Hemel Hempstead's flood risk is modest from the rivers and real from the rain. The town grew up where two chalk streams meet, and hardly any of it falls in the river flood zones. Rainwater running off the roads and slopes is a different matter. Roughly one postcode in seven is rated high risk on at least one measure. Most of the risk sits on the hillside streets, well away from the riverbank.
On the Environment Agency's river-and-sea flood zones, about 1% of Hemel's postcodes are in Flood Zone 3, the highest-probability band, and about 1% more are in Flood Zone 2, the medium-probability fringe beyond it. The Gade and the Bulbourne run through the valley floor at Boxmoor and Two Waters, with the Grand Union Canal alongside, and the post-war town was built up the valley sides and onto the plateau above. The new town's builders put most of the houses uphill, which happens to have been sound flood planning.
Surface water is what those zones leave out. Rain that can't drain away fast enough collects in dips, underpasses and the bottoms of slopes, and that can happen a long way from any river. The Environment Agency maps that risk separately, and it does most of the work in the postcode figures further down.
Allowing for climate change, the Environment Agency's projected flood zone takes in about 3% of postcodes. It is the agency's planning allowance for heavier future rainfall and higher river flows. It describes a possible future, and it shows the river zones growing a little, not dramatically.
For insurance, a home outside the river zones usually gets a straightforward quote, and homes in Hemel's high-risk surface-water spots are where questions about past claims and premiums come up. An indicative insurance quote on the specific address is the sensible check. It takes minutes, and the insurer will be reading the same Environment Agency maps.
Counting every measure together, including surface water, about 14% of Hemel's postcodes rate high risk on at least one indicator, another 14% medium and about 35% low. A postcode only needs one measure to flag it, so these shares cover far more of the town than the river zones do. The river zones and these postcode shares measure different things.
The Environment Agency's rivers-and-sea mapping lists only the 54 postcodes that carry some river risk, and they contain 1,365 properties. Of those, 43 are at high risk, 30 of them homes, five at medium risk, 189 at low risk and 655 at very low risk. A town of this size with 43 properties at high risk from its rivers has a small river problem.
About 1% of postcodes fall inside the high-risk river flood extent without any modelled defence, and none fall inside it with one. In the Environment Agency's model nothing is defending the handful of exposed spots. A modelled defence would only have meant a wall or bank appears in the model anyway, which is a long way from saying a house stays dry.
The nearest tide gauge is at Portsmouth, where sea level rose about 2 millimetres a year between 1962 and 2023. That is the rate measured at the gauge over those years, and Portsmouth is the nearest gauge, which says plenty about how far Hemel is from the sea.
No flooding is on record for Hemel. That is reassuring, but the record only lists events somebody logged, so it can't prove nothing ever happened. It does mean the town has no flood history for a buyer to inherit.
Hemel's flood risk is middling. The rivers are a minor worry, and on the wrong slope, surface water is the one that matters.