Swanley's flood risk falls out of the sky. Rivers and the sea leave it alone. About a fifth of its postcodes still rate high on at least one flood measure. The culprit is rain with nowhere to drain. Nothing is on record as having flooded. Dry on the river map, and damp on the drainage one.
On the Environment Agency's river-and-sea mapping, none of Swanley's postcodes falls in Flood Zone 3, the highest-probability band, and none falls in Flood Zone 2, the medium-probability fringe beyond it. For a town on the edge of the M25 with no river worth naming in its history, that is the expected result. It also means the phrase "not in a flood zone" will be perfectly true on almost any Swanley listing, and it answers only the river-and-sea question.
The risk that does exist here is surface water: heavy rain landing on roofs, roads and car parks faster than the drains can take it away. That kind of flooding follows the shape of the ground, so it gathers in dips, underpasses and the low end of a street while the house fifty metres uphill stays dry. Swanley was mostly built between the war and the late 1980s, and a town laid out in concrete and tarmac sheds water quickly.
The Environment Agency's climate-adjusted river-and-sea zone, which models how those flood extents could spread under future rainfall and sea levels, also covers none of Swanley's postcodes. On that mapping the town stays outside the line on the forward look as well as today's.
Insurers price surface water alongside rivers and sea, so a clean flood-zone result does not settle the premium. A home on a low-lying street or near a known drainage pinch point can attract higher excess or a pricier quote, even in a town the river maps ignore. The practical move is to put the exact address into the Environment Agency's long-term flood risk service and read the surface-water result, then get an insurance quote on that address early in the purchase.
Looking at every flood measure together, including surface water, about 19% of Swanley's postcodes rate high on at least one of them, about 21% rate medium and about 35% rate low. Those shares count a postcode once it trips any single measure, which is why they look so different from the empty river-and-sea zones. The risk is real and patchy, and it changes from one end of a road to the other.
The Environment Agency's modelled extents put 0% of Swanley's postcodes in a high-risk river-and-sea area, whether defended or undefended, and 0% in the low-risk undefended extent. There is nothing for a flood wall to hold back here, because the water in question comes down the road rather than over a bank, and no defence model covers a blocked drain.
The nearest tide gauge is at Dover, which recorded sea level rising by about 2.2 millimetres a year between 1961 and 2010. That is a measured past rate on the coast, a long way from a town whose water problem arrives by downpour, and it says little about Swanley's own risk.
No historic flooding is recorded anywhere in the area. That is reassuring as far as it goes, though surface-water flooding on a single street often never makes it onto a map at all, so a quiet record means nothing was logged, which is a different thing from nothing having happened.
Swanley's flood risk is below average for a town that never goes near a river: the zones are clear, but surface water reaches enough postcodes that the exact street matters more than the town's clean river map suggests.