In Episode 14 of the Water Matters podcast, Matt and the team discussed rainwater harvesting. They focused on “re-use” – capturing rainwater so that you can use it in place of mains water, reducing your water bills and environmental impact, and increasing your resilience to drought, hosepipe bans, and mains water failure.
But there’s another side to rainwater harvesting that’s worth talking about, and that’s how you can help slow the flow of rainwater into our drainage networks.
As our climate changes, we’re experiencing more extreme weather, which means more frequent and severe periods of drought, but also more frequent and severe storms. This winter has been gruellingly wet – England saw 35% more rainfall than average, and where I live in Cornwall, it’s been the second wettest winter on record.
One up-side to swollen rivers - it makes training more fun!
All this rain doesn’t just disappear once it’s fallen; it has to go somewhere, and that’s usually soaking into the ground and recharging aquifers, running off into natural watercourses like rivers, or being carried away by sewers and drains. In really wet weather, these systems can’t cope with the amount of water – the ground gets saturated and big puddles start to form, rivers burst their banks onto the surrounding land, and sewers and drains back up and surcharge onto roads, into rivers, and into homes.
Cornwall Wildlife Trust has recently reintroduced wild beavers to West Cornwall © Cornwall Wildlife Trust 2025
This has been in the news a lot recently, because water companies are discharging untreated water into watercourses more frequently than society is comfortable with. These companies get a lot of the blame for this because it’s ultimately down to them to make sure that their systems can cope with our day-to-day weather, but they’re facing a huge challenge. We’re urbanising rapidly – building homes, roads, and infrastructure to cope with growing populations – and all this development paves over green land with a high capacity to soak up rain, replacing it with impermeable surfaces like concrete and tarmac that water runs straight off.
You might have heard Episode 21 of the Water Matters podcast in which Matt and Tim talked about the amazing impacts that beavers can have on managing flood risk. They do this by building natural dams in rivers, which hold back water behind them. This gives it longer to soak away into the ground and recharge our aquifers, and it also holds back high volumes when the river is high and slowly lets it leak out once the storm has passed. This is great, but we can’t introduce beavers to build dams in our gutters and downpipes. What we can do is be smart about how we use our water-butts.
Your water-butt or rainwater harvesting system is just like a beaver dam - it’s a big storage volume that holds back rainwater and stops it from flowing straight off into drains where it can cause flooding and sewer overflows. But when it’s full, there’s no extra capacity in the tank to accept more rainfall. So what can we do about that?
The simple solution is to make your water-butt leak, so that it slowly empties over time. That way, it’ll always be opening up more capacity for severe rainfall. This is called an “attenuation system”.
This works brilliantly on downpipes that, for whatever reason, you can’t connect to your rainwater harvesting system. Maybe they’re on the wrong side of the house, or on an outbuilding. Stick a leaky water-butt on one of these, and you will be easing the burden on our drainage systems, helping reduce flooding, and reducing the carbon footprint of our water treatment systems.
In the winter, when I don't use my watering can, I open the tap a sliver to turn my water butt into and attenuation tank
But what about on the downpipes that feed our rainwater harvesting systems, like our water-butts or our below-ground tanks? We want to use the water in our rainwater harvesting system in our homes and gardens, so we don’t want it all to leak out!
There are a couple of ways to address this.
Option 1: Add an attenuation system that handles the excess from your rainwater harvesting system. The idea behind this is that when it rains, your rainwater harvesting system catches it and stores it for re-use, but when it’s full, the excess flows off into an attenuation system that slows it down to help the drains cope. The downside to this is that it takes up more space, and it’s more space that you could be using for your rainwater harvesting system!
Option 2: Use your rainwater harvesting system in a smart way. We know that we use use more water at home in the summer, and we get most of our wet weather in the winter, so in the winter you could open the tap on your water-butt a little when you won’t be using it to allow the water to slowly drain, then close it again in the summer when you want to start harvesting it again. There are even some systems that use weather forecasts to proactively empty rainwater harvesting systems – but it’s quite new and pricey technology without much use domestically.
The Flood Resilient Garden with it's active control tanks that make space in advance of rainfall © FloodRe2025
A great example of a system like this is FloodRe’s medal winning Flood Resilient Garden from the 2024 Chelsea Flower Show. The tanks and pond in this garden can hold more than 10 bathtubs full of water, and the tanks are fitted with a smart control system which can automatically empty the tanks in advance of storms, creating more space for water.
That kind of thing isn’t accessible to me in my little bungalow, so I’ve fitted attenuation taps on all of the water-butts on my downpipes. In the winter, I open the tap and let the water leak out, with the hose fed back into the drains. In the summer, I close the tap and store the rainwater for washing the car and watering the garden. As an added bonus, now when I look out of the window and see that it’s raining yet again, I get a little hit of excitement, because I know that the rain falling on my roof isn’t being wasted or causing problems, but is being stored and slowed, to reduce my impact on my local environment.
Ari Cooper-Davis is the Technical Director at Our Rainwater
Ari joined Our Rainwater in April 2023 as a Geospatial Data Scientist. He has recently completed his PhD at the University of Exeter, where he developed AI models to help combat sewer overflows - a topic close to his heart as a keen caver and sea swimmer.
Our Rainwater is here to lead the transformation of rainwater management, using a data-driven, innovative approach that unites individuals, communities, organisations, and businesses on a scale not seen before.
You can find out if you are eligible to take part in any of Our Rainwater’s water saving schemes by clicking on the QR code and registering your interest.
