Retrofitting a water butt as a ‘mini leaky dam’ for storm water attenuation
An easy win for those at home who would like to reduce their run-off to water courses from their roofs is to leave the tap open on our water butts and let them empty when the storm has passed. The water butt stores the water during the storm and discharges it slowly through the open tap to a flower bed. That’s all very well, but what if you are a keen gardener and conscious of last year’s long dry summer when a ready supply of water for watering plants would have been a priority for many, and you don’t want to have to try and remember to open and close the tap whenever it rains? As the spring is upon us, how can one balance a handy supply of water close to the flower beds against a need to try and reduce our run-off?
A simple answer is to modify your water butt by adding another tap halfway up the butt. This will allow you to store the bottom half for watering the garden and use the top half with the tap left open for storm water storage. In extended dry periods like we had in 2018 one could of course close the top tap to collect more water from the odd shower, but when heavier rainfall events are forecast remember to open it.
How to retrofit an extra tap
You can buy a tap for about £4.00 from DIY stores or garden centres, the taps come with a rubber washer and a plastic nut. All you need is a drill and a hole saw, you could also use a speedbit (as shown in the photos) with a bit more care. I modified three of my water butts in this way and it took about an hour to complete the task. I used a speedbit and I found you can get a slightly odd shaped hole if you operate the drill at high speed, a better result is obtained by slowing the speed of the drill. The hole diameter I used was 25 mm, I then placed the rubber washer over the threads and for a better seal I wrapped the threads with about three turns of a roll of PTFE, which you can buy cheaply at any plumbers merchant. The tap is then simply pushed through the hole and the plastic washer tightened by hand on the inside of the water butt. Collecting water in this way is easily done by also modifying your downspout with a rain diverter device (about £6.00 from DIY stores). When the water butt is full the device continues to discharge water to the drainage system.
The maths: this can slow the flow of 20% of the water that lands on your roof
A typical water butt has a capacity of 210 litres or 46 gallons. If we can store half that, say 100 litres, during a storm, what might the effects be?
Let’s assume the water butt collects water from 20 m2 of roof, which is fairly typical for a terraced or semi–detached property. For a fairly heavy downpour, let’s say with a 30 year return period, we might expect 50 mm of rain per hour, but that intensity is more likely to fall over a shorter period of say 30 minutes.
Over 30 minutes we could expect around 500 litres of rain to discharge off this typical roof, with the first 100 litres captured in about 6 minutes by your modified water butt, leaving 400 litres to discharge to the drainage system. The approximate discharge rate from the open tap is about 0.15 litres per second. Simply put, our water butt has delayed the delivery of the stored 100 litres of water to our rivers by some 667 seconds or 11 minutes. However, this assumes the water discharging from the tap goes straight to the drainage system. If instead it discharges to a ‘rain garden’ (can be as simple as a soggy flower bed), this period is massively increased. Even if it discharges to the pavement outside your home, it still has to make its way to the drainage system, so whichever way we look at it, 1/5th of the run-off from your roof is slowed from entry to our river system.
If in winter both the top and bottom taps are left open, then the effects double to 2/5th. If everybody did this then the effects would be cumulative and might just help us avoid a flood at very little cost indeed. With the two taps system, keen gardeners are also kept happy!
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Stuart Bradshaw BSc(Hons) MSc DIC CEng MIStructE M.ASCE FGS