Frome Overflow Watch

Overflow Watch tracks combined sewer overflows (CSOs) on the River Frome and its tributaries around Frome, Somerset. It checks Wessex Water’s live overflow data every 15 minutes and keeps its own history. It also shows the Environment Agency’s official yearly spill figures.

This watch covers the Frome catchment only and doesn’t try to replace the national trackers. The aim is a hyperlocal window onto current and past sewage overflows.

Reading the map

Each pin marks a CSO, a point where sewage can be released into the river. Each one is watched by an event duration monitor (EDM), which reports when it starts and stops discharging.

Red: discharging now. Amber: discharged in the last 48 hours. Green: no recent discharge. Grey: monitor offline.

Purple squares are swimming spots, with the latest water quality and river flow readings where available.

Reading the timeline

The Timeline lists every CSO. Each has a strip showing the last 90 days, one bar per day: red for a discharge, amber for the 48 hours after, green for none, grey when the monitor was offline, and striped before we started watching.

A red bar with black cross-hatching is a discharge that began after no rain. The blue bars above the list are daily rainfall (see below).

Switch to History for the Environment Agency’s official yearly figures, from 2024 onwards. (Earlier years use an older ID system that can’t be reliably matched to each CSO.)

Underneath are two averages. The first is the Environment Agency’s long-term average number of spills a year. The second is our own average total discharge time a year, since the EA doesn’t publish one.

Dry spills

A discharge is marked as beginning after no rain when the Frome rain gauge recorded 0.25 mm or less in the 24 hours before it started. That’s a fraction of a millimetre, about what a heavy mist leaves behind. We look back from the start, so the answer is known as soon as a discharge begins. Rain that falls during it isn’t counted. A card’s summary line says how many of its discharges began this way, and only when there are any.

The rain comes from one Environment Agency gauge in the town, in 15-minute readings. A shower over Mells or Nunney can be missed, and if the gauge has too many gaps in those 24 hours we don’t make the call at all. The daily totals above the list are for the calendar day, so they can differ a little from the Environment Agency’s own figures, which run from 9am to 9am.

This is a flag, not a finding. It points to a discharge that began without recent rain at one gauge. It doesn’t show that a rule was broken.

Safety

This site shows when overflows discharge. It doesn’t measure water quality, and it can’t tell you whether the river is safe to enter.

Farleigh Hungerford shows Wessex Water’s latest water quality sample, taken about weekly in the swimming season, so it’s a recent snapshot, not a live reading. Tellisford Weir shows river flow only.

No recent discharge doesn’t mean clean water. Rivers carry bacteria from farmland, wildlife and roads all the time, and cold or fast water is a risk whatever the overflows are doing. If you swim, paddle or let a dog in, judge each spot for yourself.

For official bathing water ratings, see the Environment Agency’s Swimfo. For overflows and pollution alerts beyond the Frome, see Surfers Against Sewage’s Sewage Map.

What it can’t tell you

Monitors record when an overflow starts and stops, not how much is released or what’s in it. Debris can also set them off, so an alert suggests a discharge rather than proving one.

The same goes for “began after no rain”. It compares a discharge’s start with one rain gauge in the town. Groundwater, melting snow or a local shower the gauge missed can all cause a discharge with no rain recorded, and whether a particular discharge was permitted depends on things this site can’t see, such as the works’ own flow. It’s a prompt to look closer, not proof of wrongdoing.

Why our numbers differ

If a CSO discharges twice between our checks, we only see the second, so our totals are a minimum. They won’t match the Environment Agency’s yearly figures, which are counted differently and published about a year later.

Sources

Data

Overflow activity: Wessex Water’s live data, checked every 15 minutes and filtered to the Frome catchment. Also on Water UK’s National Storm Overflow Hub. © Wessex Water, CC BY 4.0.
Event history: the live data shows only each overflow’s latest spill, so one that ends between two checks can be replaced before we see it end. Its timeline then reads “by” and “up to” – the latest it can have run to – until Wessex Water’s verified event history (published about a month behind) gives the real end, and adds any spill we missed entirely. © Wessex Water.
Yearly spill figures: the Environment Agency’s EDM Storm Overflow Annual Return, also on their Storm Overflow Explorer. Contains Environment Agency data © Environment Agency copyright and/or database right, Open Government Licence v3.
Rainfall: 15-minute readings from the Environment Agency’s Frome rain gauge (station 531108), via its flood monitoring and hydrology APIs. Unchecked readings. Contains Environment Agency data © Environment Agency copyright and/or database right, Open Government Licence v3.
Swim spot readings: from Wessex Water’s Farleigh Hungerford Dashboard. Water quality © Wessex Water; river flow from the Environment Agency.

Map

Rivers and streams: the EA Statutory Main River Map and OS Open Rivers. Contains OS and Environment Agency data © Crown copyright and database right, Open Government Licence v3.

An independent project by Smiths.Studio