rain gardens

Rain gardens are a whole new philosophy on how to deal with water in the environment. In the last five years, there has been a big rise in the use of SUDS (sustainable urban drainage systems), the practice of delaying the entry of rainwater into the drainage system by the use of swales, ditches and ponds. However, this is generally the domain of engineers who are only concerned with their pipework; rain gardens, on the other hand, do the same thing, but are equally concerned with aesthetics and ecology - and so are far more exciting. Easily applied to the domestic situation, but the concept works just as well in urban and commercial design.

Having built many water gardens in my life, I decided it was time to build a rain garden, in my own home, where I can enjoy it and also monitor its performance. These pictures show the just-completed garden, only one day old; it also rained right on cue and appears to be working well!

So what is the “philosophy” of a rain garden: why build one? Well, flood prevention is one answer; if you have ever experienced floods in your area, you have directly or indirectly contributed to them. If the rain didn't fall on your actual roof, it fell on part of the urban fabric that has been built to support you. Another answer is to re-charge ground water supplies; many urban and rural areas have groundwater levels that are dropping due to the fact that rain cannot permeate the land where it falls and also because we tend to be abstracting water far more quickly than it is being replenished.

Rain gardens are a great way to re-connect with nature, opening you up to the experience of natural rhythms and process. It will sit there quietly in hot weather, dry, yet still a micro climate for flora and fauna that like a little extra moisture. When it rains, though, the garden comes to life; water from the roof of your house, instead of disappearing down the drain, starts running into the area of dips and dry ponds you have created, perhaps having topped up your rainwater butts first. Gradually pools start appearing and maybe in a heavy downpour, water starts running between them. How long it then takes to dissipate will depend upon your soil type; I'm on an alluvial soil, so it is very free draining; on heavy clay it might take days for the water to disperse, and this might mostly be from evaporation. This is good too as it helps re-charge the local hydrological cycle, which is also severly lacking sufficient moisture content, and may well be the true cause of global warming. If you have limited space or can't allow water to rise beyond a certain level (after all, you don't want to move the flood potential from somewhere further away, to your own home!), then you might need an overflow which puts any surplus water back to drain, or perhaps (and preferably) to another part of the garden. You will have still considerably delayed the timing of water going to drain, as well as the volume.

In the garden illustrated, I have disconnected one of the main roof downpipes (which it turned out was blocked) and used an old steel channel I found on a demolition site. We have old cast-iron downpipes so I bought a 90° bend (my only cost on this project - £16 - I already had everything else, one of the benefits of being an ex-landscaper!) and fitted that to direct the water into the chute. I then dug a channel and partly lined the bottom with plastic, because our ground is very free draining and I wanted to connect this to an existing small water feature, so that this was topped up by rainfall. Surplus water is then dispersed to the sides, through the planting. If I were designing this from scratch, I would put the pond before the raingarden, so this was topped up first. Thaving said that, this section of the garden has always been incredibly dry and I'm hoping that the ground will, over time, recharge itself and things will grow better. This dryness is evidenced by the fact that we have a young fig growing well, right by the downpipe, but I'm hoping that area will remain relatively dry still!

In periods of heavy and prolonged downpours, it may be that the pond will overflow; this will happen at the back and will disperse out away from the house under the bushes. With our soil, I don't see the need for any further overflow drainage.

The roof section that feeds this downpipe is about 50m2, south facing. We get on average 50cm rain per year, so this should capture 25m3/year. This morning in light/medium rainfall, the chute was delivering 3 litres/minute (nowhere near the rate of a hosepipe). The rain garden is about four metres long and I'm not sure how to measure the drainage rate of soil, apart from having the plasticity index measured in a lab (clue: we have gravel pits close by!), but over time I will use these figures to try and calculate how much water is passing through the system; in theory 25m3/year! Of course, the larger part of this will fall in the winter.

When I first built it and tested it with a hose, the family wanted it to run all the time! This however, is not its purpose and we have a bubble feature that runs all day. The whole joy is that this is dependant on the weather, something we usually just complain about (we should realise how lucky we are to live in such a green and moisture-filled land), and turns into an unexpected source of joy.

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I intend to do a construction note on this subject and re-post this whole section of my website, which is not up at the time of writing. I also intend to write a design note on the same, but from a slightly different perspective. Finally, I have written a book review on this subject.

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