Thornbury's flood risk is real but modest, and it comes from the sky rather than the Severn. Only a sliver of the town sits in the mapped river and sea flood zones. Surface water is the bigger story, with about seven in a hundred postcodes carrying a high risk from heavy rain. There is no recorded history of significant flooding here. This is a town to check street by street, not one to cross off the list.
Start with the rivers and the sea, because that is what most buyers picture when they hear the word flood. Around one in a hundred postcodes fall in the medium river-and-sea flood zone, and well under one in a hundred sit in the highest zone. For a parish whose old boundaries once ran all the way to the River Severn, that is a reassuringly small footprint. The tidal water stays far enough from the built-up streets that the classic river-flood scenario barely registers on the maps, and the town centre is not the place that keeps a flood-risk officer awake.
Rain is the risk that actually matters here. About seven in a hundred postcodes carry a high risk of surface-water flooding, the kind that arrives when a downpour overwhelms the drains rather than when a river bursts its banks. Another one in five carry a medium risk, and roughly four in ten sit in the low band, which together means surface water is a broad, low-level fact of life rather than a concentrated emergency. It is the sort of risk that turns a specific dip in a specific road into a problem a couple of times a decade, while the house up the slope stays dry through the same storm. The map matters more than the town name.
The forward look nudges those numbers up rather than transforms them. Once the standard climate allowance for future sea levels and heavier rainfall is factored in, the share of the town in the higher river-and-sea flood zone rises to about one in fifty. That is a small increase on a small base, and it is a reason to read the long-range mapping for a particular address rather than a reason to picture the streets under water. The direction is upward, gently, as it is almost everywhere near a tidal river.
For insurance, the practical effect is uneven rather than alarming. Most homes here sit outside the meaningful flood bands and will be quoted standard cover on standard terms. The minority in the high surface-water pockets can expect closer questions from an insurer, a possible excess on the flood portion of a policy, and a request to know what drainage or defences protect the property. None of that is unusual for an English town near a river valley, and none of it is a wall to buying, but it is worth a quote before you commit rather than a surprise afterwards.
Thornbury's flood risk lands in the sensible middle: low for most, worth a careful look for a few. Get the specific address checked against the surface-water map, and the town gives you no reason to walk away.