Blackfen does carry a flood risk, and very little of it comes from a river. Hardly any postcodes fall inside the river-and-sea flood zones. About one in six rate high on at least one flood measure. Only two homes face a high risk from rivers or the sea. The rest of the risk is rainwater, and the zone maps leave it out.
The Environment Agency's river-and-sea flood zones cover very little of Blackfen. About 1% of its postcodes are in flood zone 2, the medium-probability band, and none are in flood zone 3, the highest. The Environment Agency holds seven recorded flood events for the area. The latest was in September 1968, when a main river overflowed its channel, and about 1% of postcodes fall within a recorded historic flood outline. When the particulars say flood zone 1, they are telling the truth about the rivers and nothing at all about the rain.
Surface water is where Blackfen's risk sits. This is rain that lands on roofs, roads and paved-over front gardens faster than the drains can carry it away, and it settles on low ground. The river zones do not model it, so a house can sit well outside every flood zone and still be on a street that floods in a downpour. Look up the exact address on the Environment Agency's surface water flood map, which shows the street in a way an area average cannot.
The Environment Agency's climate-adjusted zone, which models the river-and-sea risk under future conditions, still covers only about 1% of postcodes. On that measure, nothing much changes for the rivers.
For insurance, the low river risk helps and the surface water risk can count against you, because insurers price the address and many now look at surface water as closely as rivers. Get a quote on the specific property early, because a premium is easier to accept before you have paid for the survey.
The wider measure folds surface water in with rivers and sea, and counts a postcode as at risk if it rates high, medium or low on any one of several flood indicators. On that basis, about 16% of Blackfen's postcodes rate high on at least one, 26% medium and 48% low. The one-in-six high figure comes almost entirely from these other sources, because the river zones barely touch the area.
The Environment Agency also counts properties at risk from rivers and the sea. Only the 37 postcodes with some risk are listed at all, and they contain 1,226 properties. Of those, 2 are at high risk, 57 at medium risk, 202 at low risk and 3 at very low risk. Both high-risk properties are homes. For river flooding, a count that small means you would have to go looking for the problem.
The Environment Agency's modelled flood extents put about 1% of postcodes in the high-risk extent when defences are ignored, and about 1% when they are included. "Defended" here only means a defence appears in the model. It says nothing about how well the defence holds, and in Blackfen, adding the defences to the model changes nothing.
The nearest tide gauge, at Dover, recorded sea level rising by about 2 millimetres a year between 1961 and 2010. That is what the gauge measured over those years, and Blackfen is a long way inland from it.
Flood risk is Blackfen's weak point, and it is below average. The rivers are hardly a factor, but enough postcodes rate high for surface water that every purchase needs its own check of the street.