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Conditions are worth watching: the Environment Agency has a Flood Alert in force for Tidal Thames riverside from the Thames Barrier to Putney Bridge — flooding is possible from 01:00 AM to 03:00 AM on Saturday 10th October. Flooding is possible — be prepared and keep an eye on updates.
Updated 2026-10-10 01:06:23 UTC · Official GOV.UK warnings · Flood preparation & help guide
South Mimms is a village in the civil parish of South Mimms and Ridge, in the borough of Hertsmere in Hertfordshire, England. The village is near the junction of the M25 motorway with the A1(M) motorway; it gives it name to the South Mimms services at that junction. At the 2021 census the built up area had a population of 661 and the civil parish of South Mimms had a population of 874. The civil parish subsequently merged with the neighbouring parish of Ridge in 2023 to form the new parish of South Mimms and Ridge.
Towns and villages near South Mimms with a page of their own: the homes in a flood risk area there, the nearest river gauges, and recorded floods.
Place names: Contains OS data © Crown copyright and database rights 2026. Contains Royal Mail data © Royal Mail copyright and database right 2026. Contains National Statistics data © Crown copyright and database right 2026. Licensed under the Open Government Licence v3.0.
Two mapped watercourses are about the same distance from the middle of South Mimms's gauges: Mimmshall Brook (about 10 m) and Potters Bar Brook (about 20 m), so this page cannot say which one is nearer to you.
Distances along the river are from the point on each watercourse nearest the middle of South Mimms's gauges. From Ordnance Survey’s river map (OS Open Rivers), which leaves out small streams, ditches and culverts, so a nearer one may exist. This describes where water goes on the map; it says nothing about flooding at any address. Contains OS data © Crown copyright and database rights 2026.
South Mimms is monitored by 1 Environment Agency river level station on the Mimmshall Brook, in Hertfordshire. Water levels are updated every 15 minutes, providing real-time flood monitoring for the area.
The Mimmshall Brook is monitored by 1 station (Mimmshall Brook). Each station records water levels continuously, with data synced to FloodRadar every 5 minutes.
There are 8 Environment Agency flood warning areas within 10km of South Mimms. When the EA issues a flood warning for any of these areas, it will appear on this page. These include: The Mimmshall Brook at Warrengate Road including Water End; The Mimmshall Brook at Warrengate Road including South Mimms, Potters Bar, Brookmans Park and Welham Green; The River Colne at Colney Heath including North Mymms; The Upper River Colne and Radlett Brook at Colney Heath, London Colney, Borehamwood and Radlett.
There is 1 storm overflow within 5km of South Mimms. None of the 1 with a reporting monitor is currently discharging. Enable "Sewage" on the map above to see their locations.
It has its own page of live status and past discharges:
If flooding is forecast in South Mimms, call Floodline on 0345 988 1188 for the latest information. Move valuables upstairs, turn off gas and electricity if safe to do so, and avoid walking or driving through flood water. Sign up for flood warnings on any station page above to receive email alerts when water levels rise. For emergency numbers, a preparation checklist and insurance guidance, see our flood help & preparation guide.
South Mimms gets about 685 mm of rain a year on average (wettest month November, 74 mm; driest March, 41 mm). That is 41% drier than the UK as a whole, which averages 1,163 mm a year. See the rain gauges →
A thirty-year average for the area, not a forecast and not a flood risk on its own. Source: Met Office HadUK-Grid 1991–2020 (5 km grid), 0.6 km away. Average rainfall: HadUK-Grid v1.3.2, Met Office. Contains public sector information licensed under the Open Government Licence v3.0.
Floods whose mapped extent (EA Recorded Flood Outlines) reached within ~5 km of the town centre — most recent first. These are records of mapped extents, not a full list of floods. See them on the map →
Records, not floods: no outline on the record is not evidence a place has not flooded.
Current verdict: Watch. Conditions are worth watching: the Environment Agency has a Flood Alert in force for Tidal Thames riverside from the Thames Barrier to Putney Bridge — flooding is possible from 01:00 AM to 03:00 AM on Saturday 10th October. Flooding is possible — be prepared and keep an eye on updates.
South Mimms has river level monitoring on Mimmshall Brook — 1 monitoring station in total.
Use the GOV.UK Check for Flooding service (check-for-flooding.service.gov.uk) or call Floodline on 0345 988 1188 (24 hours). You can also set up free FloodRadar email or browser alerts from any station page above.
Follow official advice: move valuables and vehicles to safety, never walk or drive through flood water, turn off gas and electricity if safe, and check on vulnerable neighbours. See our flood preparation guide at floodradar.co.uk/flood-help.
Yes — the Environment Agency's Recorded Flood Outlines dataset shows 5 recorded flood extents around the town centre since 1703, most recently in 2000. That is a count of mapped extents, not of separate floods. Records, not floods: no outline on the record is not evidence a place has not flooded. See the flood history link below.
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