Cholsey's flood risk is middling, and most of it comes from rain on the ground more than from the Thames. The river zones cover a sliver of the area. About one postcode in nine is high-risk on at least one measure. The worst flood on record came up through the drains. It is a village with a river next to it, and the water it has to deal with mostly falls from the sky.
On the Environment Agency's river-and-sea flood maps, about 1% of the area's postcodes fall in Flood Zone 3, the highest-probability band, and about 1% more in Flood Zone 2, the medium-probability fringe beyond it. The two zones do not overlap, and together they cover very little of the area. The village grew up on an island in marshy ground beside the Thames, Ceol's Isle, and whoever chose the spot seems to have known which part stayed dry.
Surface water is the other half of the picture. The Environment Agency has four recorded flood events in the area, the most recent in January 2003, caused by local drainage and surface water. About 1% of postcodes fall inside a recorded historic flood outline. That record only covers floods someone mapped. A garden that floods every wet winter without anyone reporting it does not appear on it.
Allowing for climate change, the Environment Agency's adjusted flood zone takes in about 3% of postcodes. That is its modelled view of how far the zones could reach under future rainfall and river levels, and it does not say when.
Insurers price from address-level mapping, and they pay more attention to surface water than buyers tend to. A home outside the river zones can still be quoted a higher premium for rainfall risk, and a home on the edge of a zone can be priced as if it were in it. The Environment Agency's long-term flood risk service gives a separate surface water reading for every address, and in Cholsey that reading is the one to look up before instructing a solicitor.
On the postcode-level indicators, which count a postcode if it scores on any one of several measures including surface water, about 11% of the area's postcodes rate high, about 12% medium and about 26% low. Because any single measure can put a postcode in a band, these shares run well above the river-zone figures. They are a broader net, and they measure something different.
Counting homes gives a sharper picture. The Environment Agency's rivers-and-sea mapping lists 33 postcodes in the area that carry some risk, holding 823 properties between them. Of those, 66 properties are rated high risk, 21 of them homes, plus 21 at medium and 48 at low. Postcodes with no river or sea risk are left off the list entirely. Out of the whole village, a few dozen addresses carry most of the river risk.
In the Environment Agency's modelled flood extents, about 1% of postcodes fall in the high-risk extent with defences left out, and about 1% with defences included. Adding defences to the model barely moves the number. Whatever protection exists around Cholsey is either small or covers places that would not flood anyway, and "defended" on a map only means a defence appears in the model. It says nothing about how well that defence will hold.
The nearest tide gauge is at Portsmouth, which recorded sea level rising by about 2 millimetres a year between 1962 and 2023. That is the rate measured on the south coast, a long way downstream. In Cholsey it describes somewhere else's problem.
Flooding in Cholsey is a middling risk: low from the river for most homes, with surface water worth checking for any particular house before committing to it.