About SwimSignal

SwimSignal is a free five-day forecast of sewage-overflow risk at river and lake swim spots in England. It covers 89 spots, 38 of them designated bathing waters. It uses the water companies' live storm-overflow data, rainfall forecasts and the river network. It is a forecast, not a water test.

How it works

For each spot, SwimSignal traces the river network upstream and finds every monitored storm overflow whose water can reach it. It takes what those overflows are doing now from the water companies' live feeds. It forecasts how likely each one is to spill on each of the five days, from the rain forecast and the overflow's own spill record. It then allows for how long sewage takes to travel downstream, how fast bacteria die off and how much the river dilutes it, so an overflow 2 km upstream counts for more than one 40 km up a tributary. On rivers it also estimates the chance that a water sample would fail the E. coli limit for bathing water, and at designated bathing waters it adds the Environment Agency's rating and its sampler's latest algae check.

What it cannot see

• The spill forecast covers monitored storm overflows only. The E. coli estimate allows for rain washing in pollution from other sources on average, but knows nothing about a particular farm, road, boat, misconnected drain or incident.
• Dŵr Cymru Welsh Water publishes no live overflow status. Its overflows, which matter at the spots on the Dee and the Wye, are forecast from rain and their yearly spill record, with no "right now".
• It does not show the Environment Agency's short-notice advice against bathing after pollution or an algal bloom.
• It says nothing about currents, cold water or other dangers.
• A "low" reading means nothing the site looks at flags the day. It does not mean the water is clean, or that it is safe to swim there.

How good it is

Forecasts are scored afterwards against what the overflows actually did and against the Environment Agency's water samples. The results are published on the accuracy page, including where the forecast was wrong. The water-quality estimate is tested on rivers in the May to September sampling season only. Please take any figures from the accuracy page: they change as new results come in.

Who runs it

SwimSignal is a free, non-commercial project run by one person, not a company. It is not run by, connected with or endorsed by the Environment Agency, any water company, Water UK or the Met Office. The code is open source under the MIT licence. See the terms of use.

Contact

Email ethan@ethanbuckley.me.uk. To report a problem or ask for a spot to be added, you can also use the feedback form or the spot request form. These prepare a private email for you to review and send; no GitHub account is needed.

Data

Storm overflow data from the water companies via the National Storm Overflow Hub, and data from the Environment Agency, Ordnance Survey, Open-Meteo, the Met Office and OpenStreetMap. Sources, credits and licences.