Why We Use Swales and How to Do It Appropriately

A swale is one of those permaculture terms that probably gets thrown around to readily (or inaccurately) and perhaps implemented irresponsibly. Only a few years into my permaculture career, I have certainly been guilty of this, and I have distinct memories of mistakes I made with regards to both attempting to construct swales and putting them in the wrong place.

In my defense, and perhaps to my own credit, I had read enough material about swales to get the general gist, but as I’ve learned since, a general gist doesn’t always cut the mustard. I put swales in beyond questionable locations. I was sloppy in my installation, leveling by eyeballing and ignoring contour lines. In short, I had the idea, but I lacked the execution. I missed the message on what exactly the entire swale-system accomplished and required.

For me, this never amounted to a major problem. I was working by myself with shovels and hoes, which meant the results were small and any failures easily remedied. But, I know now that I could have done a much better job, and I hope that, by sharing a bit of what I’ve learned, this might help to lead people towards more successful swales and, in turn, permaculture systems.

Why We Use Swales

Video: Harvesting Water DVD –The Swale Plume

There are a few reasons, with regards to water, that we use swales. By putting them on contour lines and giving them level bottoms, we are able to pacify water flows that might otherwise be destructive to the landscape. In this way, we are preventing the erosion, and we are capturing nutrients, silt, and organic matter. As that water is pacified, swales also spread it out over the landscape rather than having it concentrate only in specific topographical areas, and they hold the captured water in place until it is able to soak into the soil. Over the next several years, the landscape will become fully hydrated, which will make it drought-resistant, and the groundwater sources will begin to recharge. Swales, like any catchment, are a means of stopping water runoff, especially from roads and hard surfaces, and putting it to use rather than having it drain away. Using swales for these reasons can prevent both floods and droughts, which make them a pretty powerful tool.

Swales are also commonly used in conjunction with other water catchments, especially dams, ponds and gray water systems. They can serve a few major functions in these combinations. By attaching swales to dams, we are able to increase the water catchment area feeding the dam, such that the swale will fill the dam as with water that would normally go elsewhere. Swales also perform the opposite function, taking on the overflow of dams that have reached capacity and spreading and soaking the excess water across the landscape. In these overflow systems, swales allow for the specific placement of level sill spillways, which set the water level on the dams, and this allows the overflow to be passively released wherever is advantageous for design. Otherwise, spillways are simply put beside the dam wall and allow water to continue its journey downhill. Using swales can vastly improve the water availability of dams, as well as the usefulness of overabundant water harvesting in them.

But, water isn’t the only function of swales, and it is, in fact, imperative that they be regarded as a tree growing system. Swales without trees can possibly be even more damaging than the flow of water they have pacified. In some climates, they can potentially oversaturate the landscape, leaving a designer with difficult growing conditions. Trees, however, will moderate the saturation levels, utilizing the water deep into the soil as opposed to having it collect and cause problems. The other reason trees are vital to swale systems is that their roots stabilize the landscape, especially the berm, that loose pile of soil build on the downhill side of the swale, and the backside of the excavation. In effect, swales only make sense when they are used to cultivate trees, as in a humid climate they would probably overcharge the ground with water and in a bare, arid climate the system would likely fill, erode, and evaporate very quickly.

How to Use Swales Appropriately

Video: Permaculture Behind Greening the Desert with Geoff Lawton

While swales don’t have any particular size or measurements, there are certain rules that must be adhered to when building them. Swales—in permaculture terminology—are built on contour, which means they run level across the landscape. Even more importantly, they must be excavated to have level bottoms so that the water rests evenly within in them, soaking throughout the terrain rather than congregating in a particular area. The berm is on the downhill side of the swale and should be planted with both trees and groundcovers so that the soil is stabilized. The swale also must have a level spillway so that, in times when water is overabundant, it can release safely and passively in an appropriate location without damaging the berm.

Where we place swales is also significant, and it’s important to remember their relationship with water when doing so. Swales are not appropriate on steep landscapes. Any area with more than a fifteen-degree slope (about 1:3.75) isn’t appropriate for installing swales, as the water saturation may cause slides, which could be dangerous. However, like dams, the higher we can get swales in our landscape the more effective they’ll be, as the water will plume beneath them, continuing to use force of gravity to permeate the subsoil at a right angle down from contour (i.e. water can’t move uphill without a pump). Lastly, swales should have trees downslope to regulate the water and stabilize the soil, which means putting a swale just uphill of a building (and its foundation), with nothing between them, probably isn’t a great idea.

Lastly, swales are used to grow trees. Even in the case of using them to extend catchments for dams or to provide an overflow system, trees should still be planted, especially on the downhill side of the swale, and it is advisable to plant more just along the uphill edge or backside (nitrogen-fixers are great for this). When used for rehydrating and reforesting, swales can more or less be left to naturally fill with sediment and organic matter as the forest grows. By the time they are full, the forest should be fully hydrated and regulating its own water needs, and the soil will have plenty of detritus from leaf-fall to prevent it from eroding and to help it with soaking up moisture. Without the trees, the swale is missing and integral part and function, not to mention that trees are equally as important to maintaining water cycles.

Parting Thoughts and More Reference Material

Video: Bill Mollison shows swales near Tucson, AZ

As I said in the beginning, I made many mistakes when attempting my first swales, but because I tend to work slowly and by hand, nothing catastrophic ever came of it. Often I was building diversion ditches (the terminology gets muddle in the migration between the US and Australia), and these have slight pitches rather than level bottoms, such that they would slow and catch water like swales, even allowing some of it to soak in, but ultimately move it towards a particular destination. This was probably a good thing because the slope wasn’t always appropriate for what I thought I was doing. The “swales” I made tended to be short, not really all that useful for spreading water across the landscape, and they lacked the trees necessary anyway. But, I learned to move water around, gained a little trial-by-error experience, and continued to research, ultimately realizing the preliminary and finishing work necessary for installing them correctly.

Consequently, I thought it might be useful to others to share in this folly, discuss swales in basic but important terms, and provide some reference material for further exploration into the how it is and is not done.

Don’t Try Building Hugel Swales – This Is a Very and I Mean Very Bad Idea
To Swale or Not to Swale
How to Build a Swale on Contour Successfully
Infrastructure: Dams, Roads and Swales
Bunyap Water Level: Measure Contour Lines & Swales
Urban Swales
Experimenting with Overflowing Circles and Slow-Flow Swales (My first experiments)

Feature Photo: Courtesy of Brian Boucheron

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5 thoughts on “Why We Use Swales and How to Do It Appropriately

  1. Thanks for the article here. To add to this, many cross section diagrams you find of swales with trees show the tree either on or at the toe of the berm. In my experience, if the tree is to be managed or harvested as in a fruit tree, they are better placed a bit below the toe of the berm. I will align the anticipated drip line of the full grown fruit tree with the toe of the swale berm. Then use shrubs, grasses and clovers to hold the berm of the swale in place.

  2. Hi,

    Hoping someone can help with some advice.

    Can swales be used on a steep slope to stabilise the soil? Proposed area drops 12m over a run of approx 15m. The length of the proposed swale is over 30m.

    We are in the Nilumbik area and on an acre of steep, rocky clay soil, lots of native Eucalyptus.

    What natives and also edibles would be suitable, and what other considerations should we have in using swales as option. Not really keen on building a second great retaining wall of China in a beautiful bush back yard.

    Thanks in advance

    Kelly

    1. Hello Kelly, as stated in the article any slope greater than 15 degrees is unsuitable for swales. You could however plant trees on your steep slope to help stabilise it, this could be achieved by doing terraces and retaining wall type structures or by doing a net and pan configuration when planting your trees.

  3. Everyone seems to have their own opinion as to the functions and proper applications of a swale, and of the downsides. Even Mollison himself had shifting ideas. Those aren’t, I think, good signs.

    Some additional things to keep in mind with swales:

    In drylands: here, your best general water-conservation strategy for trees is to clump them into a little forest, where they can shade, wind buffer, and recycle transpired moisture with each other. Stretching them out in lines may be a bad idea for a small to medium sized orchard (or fuel, timber forest, etc.)

    In wet areas without an annual summer drought, you don’t really need to worry about watering your trees much – mainly weeding and mulching. Questionable use of time in such cases.

    In wet areas with an annual summer drought, you must therefore have a lot of winter rain – in order to still fall into the “wet” category. That usually means you’re looking for good drainage for your young trees, not water-logged soil. Here, swales won’t help you. Also, soaking in more water in the wet season will wash away more N, which tends to leach in these climates especially. If you do have a wetlands species, a simple depression – natural, or dug with a spade – will serve the same purpose as a swale (and what’s more, requires no calculations or risks).

    Then there’s the stories (mostly from tropical folks), that planting in swale depressions saves them water with their annual crops.

    But, a few weeks into the dry season, the plume – unless trapped by clay or humus – is out of reach of the short roots of most annual crops. If water is held by clay or humus, then your reduced irrigation needs have little to do with the swale itself (unless your swale was filling with runoff).

    Now, about runoff… Swales do “spread and soak” runoff, but people are often surprised to find that they collect less than expected. I think the main confusion is thinking that runoff is always a bad thing. Runoff really is only a problem in rather particular circumstances, and can often be solved fairly easily with trees, ground covers, organic matter or other soil amendments. Very rainy climates are usually quite lush and rounded landscapes, and there is already more rainfall than you can store. Here, runoff flows over vegetation and normally doesn’t pick up much sediment. In arid climates that have runoff, you need to capture and pacify it, but a gabion silt field would be better suited for such a landscape.

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