Harpenden has a small, specific flood risk, and very little of it comes from rivers. Hardly any of the town is in a river flood zone. About one postcode in eleven is rated high on at least one flood measure. Rain running off the land does most of that work. The River Lea runs through Batford, but surface water is the bigger worry.
On the Environment Agency's river-and-sea flood zones, about 1% of Harpenden's postcodes fall in zone 2 and under 1% in zone 3. The zones are separate bands: zone 3 is the higher-probability land nearest the water, and zone 2 is the medium-probability fringe beyond it. The town straddles two valleys, the dry one in the south-west holding the town centre and Harpenden Common, and the Lea valley in the north-east holding Batford. The dry valley lives up to its name in river terms, at least.
Surface water is where Harpenden's risk shows up. The river zones barely register, but rain that falls faster than the drains and soil can take it has to go somewhere, and in a town built across two valleys it collects at the low points of roads and gardens. That kind of flooding ignores river maps entirely, so the Environment Agency's long-term flood-risk check for the exact address, with the surface-water layer switched on, is worth more than any town-wide share.
Allowing for climate change, the Environment Agency's modelled flood zone covers about 1% of Harpenden's postcodes, barely different from today's maps.
For insurance, a home outside the river zones and clear of surface-water flags should be routine to cover. A property on one of the flagged low spots may face more questions from insurers, and a past flood claim on the house will matter more than anything the town-wide figures say.
On this site's postcode indicators, which count a postcode if it is flagged on any one of several measures including surface water, about 9% of Harpenden's postcodes rate high, about 10% medium and about 27% low. A postcode only has to fail one test to be counted, which is why these shares sit so far above the river-zone figures.
The Environment Agency's rivers-and-sea mapping lists just 11 Harpenden postcodes with any risk at all, holding 219 properties. Of those, 13 are at high risk, none at medium, 40 at low and 19 at very low. None of the 13 high-risk properties is a home. Postcodes with no river or sea risk do not appear in that count at all.
Modelled river flood extents put under 1% of postcodes in the high-risk area without defences, and the same small share with defences included. A defence being present in the model means something is drawn on the map between the water and the buildings, and that is all it means. The Environment Agency has 11 recorded flood events for the area, the latest in May 1992, from a main river that overtopped its channel where there were no raised defences.
The nearest tide gauge to Harpenden is at Portsmouth, which has measured sea level rising at about 2 millimetres a year between 1962 and 2023. That is a long way from a Hertfordshire commuter town, and the sea is the one flood source Harpenden can safely leave off its list.
Harpenden's flood risk is middling: the rivers leave almost every home alone, and the surface-water flags on about one postcode in eleven mean the street matters more than the town.