Arlesey's flood problem is the kind of flooding a river-and-sea flood zone map leaves out. Hardly any postcodes fall in the zones for flooding from rivers or the sea. Roughly one in six rate high on at least one flood measure once rain running off the ground is counted. No flood has been recorded. Rain is the thing buyers here should ask about, more than the river.
About 3% of Arlesey's postcodes are in the Environment Agency's river-and-sea flood zone 2, and none are in zone 3. Zone 3 is the higher-probability band and zone 2 is the medium-probability band beyond it, so the two shares do not overlap. A seller can truthfully describe almost any house in town as outside the high-risk flood zone. That statement covers rivers and the sea and nothing else.
The river-and-sea zones leave out surface water entirely. That is water from heavy rain that cannot drain away fast enough and collects in low ground, in gardens and at the bottom of driveways. About 4% of the town's postcodes fall inside the Bedfordshire and River Ivel Internal Drainage Board's district, where the board manages ditches and water levels. One archaeological reading has the medieval manor at Etonbury starting life as an inland harbour on the River Hiz. Today the Hiz threatens very few houses, and the risk comes from rain.
The Environment Agency's climate-adjusted zone, its estimate of how far zone 2 could spread as rainfall and river flows change, covers about 3% of postcodes. That is the same share as today, so climate change makes no visible difference to the river picture here.
Insurers price each address separately, and many now use surface-water mapping as well as the river-and-sea zones. A house can be outside every river zone and still get a higher premium if its postcode floods in heavy rain. The useful check is to get an insurance quote for the exact address early on, since the premium reflects the insurer's view of the water risk.
The postcode-level measure combines several flood indicators, including surface water. On that measure, about 16% of Arlesey's postcodes rate high on at least one of them, about 26% medium and about 52% low. A postcode counts if any single measure flags it, which is why these shares are much larger than the river-zone shares above. The two measures answer different questions.
Counting homes is more useful than counting postcodes. Only nine of Arlesey's postcodes carry any river-or-sea risk at all, and they contain 250 properties. The Environment Agency rates 5 of them high risk, 26 medium and 58 low, and only one of the five high-risk properties is a home. Postcodes with no river-or-sea risk are left out of the count.
None of Arlesey's postcodes fall inside the high-risk modelled flood area, whether defended or undefended. About 3% are inside the wider low-probability area with no defence modelled. In the model, "defended" only means a defence such as a wall or embankment is present, and it never means a house is safe. Here, the question of defences barely comes up.
The nearest tide gauge to Arlesey is at Portsmouth. It recorded sea level rising at about 2 millimetres a year between 1962 and 2023. The sea matters very little to a town whose nearest tide gauge is on the south coast.
The Environment Agency has no historic flood on record in Arlesey. That is good news, but it only covers floods that were reported and logged, and surface-water flooding often goes unrecorded.
Flood risk is a weak point for Arlesey, because the rain causes problems that the river does not. Whether it matters for a particular house depends on which street it is on.