Downstream Thinking & Sustainable Drainage

Banner image

Downstream Thinking is taking a new approach to tackling flooding from sewers.

We know how devastating sewer flooding is and we are committed to reducing the risk of it happening in our region in future.

Working with communities and our partners, we are looking at innovative and sustainable ways to manage the amount of surface water that enters our sewer network.

Surface water comes from the rainwater from roofs, roads and other hard (or ‘impermeable’ surfaces). As the amount of impermeable surfaces increases, the amount of surface water is also increasing, putting our sewer network under pressure.

Reducing surface water will help to reduce the risk of sewer flooding to homes and businesses, and protect our environment from pollution.

At South West Water, we call this programme Downstream Thinking.

How do we manage rainwater?

We invest millions of pounds every year to maintain and upgrade our sewer network and safeguard our environment well into the future. We prioritise areas that have suffered from sewer flooding.

If sewer flooding is caused by our sewer, we are responsible, but we also work in partnership with other organisations, such as the Environment Agency and local authorities.

The Environment Agency will need to be involved when flooding is from a river or watercourse, while flooding from the highway caused by blocked gullies (slotted grills at the edge of the road) is the responsibility of the local authority. Often, flooding can be a combination of all of these and what is need is an investigation into the whole catchment.

Traditional engineering solutions to manage rainwater such as bigger pipes and underground storage tanks, are expensive and not always the right answer to managing rainwater.

In 2014, the main flood risk authorities of the South West came together to give a shared commitment to building a water sensitive South West recognising the benefits of non-conventional solutions and partnership working. Those methods, known as ‘water sensitive urban design’, includes use of sustainable drainage solutions (SuDS).

Sustainable drainage solutions

SuDS remove, store, divert or delay the speed that rainwater enters our sewer network naturally.

By managing rainwater naturally, we can avoid wasting energy and money pumping and treating clean rainwater in our sewage network.

South West Water is piloting different sustainable drainage solutions throughout the region  and we plan to apply the ‘lessons learnt’ from the pilots to implement a bigger programme in the next few years.

SuDS may include rainwater harvesting, raingardens, attenuation ponds or grassed detention basins. SuDS have wider social and environmental benefits, including providing green spaces for people to go to in urban areas and creating valuable wildlife habitats.

They help to prevent surface water entering the sewer network by redirecting it to a natural watercourse, or slowing down rainwater entering the drainage system and only allowing it to enter when there is room to do so.

They include:

  • Rainwater harvesting – water butts have evolved, and in the South West we are installing dual-purpose rainwater harvesting tanks that provide both a free supply of water and storm storage capacity.
  • Detention basins – open areas of grass that are normally dry, except after major storm events. In heavy rainfall they fill with water for a short time before slowly soaking away.
  • Swales – long, shallow, grassed drainage channels that provide temporary storage for rainwater. They can be designed to promote infiltration into the soil where possible.
  • Rainwater garden  – a shallow depression with absorbent soil and planted with vegetation that can withstand occasional temporary flooding by rainwater from roofs. Property downpipes are often disconnected from the drainage system and redirected to these.
  • Raingarden planters – these special planters are attached to downpipes and include an overflow and good drainage. They can be planted with suitable plants and are self-watering.
  • Geocellular storage – installed beneath roads or areas of land and used to control rainwater surface water runoff either as a soakaway or as a storage tank.

The benefits of Sustainable Drainage Systems

SuDS are a natural approach to managing rainwater in urban areas and mimic the ways in which water drains away naturally. They:

• Slow down surface water run-off and reduce the risk of our sewer network becoming overwhelmed in times of heavy rainfall.

• Provide valuable habitats for wildlife in urban areas.

• Create green spaces for people in urban areas.

• Improve the quality of surface water entering rivers and streams.

• Can reduce the impact of climate change by lowering the temperature during a heat wave.

For more information about SuDS please visit

What can you do? Our top tips

You too can reduce the amount of water getting into our sewers and help prevent sewer flooding and pollution.

This will help to improve your local environment, as well as saving you water and money.

The downloadable guides on the right show you how to do some of these.

• Reduce the amount of water getting into our sewer system by using water wisely.

Top tip: Switching the tap off while you brush your teeth can save up to six litres of water each time. Click here for more water saving tips.

• Connect the downpipe from your rainwater guttering to a water butt to capture rainwater.

Top tip: You can use this to water your plants or wash your car.

• Reduce the amount of hard surfaces in your garden or yard by using rainwater planters or de-paving your drive. This will reduce the amount of rainwater going directly into the drains.

Top tip: The rainwater planters can be used to produce herbs and vegetables for easy use in cooking.

• Use permeable paving when renewing patios and drives which absorb rainwater and allow it to filter naturally through the ground.

• Slow the flow of rainwater getting into the sewer system by creating a rain garden (a planted, shallow depression) that rainwater can flow into.

Top tip: Plant up with native vegetation that is happy with occasional inundations.

• Blocked pipes can increase the risk of sewer flooding. Only toilet paper, pee and poo go down the loo and when cooking, put fats, oils and greases in the bin, not the sink.

Have a look at our Love Your Loo and Think Sink!  Campaigns for more tips on how to do this.

Why do we need to manage rainwater?

More people, more hard surfaces and more rain mean our sewer network is under increasing pressure.

During heavy rainfall, the network may be overwhelmed by the amount of water getting into it and we are at increasing risk of flooding from our sewers.

This can have a devastating impact on homes and businesses. Sewer flooding can also be harmful to the environment - if sewage gets into rivers or streams, it could kill fish and other wildlife.

Blocked pipes, caused by people putting unsuitable products down the toilet or sink, also increase the risk of sewer flooding. We continue to raise awareness about this through our campaigns, Love Your Loo and Think Sink!.

Flooding is often a complicated combination of factors that require action by more than one agency. A key part of Downstream Thinking is developing a partnership approach to tackling problems on a whole-catchment basis.