Flood risk in BS35 (Thornbury)
Of the 757 postcodes in BS35 – covering Thornbury, Alveston and Severn Beach – 135 (18%) have their centre in Flood Zone 3 and 15 (2%) in Flood Zone 2, on the Environment Agency’s Flood Map for Planning.
The Zone 3 share is about 3.2 times the England average of 5.6%, and about 6.2 times the Gloucestershire figure of 2.9%.
With 20% of postcodes in Zone 2 or 3, river or sea flooding is a consideration for many homes here. Flood zones are drawn as if there were no defences, so where walls, embankments or barriers protect the area the actual chance of flooding can be much lower – the Environment Agency’s long-term flood risk service shows risk with defences taken into account. Check the exact property before you buy or extend.
Flood zones only cover rivers and the sea, and ignore flood defences. Surface water from heavy rain can affect any street, including in Zone 1.
Flood zones by postcode sector
Each sector is the first part of the postcode plus the first digit after the space. BS35 4 has the highest share in Flood Zone 3 (63% of its postcodes).
| Sector | Postcodes | Zone 3 | Zone 2 | Breakdown |
|---|---|---|---|---|
| BS35 1 | 235 | 12% | 2.6% | |
| BS35 2 | 177 | 0.6% | 0% | |
| BS35 3 | 153 | 1.3% | 0% | |
| BS35 4 | 158 | 63% | 3.2% | |
| BS35 5 | 28 | 14% | 14% | |
| BS35 9 | 6 | 0% | 0% |
Main rivers in Gloucestershire
River Severn – Gloucester, Tewkesbury and the Vale of Gloucester
Warwickshire Avon – Joins the Severn at Tewkesbury
River Wye – The western edge of the Forest of Dean
More on the area in our Gloucestershire flood risk guide.
How these figures are worked out
We took the centre point of every live postcode in BS35 from OS Code-Point Open and tested it against the Environment Agency’s Flood Map for Planning, as published in the flood-risk-zone dataset (version of 5 November 2026). A postcode counts once, whether it has one home or forty, and a postcode whose centre is just outside a zone still counts as Zone 1. Updated November 2026.
For one address, the free postcode check looks at the zones around the postcode, and the full report adds surface water, reservoirs, recorded floods and defences.
