsmouth, where sea level rose by about 2 millimetres a year across its record from 1962 to 2023. The gauge measures the south coast, and its distance from Hitchin is a fair indication of how little the sea has to do with the town's flood risk.
Flood risk is the poorest part of the Hitchin picture, because the problem is rainwater on ordinary streets, and the river maps most buyers check were never built to show it.