Yes, Hertford has a real flood risk, and it is concentrated along the rivers. Four rivers meet in the middle of town. About one postcode in sixteen is in the highest river flood zone. Roughly one in six is high risk once surface water is counted too. The river streets carry most of the risk, and the hill streets carry very little of it.
The Rib, the Beane and the Mimram all join the Lea at Hertford, and the town grew up around a crossing over that river. In the Environment Agency's river-and-sea flood zones, about 6% of Hertford's postcodes are in Flood Zone 3, the highest-probability band, and about 7% are in Flood Zone 2, the medium-probability band just outside it. The two bands don't overlap, so the second figure is extra land at the edge of the first one. The town is named after a ford, and a ford is where a river is shallow enough to cross and low enough to spread.
The river zones leave out rain that falls on roads and roofs and has nowhere to drain. That surface water only appears on the Environment Agency's wider postcode measures, and those are covered below.
The forward look is bigger. Once the Environment Agency adds its allowance for climate change to the river-and-sea zones, about 20% of Hertford's postcodes fall inside them. That is a modelled future, not a record of anything that has already happened.
Insurers rate individual addresses, so two houses on the same street can get very different quotes. A property in Flood Zone 3 near the Lea can cost much more to insure than one on the slope behind it, and some will be harder to insure at all. The Environment Agency's long-term flood risk service lets anyone enter an exact address, and its answer for that house is the one worth bringing into negotiations.
The postcode picture is wider again, because it counts a postcode as at risk if any one of several flood measures flags it, surface water included. On that basis about 16% of Hertford's postcodes are high risk on at least one measure, about 18% medium and about 38% low. About 5% are inside outlines of floods the Environment Agency has recorded in the past, and it has logged 75 flood events in the area. The most recent was in February 2009, when the main river overtopped its channel at a spot with no raised defences.
Counting buildings makes the risk more concrete. The Environment Agency's rivers-and-sea mapping lists 156 Hertford postcodes with some level of risk, covering 3,385 properties. Of those, 486 are at high risk, 399 medium, 897 low and eight very low, and 364 of the high-risk properties are homes. Postcodes with no river or sea risk don't appear in that list at all, so these counts describe only the part of town the rivers can reach.
The question of defences is the awkward one. About 6% of postcodes are inside the Environment Agency's high-probability flood extent whether the model includes defences or not, and the share is the same either way. "Defended" only means the model has a defence in it somewhere, and in Hertford's case including those defences doesn't shrink the high-risk area at all. A flood wall that doesn't change the map isn't protecting the house you're buying.
Flood risk is one of Hertford's weaker points. Most of the town is safe enough, but the riverside streets are a real risk, and buyers should check any home near the water carefully.