Sewer Flooding

What is sewer flooding?

Sewer flooding occurs when large volumes of rainwater enter the public sewer system or when the sewer system becomes blocked. Sewer flooding can occur anywhere and at any time under certain circumstances and there is little available information to inform you if you are at increased risk. Public sewers can be categorised into three types:

1. Surface water sewer

Surface water sewers accept only surface water from roofs, drives, roads, etc. and are usually laid in conjunction with a separate foul sewer to take the waste water. The advantage of separate surface water and foul sewers is that during extremely heavy rainfall, no additional rainwater is directed to the foul sewer and so any sewer flooding should be rainwater only. If you experience a flood event with foul water flooding it could be that foul water has been unintentionally piped into a surface water sewer or that your area is served by a combined sewer. Severn Trent Water may be able to investigate foul water flooding following a flood event.

2. Foul water sewer

Foul water sewers carry wastewater, including water from toilets, sinks, dishwashers and washing machines, from properties and businesses to sewerage treatment works. No water from roads, roofs or any other surface water should enter a foul water sewer. If you do experience foul water flooding during a rainfall event it is likely that there is a combined sewer or possibly that surface water has been accidentally routed into the foul sewer. In normal weather conditions it could be that your foul sewer is blocked. In any case, you should contact Severn Trent Water to report the issue on 0800 783 4444 or visit

3. Combined sewer

Combined sewers are not installed routinely today although they are often found in older developments. Combined sewers accept foul water and surface water into the same pipe, with all of the contents transferring to a sewage treatment works. The main disadvantage of combined sewers, and the reason why they tend not to be used today, is that during heavy rain storms flood water will contain foul water and may cause a health hazard.

In Derby, there are areas served by both combined and separate (foul water and surface water) sewers. If you have any queries about the public sewers serving your area, you are advised to contact Severn Trent Water on 0345 602 2777

If you think you have a blocked drain or if you’d like to report an incident of sewer flooding, please contact Severn Trent Water on 0800 783 4444. Alternatively, Severn Trent Water has published a guide to sewer flooding or you can contact Severn Trent Water directly.

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