What causes sewer flooding?

Climate change, population growth, and the paving over of green spaces that provide natural drainage are putting increasing pressure on our sewerage network.

This is often made worse by people putting unsuitable products down the sink or toilet, increasing risk of sewer flooding.

Many areas have separate sewers to take foul waste and rainwater, but in much of London and some other urban areas, the sewer system is combined, meaning that foul sewage from kitchens and toilets mixes with rainfall.

This means that, during a heavy storm, the flow in the sewer is much greater and can reach maximum capacity.

Our sewers are designed to cope with the vast majority of storms but occasionally rainfall can be so heavy that it overwhelms the system.

When this happens, sewage can overflow from manholes and gullies and flood land, rivers and gardens. In the worst cases, sewage can even flood homes.

The effect of groundwater

Outside of London, our sewers are only designed to take the water from our sinks, toilets and washing machines. These can be affected by groundwater and surface water, which in turn may lead to sewer flooding. Flooding caused by seasonal groundwater can be especially prevalent in areas with chalk valleys, mainly found in the South and East of England. However, flooding from groundwater can also happen in other areas.

In a typical summer, groundwater remains well below the surface and does not affect the sewers. However, in a typical winter the water table and groundwater levels can rise. This can cause the appearance of springs or winter rivers, known as winterbournes. Winterbournes may look like a dry ditch in the summer but become a flowing stream or river in the winter months. In very wet winters the number of springs tends to increase, and these will flow for longer. In some areas this water may flow down from the land and cause flooding to highways.

When we have wet winters and the groundwater levels rise, sewers become vulnerable to infiltration, especially when the sewers are designed for foul water only. Infiltration occurs when the groundwater level rises, making its way into the sewers and the private lateral drains of homes and businesses before they connect to the main sewer. When this happens, groundwater will force itself through small cracks either in the sewers, private connections and through the manhole chambers and can also inundate through the manhole covers.

Where there are many of these small flows, they can be cumulative and in time overwhelm the sewer, resulting in the sewers filling with groundwater. This can lead to flooding, as these sewers are not designed to carry groundwater or excessive rainfall runoff from roofs, driveways and highways.

The springs may also appear as surface water. It is important to remember not to lift manhole covers in the event of flooding from surface water, as this can lead to sewer flooding downstream. This excess surface water and groundwater in the sewer may also cause the sewer to back up into homes or prevent people from being able to flush their toilets. This is called restricted toilet use.

One of the biggest challenges in dealing with groundwater is its quantity, as we cannot intercept this water before it reaches the ground. Pumping this water to rivers would also fail to reduce the groundwater levels and may also increase the chances of flooding from the river.

As well as being disruptive and very costly, increasing the size of the sewers would also prove ineffective, as groundwater would simply fill these larger sewers. Increasing the size of the sewers would also present a problem when groundwater levels are lower, as there would not be enough flow in the sewer. This would ultimately lead to a higher risk of blockages and sewer flooding throughout the year.

In locations with high groundwater, we may occasionally need to tanker the area to take the pressure off the sewers and reduce the risk of foul sewer flooding.

What we're doing

When we get these wet winters and high ground water, we investigate where the water may be forcing its way in. We do this using CCTV, Impermeable Area Surveys and flow monitors, as well as physically lifting and looking in the manholes to identify problem areas.

Where we have some sources, we have been using different techniques to stop the water getting in. This could include:

  • A specialist no-dig technique of lining the sewer. This uses a specialist material which prevents the water getting in.
  • Low leak manhole covers, which prevent inundation through the top of the manhole

Sewer Flooding: what to do? How we can help