French Drains for Urban Storm-water Infiltration
Biological Cleaning, Conservation, Storm Water, Waste Water, Water Harvesting — by Campbell Wilson July 9, 2009
When rainwater harvesting is mentioned, most people think of tanks straight away. That’s a great start, but there is a much bigger storage you have available to you on your land – the soil. (Check out Brad Lancaster’s fun U-tube video on this site, ‘The muffin tin and the sponge’ for a good intro)
So when your gutters flow and your tanks are full, don’t send all that valuable water straight the drain like so many do. Instead, have a go at utilising that water in your garden by creating a system similar to this.
Rural and semi-rural permaculture systems are often built with swales incorporated. A swale is a water harvesting ditch on contour which intercepts runoff, then fills up and holds onto the water long enough for it to soak into the soil for the use of tree crops (You can see plenty of examples of swales on this website).
Swales are fantastic, however, in an urban situation not everyone wants to have a garden that resembles a motor-cross track, nor gaping wide trenches that Granny can get lost in. The French-drain design explained in this article is an alternative, which has the same benefits of a swale, whilst being pretty much invisible.
Designing
Just as with swale systems, whenever we are infiltrating large amounts of water into our soil, it’s very important to include trees as part of the design; if not, you risk water logging and quite possibly salting the landscape.
Don’t just wack one of these in; you can put a whole series of them throughout your forest garden for example. When you’re deciding where they’ll go, it’s good to be aware that most fruit trees like to drink and eat at their dripline, so therefore this is where it makes sense to position your trenches. For young trees you’ll need to decide where the end dripline will be and position them there. It’ll take a while for the young tree’s roots to reach out to the moisture you’re infiltrating for them, so in the meantime, you can dig a finger off the side of your trench to direct water closer to it’s young root system.
Gravity is your best friend when playing with water, so start off by directing your downpipes or tank overflow to the top corner of your land through some 90mm PVC pipe (even if you think that your land is flat, there will always be some fall. If not, you’re either living in the middle of a salt-pan or an indoor basketball court). From here, work your way down the hill and position a trench wherever you have trees and shrubs that can utilise the moisture. Just make sure that you’re infiltrating water at least a few metres away from building foundations or else you can undermine them.
Size and shape
Each trench should be about 50 cm deep. Any more than that and you’ll be infiltrating the water below the main feeder and drinker roots of your fruit trees.
If your land is relatively flat, you can get quite creative and make them basically any shape you like. However, if you are on a slope, you’ll need to make sure that they are positioned across the slope, that is, on contour like a swale. Just remember, the bigger your trenches, the more water it can hold, and therefore, the more water it can infiltrate.
The construction is quite simple as the illustration shows:
90mm PVC pipe. Begin with some 90mm PVC pipe heading from your water source with an open 90mm Tee pointing downwards at the centre of your trench. The top of this pipe should be about 10 cm below the surrounding soil level.
Reln drain. Over the top of the PVC pipe you place an impervious reln drain with a gap of about 5 cm above the pipe (a reln drain is a corrugated half-pipe about 40cm in diameter and comes in 1.5m sections. They’re designed for use in septic tank leach fields and cost about AUD $15 at plumbing supply places). The reln drain should be well supported using old blocks or bricks to prevent it sinking over time, and to prevent pressure on the PVC pipe if it’s walked on.
Budgie wire (the red line on the picture). This is placed over the base and ends of the reln drain and also at the opening of the tee. This is to prevent the mulch (which the trench is filled with) from floating up and entering the pipe.
Old plastic (the yellow line on the picture). This is used to line the base of the trench, which encourages water to infiltrate laterally towards the trees’ main drinker roots, rather than straight down.
Mulch. The trench should be filled with either coarse mulch or aggregate. The purpose of the reln drain is to create an air gap around the pipe, which stops tree roots from clogging it up (see the front view below). If you decide on mulch to fill your trench, it will need to be topped up from time to time, but will eventually become a lovely big humus sponge. If you decide on aggregate, you’ll need to line your trench with Geo-textile matting to stop tree roots and soil from clogging it up.
So basically, what happens is the water flows along the pipe until it comes to your open tee where it has no choice but to pour out into your trench.
Once the trench fills up to the height of the tee, the water will then continue along the pipe until it gets to your next trench which it will dutifully fill etc etc.
On urban properties, a lot of water can run off all those hard surfaces during a wetter period or during a large rain event (A note to Victorians: They will come again one day, I promise), far more than our gardens can infiltrate. So it’s really important to be mindful about the possibility of flooding your neighbour’s or even your own house.
To avoid this problem (quite possibly very costly), once you’ve caught and infiltrated all the water you can/need, you’ll need to divert it back into the stormwater drains before it rushes onto buddy-next-door’s place (by law in Oz, you’ll need a plumber or landscaper to do this job, but before you do, have a chat with your neighbour cause he might like to use the runoff himself).
Similarly, there will be times when your soil is already as wet as it needs to be and if you send in much more, you’ll just end up making your soils anaerobic and you’ll also leach valuable nutrients from your system. To avoid this, plumb your system so that during these wetter times you can just send the water down the drain.
Good luck and happy drought-proofing!
Cam Wilson, www.forestedgepermaculture.com
Comments (2)
I’m already doing a similar system for my Vegetable patch. the paths between the beds are all shallow ditches whic coarse mulch in them. I’ve then diverted water from the downpipe onto the path area.
Now if only i could get some decent rain to give it a good test.
Comment by Geoff Griffiths — July 10, 2009 @ 2:11 pm
How about fitting a float operated valve so that once the pit is full of water the float valve sends the water back into the storm water drain.
Comment by Hamish — July 15, 2009 @ 9:04 pm
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