Causes of sewer flooding

Some of the common causes of sewer flooding include blockages, heavy rainfall, groundwater infiltration and damaged pipes.


Blocked drains and sewers are often the cause of sewer flooding, and we deal with around 13,000 blockages on our network every year.

When a sewer is blocked, sewage cannot pass through the pipe as it usually would. This can then cause sewage to back up into bathrooms and kitchens.

Most blockages can easily be avoided by only flushing the three Ps (paper, pee and poo) down the toilet. You should also avoid putting fats, oils and grease (FOGs) down the sink.

Learn more about how to avoid blockages.

Heavy rainfall

Our sewers are designed to take sewage away from properties. In some cases, they also take away surface water which runs off roads, footpaths and paved areas.

Our sewerage system can cope with day-to-day flows, but exceptional wet weather can overwhelm it and cause sewer flooding.

Find out how to help manage rainwater runoff.

Groundwater infiltration

Sometimes groundwater, held within rocks and soil, gets into drains and sewers. This is known as groundwater infiltration and normally happens during or after heavy or prolonged rainfall.

Most homes are connected to a public sewer in the road through private drains which carry the foul water from baths, washing machines and toilets.

When groundwater is high, it can be forced into public sewers and private drains through small cracks in pipes or inspection chambers. Over time, these small flows can inundate sewers and cause flooding.

What we are doing about it

Increasing the size of sewers is expensive and disruptive. However, we’re lining and sealing pipes across our region every day. Additionally, we use nature-based solutions such as wetlands and reedbeds where appropriate to filter groundwater.

We also monitor new housing developments to agree suitable points of connection to sewers, ensuring rainwater from roofs and driveways does not lead to infiltration.

How we protect homes and our services

When prolonged rainfall leads to exceptionally high groundwater levels, we can set up temporary pumps to remove water from sewers and discharge it into nearby watercourses. This is permitted by the Environment Agency and called an Operational Mitigation Action Plan (OMAP).

Doing this protects customers from flooding and having to restrict their toilet use. Sampling of watercourses is carried out to ensure there is no environmental impact.

Find out how groundwater causes flooding and what we are doing to manage the problem.

Damaged pipes

Many of our sewers were built at the start of the last century or earlier and because the pipes are underground, it’s not always obvious when they are damaged.

They can be broken or damaged by tree roots or soil movement during very dry or winter weather, which can make sewer flooding more likely.