Flood risk in AL8 (Welwyn Garden City)
Of the 423 postcodes in AL8 – covering Welwyn Garden City – 2 (0.5%) have their centre in Flood Zone 3 and 0 (0%) in Flood Zone 2, on the Environment Agency’s Flood Map for Planning.
The Zone 3 share is well below the England average of 5.6%, and well below the Hertfordshire figure of 1.7%.
Most of the district is in Flood Zone 1, but the postcodes in Zones 2 and 3 tend to cluster along rivers, streams and low-lying ground – so it’s worth checking the exact postcode rather than relying on the district picture.
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. AL8 7 has the highest share in Flood Zone 3 (0.5% of its postcodes).
| Sector | Postcodes | Zone 3 | Zone 2 | Breakdown |
|---|---|---|---|---|
| AL8 6 | 214 | 0.5% | 0% | |
| AL8 7 | 209 | 0.5% | 0% |
Main rivers in Hertfordshire
River Lea – Harpenden, Hertford, Ware and the Lee Valley
River Colne – Watford and Rickmansworth
River Ver – St Albans
More on the area in our Hertfordshire flood risk guide.
How these figures are worked out
We took the centre point of every live postcode in AL8 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.
