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Up to Low likelihood
Properties in CM21 9HX sit in the Environment Agency's Low flood-likelihood band (worst property). The band is the worst found in the postcode — it describes the land around properties (defence-adjusted, rivers & sea), not any single address, and not today's conditions.
| Likelihood band | Annual chance | Properties | Of which homes |
|---|---|---|---|
| High | greater than 3.3% | 0 | 0 |
| Medium | 1% – 3.3% | 0 | 0 |
| Low | 0.1% – 1% | 3 | 0 |
| Very low | below 0.1% | 0 | 0 |
Over a 25-year mortgage, a property in a low-likelihood area has a 2–22% chance of flooding at least once.
CM21 9HX gets about 639 mm of rain a year on average (wettest month October, 70 mm; driest March, 38 mm). That is 45% drier than the UK as a whole, which averages 1,163 mm a year.
A thirty-year average for the surrounding area, not a measurement at this address and not a flood risk on its own: a dry area floods when a storm sits over it, and a wet one may drain away perfectly well. Source: Met Office HadUK-Grid 1991–2020 (5 km grid), 3.4 km away. Average rainfall: HadUK-Grid v1.3.2, Met Office. Contains public sector information licensed under the Open Government Licence v3.0.
The ground here sits at about 73 m above sea level and falls 14 m across the surrounding 800 m. Surface water reaching this point runs off ground lying to the east-south-east, 4.9 m higher.
The river map has no watercourse within 1 km of the centre of CM21 9HX.
That means this spot is not on the river map, not that there is no water here: Ordnance Survey’s river map (OS Open Rivers) leaves out small streams, ditches and culverts. Contains OS data © Crown copyright and database rights 2026.
Because properties in the low likelihood band; 6 recorded flood outlines mapped nearby; 130 official warnings since 2006.
The map above shows the real EA rivers & sea likelihood cells around CM21 9HX. The small panels are illustrative previews of the report's per-layer map views — the full report draws each layer for real, plus recorded extents, defences and a building picker on an interactive map.
Warnings cover flood areas, not individual addresses.
| Date | Mapped flood extent | Approx. distance |
|---|---|---|
| Feb 2009 | ea06FebruaryWinter2009 | ~1.6 km |
| Oct 2001 | 06OctoberAutumn2001 | ~7.0 km |
| Oct 2000 | 06OctoberAutumn2000 | ~1.3 km |
| Oct 1993 | 06OctoberAutumn1993 | ~2.4 km |
| Jan 1993 | 06OctoberAutumn1993 | ~4.5 km |
| Jan 1978 | 06MaySpring1978 | ~2.8 km |
These are extents the Environment Agency has mapped, not a list of every flood. Records, not floods: no outline on the record is not evidence a place has not flooded. The full report tests whether any extent covered the exact spot.
2013-14 UK winter floods (2013, 19 warnings nearby) ▶ replay it here · Henk (2024, 6 warnings nearby) ▶ replay it here · Dennis (2020, 3 warnings nearby) ▶ replay it here · Ciaran (2023, 2 warnings nearby) ▶ replay it here
| Gauge | River | Distance | Typical high | Highest on record |
|---|---|---|---|---|
| Sawbridgeworth | River Stort | 1.2 km | 1.63 m | 1.98 m |
| Harlow Mill | River Stort | 3.6 km | 2.02 m | 2.95 m |
| Pishiobury Brook | Pishiobury Brook | 3.6 km | 2.08 m | 2.87 m |
| Bishops Stortford, Grange Paddocks | River Stort | 7.2 km | 1.15 m | 1.80 m |
Live levels, predictions and rain-response behaviour are on each gauge's page and in the property report.
| Overflow | Discharges into | Distance | Spills | Hours |
|---|---|---|---|---|
| Little Hallingbury Wwtw | Little Hallingbury Brook | 2.4 km | 50 (2024) | 710 |
| Hatfield Heath Wwtw | Hatfield Heath Watercourse | 2.4 km | 97 (2024) | 1,892 |
| Bakers Farm, High Wych | High Wych Ditch | 4.0 km | 7 (2024) | 19 |
| Hatfield Broad Oak Cage End | Pinsey Brook | 4.6 km | 15 (2024) | 93 |
Official annual EDM figures — a water-quality signal for local watercourses, not a property flood-risk factor.
No mapped flood-defence assets within 500 m of the postcode centre. Defences reduce risk, never remove it.
The interactive flood-risk map with a building picker (your exact house, not the postcode), the 2050s climate view, surface-water and planning flood zones at your building, exact recorded-flood distances, year-by-year warning charts, watercourses including culverted streams, an insurance outlook with the Flood Re rules, and a printable document with every dataset receipt — start with the free preview, unlock for £19.
At-risk postcodes near CM21 9HX — all in the CM21 district.
© 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. Recorded Flood Outlines and warning history © Environment Agency copyright and/or database right 2025. Storm-overflow figures: EA Event Duration Monitoring (England); live overflow status © the water companies, incl. Dŵr Cymru Welsh Water and Scottish Water. Flood defences © Environment Agency (OGL v3). This page summarises public area-level data — it is not a flood-risk survey and not a substitute for the official GOV.UK service. All data sources & licences. More postcodes: the CM21 district.