Does AL9 flood?

Yes — parts of the AL9 district carry real flood risk: 15 of its 27 at-risk postcodes reach the Environment Agency's High likelihood band. The AL9 postcode district covers Welham Green and Hatfield South, Hatfield East, Brookmans Park and Little Heath, Hertford Rural, close to Water End, South Mimms, Birch Green, and surrounding villages in parts of Hertfordshire and England.

27 postcodes in the AL9 district appear in the Environment Agency's rivers-and-sea flood-risk dataset — worst bands: 15 high, 6 medium, 5 low, 1 very low. A postcode not listed here has no properties in any recorded risk band (which is good news, but never proof it cannot flood).

Free preview first · full report £19

Which AL9 postcodes are at risk?

AL9 5LZ up to High AL9 5NT up to High AL9 5PB up to High AL9 5PD up to High AL9 5PG up to High AL9 5PJ up to High AL9 6AA up to High AL9 6AD up to High AL9 6AX up to High AL9 6AZ up to High AL9 7BB up to High AL9 7PF up to High AL9 7SX up to High AL9 7TD up to High AL9 7TS up to High AL9 6JJ up to Medium AL9 7TG up to Medium AL9 7TR up to Medium AL9 7TT up to Medium AL9 7TU up to Medium AL9 7TW up to Medium AL9 5NY up to Low AL9 5RG up to Low AL9 7SW up to Low AL9 7TX up to Low AL9 7UN up to Low AL9 7TY up to Very low

Rivers and flooding history near AL9

The rivers that matter here, each monitored live on FloodRadar:

North Mymms on the Mimmshall Brook — 3.3 km away, highest on record 2.35m
Mimmshall Brook on the Mimmshall Brook — 4.4 km away, highest on record 1.80m
Waterhall on the River Lee — 6.5 km away, highest on record 1.16m
London Colney on the River Colne — 6.8 km away, highest on record 2.72m

265 official flood warnings and alerts have been raised within ~10 km of the district's centre since 2006 (8 at warning level or above), most recently 27 August 2026. Warnings cover flood areas, not individual addresses.

Recorded flood extents near the district (Environment Agency record):

February 2009 flooding — about 3.6 km from the centre
December 2000 flooding — about 3.8 km from the centre
October 1993 flooding — about 11.4 km from the centre
September 1992 flooding — about 3 km from the centre

Records, not floods: no outline on the record is not evidence a place has not flooded.

🏠 Buying or renting in AL9? The full property flood-risk report covers any postcode here — the official likelihood band, every recorded flood nearby with distances, warning history year by year, how the local rivers behave, and an insurance outlook, print-ready for £19. See what's in the report →

Flood risk in nearby postcode districts

AL10 AL8 AL6 AL5 AL4 AL3 AL2 AL1

Band = the worst likelihood band of any property in that postcode (EA Risk of Flooding from Rivers & Sea, defence-adjusted, area-level). Check any postcode in detail — or get the full £19 property report — at the property flood-risk report. © Environment Agency copyright and/or database right 2025. All rights reserved. Some features of this map are based on digital spatial data from the Centre for Ecology & Hydrology, © NERC (CEH). © Crown Copyright and Database Rights 2025 OS AC0000807064. This product is produced in part from PAF® and Multiple Residence Data, the copyright in which is owned by Royal Mail Group Limited and/or Royal Mail Group plc. All rights reserved. Licence number AC0000807064.