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How to Build a French Drain


Water management is an important aspect in the design of a sustainable garden, for without water nothing grows, as I’ve said before, it’s virtually the very essence of life itself!

In Permaculture, designing a garden to make optimum use of water is critical, especially in a country like Australia, the driest continent in the world, where there is a shortage of water more often than not.

French drains are one of the many tools utilised in Permaculture to best manage water.

Before going into the details of the construction of French drains, it’s best to understand the very element we’re dealing with – water!

The Permaculture Perspective on Water

Water is one of the critical design elements in Permaculture, we work on capturing it, using it in the most efficient way, re-using it where possible, and dealing with waste water in a sustainable way.

Without understanding how the nature of water and how it behaves, you cannot create an efficient Permaculture garden design. To understand the use of French drains in gardens, we’ll cover some water basics:

    1. Water always seeks out the lowest level – water will naturally flow downwards due to gravity, and will run from higher to lower levels until it settles down at the lowest level. This may seem quite obvious, but we need to consider what this means in the context of nature, soil and plants.Consider rainwater falling on a mountain side, it will trickle down the slope quite quickly, picking up loose soil, debris and leaves and other organic matter, and carry it with it down the side of the mountain. The land eventually levels out into a valley, where all the nutrient carrying rain ends up, resulting in a moist, fertile valley, which also probably has a lake at the bottom. The plants in the valley have all the water they can use as it seeps deep into the soil, where it can be accesses by the roots of plants and trees. Conversely, very little water has soaked into the mountainside, as it runs along the surface, with very little soaking into the soil. Furthermore, soil and nutrient rich material is potentially stripped away.If the water doesn’t flow into a valley, it forms a stream, which flows into a creek, which flows into a river, which finally flows into the sea – and with sea level being the lowest level, the water finds its way there, as it naturally does.
    2. Water takes the shortest path to the lowest level – water flowing down the side of a hill or mountain will take the most direct root to the bottom, as will any running water, such as a stream or river. By taking the shortest route, it makes the minimum amount of contact with the soil, and therefore soaks as little as possible into the soil on its downward journey to the lowest point.
    3. Water is strongly absorbed by composted organic matter – humus, which is the result of the breakdown or plant matter, makes for a rich, dark soil with good water holding properties. It is what is termed hydrophilic, that is, it draws and holds water. Soils that are dried out, leached, with little organic matter left in them don’t hold water, it runs off the surface, these are termed hydrophobic, that is, they repel water. If you’ve watered an area of depleted soil, only to dig into it afterwards to find it is bone dry underneath, then you’ve seen hydrophobic soil in action. If you want your soil to make the most of available water, then add lots of organic material to your soil, it’s that simple.
    4. Water can be “stored” in the soil – since rich, fertile soil has the ability to hold water, it can carry a certain amount of water between watering or rainfall, ensuring a constant supply of water to the roots of plants and trees by acting to buffer the cyclical nature of rain and dry periods. As water soaks into the soil, it still moves downwards into the water table, flowing into underground streams and aquifers, seeking the lowest point, as it does above the ground. But, being under the ground, it moves more slowly, so you have a slow moving water storage system that plants and trees can tap into as it slowly drifts past. So, in addition to storing water in tanks and dams, we can use healthy, rich, fertile soil with lots of organic matter in it as a secondary water storage system to increase the amount of water we can store.
    5. Water flow can be slowed down – water flowing down the side of a mountain over dry, eroded soil will run straight down across the surface and not soak into the ground at all. If we were to alter the path, to give it a “scenic route” back to the lowest point, making it flow sideways (horizontally), to cover the longest path possible before it reaches the bottom, then we maximise the amount of time it is in contact with the soil. If the soil is rich, fertile, water absorbent soil, it will draw most of the water in like a sponge, so very little will get to the bottom.How do we get water to flow sideways down a slope you may ask? Simple, we just dig trenches along the contours of the slope – that is, we can simply dig a series of horizontal trenches along level lines across the slope. To explain contours better, think about the lines on a map that show you the elevation and indicate slopes, hills and gullies. Each contour line is at the same level along its entire length on the map. If you dug your trenches at the same depth on spots marked by the contour lines on the map, the bottoms of the trenches would all be perfectly level, so water would sit at an even height if you filled them with water. These contour trenches are called swales in Permaculture.

      With swales, the dirt removed from the trench is banked up on the downhill side to create a swale mound or embankment, called a berm. The berm is usually planted up with ground cover plants, and nitrogen fixing plants. When these plants are established, the fertility of the soil is improved, and then a row of trees (usually food producing) is planted there. The water soaks past the tree roots as it moves down the slope through the depths of the soil. The swales will also collect any soil that is being eroded from the slope, and slow the flow of water down to dramatically reduce soil erosion.

Naturally, swales are more at home on rural hillsides rather than in urban back yards, though you can utilise the same concept on a reduced scale, by creating a small trench on a slope to prevent run-off. For those of us that have concrete in our backyards, which has 100% runoff and doesn’t retain any water, there is still hope! The next best thing to a swale is a French drain for capturing runoff and redirecting it back into the soil.

In the instructions below, we’ll go through a real-life example from my backyard. So, lets have a look at French drains!

What is a French Drain?

A French drain is basically a long trench dug into the ground that is filled with gravel to redirect water away from an area and carry it elsewhere. A French drain can have one or more perforated pipes (slotted Ag agricultural drainage pipes or “slotted Ag Pipes”) running along the bottom to improve drainage and carry away water faster.

Traditionally, French drains are used to carry water away from building foundations, or to function as an outlet for septic tank sewage treatment system, distributing the water over a large area so it can seep into the ground.

As you can see, it’s a fairly simple concept. Now, lets have a look at a real-life example in terms of design and construction.

How a French Drain Works and How to Build One

Pictured below is the rearmost part of my backyard, with the back fence in the background. The block of land is sloped, highest point of elevation at the back, the front of the house is the lowest point.

Now, to explain the problem here…

The blue arrows show the way the water flows when it rains. It basically flows over the concrete area pictured, down the concrete steps, down the walkway at the side of the house, then down the driveway, by which time it has travelled over thirty metres to the footpath, where it runs into the stormwater drain.

You might think this is insignificant, considering that it’s only a small space above the step where the clothesline (the green post in the middle of the picture) sits. Firstly, there’s one more factor here which needs to be considered. Now, you can probably see a line of pots along the back fence. These all run trickle irrigation, and when watering is completed, the water (carrying nutrients from the soil in the pots) runs from the bottoms of the pots down the slope to the stormwater too. Better if this was redirected back into the garden – it would save water and recycle nutrients.

If we take a step back to the question of rainwater, the small space we are concerned with here spans an area of approximately 3.5m x 2.5m = 8.75 sq. m.

If we remember our rainfall formula: 1mm of water over one square metre of roof (or surface) will give one litre of water.

In Melbourne, our relatively dry climate gives us around 700mm of rainfall per year, so each square metre of surface gives us 700 litres of water. Now, if we look at our seemingly insignificant patch of concrete, and we do our maths, we find it captures 700mm x 8.75 sq. m = 6125 litres in rainwater alone (not including irrigation run-off from the pots).

Now, the question is, would the 6000 litres of rainwater be better off redirected in my garden, or washed down the gutter into the stormwater drain. Unless you live in a swamp, the choice is simple! So let’s get building!

Step 1. Mark the concrete for the cut

Cutting concrete is a dusty and messy affair, so it’s best to mark where you want to make the cut. In the picture below I’ve marked the cut location with the dashed red line. In this example, I made the trench 15cm (6”) wide.

To mark the cut, the traditional method is to use .a builder’s marking tool called a chalk line. Pictured below is a chalk line and a bottle of “chalk”. It’s not really chalk, it’s just called that, and it comes in two varieties, temporary (washes off with water) and permanent (never comes off!!!) use the temporary one in case you slip up and make a mistake. Chalk also comes in a wide range of colours too, so just pick something you can see well!

The chalk line is a string on a winder in a container of powdered “chalk”. You fasten one end of the string down where you want to start the cut, either by pegging it down or simply getting another person to hold it down, you unwind the chalk line to where you want to finish the cut, and you stretch it tight. To mark the concrete, simply pull the taut string upwards like the string of an archery bow, and let it go, so it hits the ground with a hard “snap”, marking the line with the chalk. Do it a few times if you need to to get a clearly visible line. I do wn a chalk line, so I used it to mark the line, it takes a few seconds only.

If you don’t have a chalk line, you can do the same with a straightedge (a long, straight piece of anything), and some blackboard chalk.

Step 2. Cut the concrete

This is the hard and messy part. The professional method of cutting concrete is with a demolition saw. It’s like a big chainsaw motor with a huge cutting disk which is used to cut doors and windows into concrete walls and the like. they can do a wet cut by having water feeding into the cutter to control the dust. These big brutes are usually hired, and they can cost a bit, so you wouldn’t hire one for a very small job. Best to check to see what the daily hire rate is. Incidentally, this is what a demolition saw looks like.

Now, it wasn’t cost effective to hire one of these for a single 2.4m (8 foot) cut, so I did it the cheaper, but harder way, I borrowed a friend’s 230mm (9”) angle grinder and equipped with a masonry cutting disc, which can cut concrete. Here’s one pictured below.

Now, since it’s electrical, you cannot use water when making the cut! And as a consequence, it will kick up a hell of a lot of dust (powdered concrete), and if you think it can’t be that bad, let me reassure you, you’ve never seen this much grit and dust, ever! It will go everywhere, and if you’re not covered, head to toe, in some kind of clothing and covering, you’ll look like a statue from the dust. Needless to say, protective eyewear, hearing protection, and a respirator are a definite must. The concrete dust is seriously bad for your eyes and lungs, and the sound is deafening, not to mention that it will take ages to agonisingly cut your way along a thick piece of concrete while holding this heavy too perfectly straight.

You only need to use these two monsters if you want a neat, straight cut. If you don’t care, you can cut the concrete anyway you choose, Doing it with a club hammer and cold chisel, and prying out the bits with a wrecking bar wirks too, but is even more laborious, but not messy.

I guarantee you that after you complete the concrete cutting, you’ll wonder why we ever decided to cover all the spaces we habitate with this god-forsaken substance!!!

Step 3. Lay drainage pipe (optional) and fill trench with gravel

Anyway, after all the drama and commotion, you’ll hopefully have a neat and straight-cut trench. Just dig out the broken bits of concrete, and dig out the soil till it’s the required depth. I dug mine so it was 15cm (6”) deep. You can then lay in the slotted Ag pipe (agricultural drainage pipe) and then fill the trench with gravel, or leave the pipe out and just fill with gravel only.

Ag pipe (pictured below) greatly assists drainage, use i if you feel you need it, it is cheap, and will allow you to use less gravel when filling the trench. Make sure you use the slotted variety, it has holes or perforations between the ribs all along the walls, so water flows into it and through it very easily.

Once the gravel is in, level it with a flat piece of wood, a spade or the back of a rake. Your French drain is now complete.

The Results

This is the finished French drain, quite unspectacular, but very effective.

If we retrace the water flow, as we did before, with the blue arrows, we see how the water is redirected sideways along the trench.

Here’s a closer look at the French drain.

Obviously, the ends of the drain have to go somewhere! Here, you can see the right hand side flows under one of the mulched paths into the garden beds.

Water flow into the garden beds is shown by the blue arrows.

And the other side of the French drain also flows into a garden bed, right where the lemongrass is growing, it appreciates the extra moisture.

In summary, I will admit, cutting the concrete with an angle grinder was absolute hell, but it’s definitely worth it when I consider that the garden gets an extra 6000 litres of water a year from this small piece of work. It definitely goes a long way to making the garden more resilient and drought-proof, and saves quite a bit of water in the long term. Furthermore, just because we can’t dig swales in urban permaculture settings, doesn’t mean we can utilise the same principles in creative ways.

Creative solutions are what permaculture is all about, so step into your creativity, and don’t be afraid to get your hands dirty, the world can only become a better place from your efforts!

Want a more decorative looking French drain? No problem, you can plant it up with the right type of plants, so the water drainage won’t be affected and weeds are also prevented from growing. See what this French drain looks like planted up in my article Lawn Alternatives – Creeping Thyme.

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