Cefn Storm Overflow
Storm overflow run by Dwr Cymru Welsh Water, discharging into Tref-y-nant Brook.
"Not discharging" means the company's monitor reports no discharge right now — not that the water is clean. The feed publishes status only; no discharge volume is measured.
Year by year
Dŵr Cymru Welsh Water's annual Event Duration Monitoring return to Natural Resources Wales (12/24-hour spill count), as compiled by The Rivers Trust. 2026 is our own running count, the same way, from the live feed since 28 February (when we began keeping every discharge it reports) — so it can only be lower than the full year. The official figure is published next spring.
"No data" means the monitor was not working or not required to report that year — not zero.
Twin outlet
Dwr Cymru Welsh Water runs another overflow within 300 m that also discharges into Tref-y-nant Brook. They share the sewer and the storm, so one outlet's figures are only part of what reaches the water.
This overflow: 46 spills in 2025. Distances are between the published outlet points.
Downstream of this overflow
The next gauge downstream that the river map places is Manley Hall River Station (Dee), 13.9 km along the river.
Water-sampling points downstream
The nearest Environment Agency sampling points the river map places below Manley Hall River Station. Points between this overflow and that gauge cannot be put in order with it on the map, so they are not listed.
- River Dee at A534 Wrexham Road Bridge, Holt
49 km along the river, below Manley Hall River Station · Last sampled 7 Mar 2025 · 19 months ago - River Dee at Farndon Bridge
50 km along the river · Last sampled 10 Aug 2026 · 52 days ago - River Dee near Eccleston
64 km along the river · Last sampled 1 Jul 2026 · 3 months ago
A sample is dated: it is not the river now, and its results are on its own page. A sample taken after a discharge does not show that the discharge caused what was measured. Samples: Environment Agency Water Quality Archive, © Environment Agency and/or database right 2026, Open Government Licence v3.0.
A river gauge measures the level of the river; it does not measure this overflow's discharge, and a spill does not by itself explain a change there. Traced on OS Open Rivers (edition 2026-04), which leaves out many small channels.
Contains OS data © Crown copyright and database rights 2026.
Discharges we have recorded
296 discharge periods since 2 Jan 2020, 1 still open. 296 of them come from Dwr Cymru Welsh Water's own event-duration-monitoring records (published by the company, Jan 2020 – Dec 2023); the live feed adds anything after that. The official annual count is the figure to quote.
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Sat 30 Dec 2023, 10:30ongoing
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Wed 27 Dec 2023, 02:302 d 12 h
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Tue 12 Dec 2023, 17:3017 h 15 min
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Tue 12 Dec 2023, 01:0015 h 15 min
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Mon 11 Dec 2023, 08:003 h 0 min
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Thu 7 Dec 2023, 12:153 d 13 h
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Thu 7 Dec 2023, 07:454 h 0 min
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Thu 7 Dec 2023, 04:4545 min
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Tue 5 Dec 2023, 04:0017 h 45 min
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Tue 5 Dec 2023, 00:301 h 0 min
Show 30 earlier (of 296)
| Started (UK time) | Ended | Discharge time |
|---|---|---|
| 4 Dec 2023, 17:00 | 23:00 | 6 h 0 min |
| 4 Dec 2023, 00:45 | 15:00 | 14 h 15 min |
| 27 Nov 2023, 07:30 | 11:45 | 4 h 15 min |
| 15 Nov 2023, 08:00 | 09:30 | 1 h 15 min |
| 13 Nov 2023, 03:15 | 02:00 | 1 d 22 h |
| 12 Nov 2023, 15:00 | 15:30 | 30 min |
| 12 Nov 2023, 09:30 | 14:15 | 4 h 30 min |
| 10 Nov 2023, 07:00 | 10:30 | 3 h 15 min |
| 9 Nov 2023, 13:00 | 05:15 | 16 h 0 min |
| 9 Nov 2023, 08:00 | 08:30 | 30 min |
| 8 Nov 2023, 08:15 | 09:15 | 1 h 0 min |
| 4 Nov 2023, 11:00 | 14:15 | 1 d 3 h |
| 4 Nov 2023, 09:30 | 09:45 | 15 min |
| 2 Nov 2023, 02:00 | 02:30 | 1 d 0 h |
| 1 Nov 2023, 05:45 | 06:15 | 30 min |
| 31 Oct 2023, 03:45 | 04:00 | 15 min |
| 28 Oct 2023, 19:15 | 19:45 | 30 min |
| 26 Oct 2023, 07:45 | 09:30 | 1 h 45 min |
| 23 Oct 2023, 09:45 | 10:30 | 45 min |
| 22 Oct 2023, 11:00 | 11:15 | 15 min |
| 20 Oct 2023, 02:45 | 03:45 | 2 d 0 h |
| 19 Oct 2023, 22:00 | 02:00 | 3 h 45 min |
| 19 Oct 2023, 21:00 | 21:15 | 15 min |
| 19 Oct 2023, 16:15 | 20:15 | 4 h 0 min |
| 19 Oct 2023, 14:45 | 15:30 | 45 min |
| 18 Oct 2023, 20:30 | 10:00 | 13 h 30 min |
| 14 Oct 2023, 11:45 | 12:00 | 15 min |
| 13 Oct 2023, 02:30 | 15:45 | 13 h 15 min |
| 11 Oct 2023, 14:45 | 17:15 | 2 h 30 min |
| 11 Oct 2023, 11:30 | 13:15 | 1 h 45 min |
The year figures above use the EA's 12/24-hour counting method, so they are comparable with the official figures; the monitor log's 609 raw blocks were merged into 296 continuous discharge periods (gaps under 15 min joined).
Rain nearby last 30 days
Nearest live rain gauge: Five Fords Raingauge, 10.4 km away. Brown shading = when this overflow was discharging; ▼ = a discharge start.
Overflow shading is a timing match, not proof of cause — heavy rain and spills share a trigger. Times come from the water company’s event-duration monitor (site status only; no discharge volume is published).
Where it is
Concerned about this overflow?
Report pollution you can see or smell to the regulator straight away; the company handles complaints about the overflow itself.
Live status: © Dŵr Cymru Welsh Water, from its Storm Overflow Map. Spill-event history: © Dŵr Cymru Welsh Water's published EDM start/stop records. Annual figures: The Rivers Trust EDM compilation, © Dŵr Cymru/Welsh Water, Open Government Licence. Times are shown in UK local time. FloodRadar republishes the monitor's status; it does not measure discharges itself.