Eltham's flood risk comes from the sky far more than from any river. Almost none of it lies in the river flood zones. Count surface water, though, and about one postcode in five rates high on at least one measure. Most of the town is on a plateau, and the rain that lands there still has to drain somewhere. Flooding is the weakest thing about Eltham, and the rivers have very little to do with it.
On the Environment Agency's river-and-sea mapping, well under 1% of Eltham's postcodes fall in Flood Zone 3, the highest-probability band, and another well under 1% fall in Flood Zone 2, the medium-probability fringe beyond it. The rivers here are small: the Quaggy along the south-west edge, fed by the Little Quaggy from The Tarn in Mottingham, and the Shuttle rising in Avery Hill Park. The Environment Agency holds three recorded floods for the area, and the most recent was in 1968, when a main river overflowed its channel and there were no raised defences to stop it. That makes the Quaggy a minor actor with a long-ago record.
Surface water is a different matter. A plateau with a scarp on its western edge, built over with interwar semis, driveways and patios, sends heavy rain downhill along roads and into back gardens, and the lower ground towards Mottingham and New Eltham takes what comes off the top. This is the risk that turns up after a summer downpour, not the slow rise of a river.
The Environment Agency's climate-adjusted zone, which models how the river-and-sea flood extent could grow as rainfall changes, also takes in well under 1% of Eltham's postcodes. Any change from that quarter is small, and the change that matters is how much harder the rain falls.
Insurers price surface water and river flooding together, so a house on a street that sheds water can cost more to cover than its river-zone status suggests. Ask an insurer to quote on the exact address early, while walking away still costs nothing. That quote will tell you more about a particular house than the town's flood record does.
At postcode level, counting any of several measures together, surface water included, about 22% of Eltham's postcodes rate high on at least one, about 26% rate medium and about 44% rate low. One flood indicator is enough to put a postcode into a band, so these shares are a broad net and describe very different streets alongside each other. A postcode on the high plateau and one at the foot of Eltham Hill are not the same purchase, even with the same district code.
Counting properties is more useful than counting postcodes. The Environment Agency's rivers-and-sea mapping lists 227 properties across Eltham's affected postcodes. Of those, 72 are at high risk, 29 at medium, 38 at low and 36 at very low, and 69 of the high-risk properties are homes. Only postcodes with some river or sea risk appear in that count at all, so the rest of Eltham is simply absent from it. For river flooding, that comes to a few dozen front doors.
The modelled extents show whether anything stands between the water and the house. About 0.2% of Eltham's postcodes are inside the high-risk extent with no defence in the model, and about 0.4% are inside it with one present. "Defended" means a defence exists in the Environment Agency's model. It says nothing about whether that defence holds on the night, and nobody should read it as reassurance.
The nearest tide gauge, at Dover, measured sea level rising by about 2 millimetres a year between 1961 and 2010. That is the rate recorded there over those years. With the Thames a couple of miles beyond the town's northern edge, it matters to Eltham far less than a blocked drain does.
The river risk is small and limited to a few streets, but surface water across the town makes flood risk a real weak point for Eltham, and the street a house is on matters more than the postcode.