What are swales?
Swales are depressions in the soil that are often marshy, because they contain rainwater runoff. It can either appear naturally or be created. Most simply put a swale is a ditch used to harvest and distribute rainwater along a horizontal plain.
The basic anatomy of a swale consists of a ditch on a slope lined along with a mound on the downward side – called a berm – to keep the water in. Both sides of the swale must be covered in plant life to prevent soil erosion.
How can a swale help your garden?
- Swales can be used in conjunction with other permaculture techniques to capture runoff and spread it across the landscape to improve water infiltration in the soil.
- The quality of water will improve as a result of this, since it will first percolate through the soil. This improves the water’s mineral and microbial content which in turn provides better nutrition for your garden.
- Vegetation planted in and around the swale will benefit from the concentration of runoff. Trees and shrubs planted there will provide shade and natural mulch which will make evaporation slower and thus improve the amount of water absorbed. This is doubly useful for arid climates.
- Swales can also act as a filter for urban pollutants that rainfall absorbs, because of the fact water seeps through the soil before reaching your plants’ roots.
- Naturally all of this will result in optimum water efficiency, especially if you are building a xeriscape.
- Swales are really cheap to construct and require no maintenance once set up, which leaves you more money for other gardening supplies. With a small enough garden or big enough smartly situated swale you can potentially completely eliminate the need for supplemental watering.
How to build a swale?
1. Observe your Plot
To manage rainwater properly you want to hold it at maximum height in your landscape. This will provide maximum absorption efficiency. Typically runoff quickly escapes flat surfaces and barely any of it sinks in the soil. The swale will hold the water inside the trench and help it soak deep in the soil, minimising water loss due to runoff.
- To build one properly you need to observe how water flows in and out of your yard. Pay attention to the following things:
Where are the steepest slopes in your yard. These are instrumental, as swales need an incline to be build upon. Of course if one isn’t present you can create it artificially by piling soil in your desired spot.
- Learn the drainage pattern of your property. This is easily done by sitting in good view of your garden with a cup of tea during rainy weather and just paying attention where water gathers and where it flows out.
- Consider how much water your home’s roof captures and where your gutters deposit that water. This is helpful to have in mind when building the swale as this can be integrated in your planning stage to make the most out of rainfall. Think of your roof as one big funnel for rainwater.
2. Choose the Perfect Spot
Here are a few things to have in mind when choosing the site for your swale:
– Keep it at least 3 meters away from your house. Water must drain away from your home, otherwise you may have some problems with moisture, especially in your basement.
– The swale has to be at least 5 meters away from steep slopes or septic drain fields
– The soil at the spot you choose, or the soil you use to create an artificial slope for your swale must have good drainage to make sure water absorbs properly.
3. Mark the Land
Mark the contour line of your swale. You have to make sure it’s properly leveled so it will hold water without spilling it. The swale’s maximum capacity is as big as its shallowest end. This is why you want to make the depth and width as uniform as possible.
4. Dig the Swale
Now it’s time to get digging. You don’t want to make the swale too dip. A maximum of half a metre in depth is typically enough. You’d want your swale to be wider than it’s depth. This will make it easier for water to sink in the soil and enrich a wider area.
Mound the soil you’ve dug up from the trench on the downhill side to create the berm. Make sure it’s at least as high as the other side to hold water properly.
5. Testing and Adjustments
Once you’re done building the swale you have to test if the bottom is level. If there are uneven spots fix them. The most sure way to see if your swale is working properly is to observe it during rainy weather. If you can’t wait for such an event you can always test it by manually filling it with your garden hose. Although this option may not be entirely trustworthy as you can’t exactly replicate rain conditions with it.
If the swale overflows you can either make it longer, deeper, or wider. Another option is to dig another swale further down, and make a spillway in the top one’s birm. Lining this spillway with rocks will prevent soil erosion. This way the overflow will be captured in the second tier. Figure out which option is most applicable to your garden.
6. Start Planting!
Once your swale is complete and you are sure it won’t overflow you can start planting on it. The berm is the best place to do that. You can either use it for decorative vegetation or crop yielding plants. Perennials are the best choice. Fruit bearing trees are a great option as their foliage will shade the swale and slow down evaporation, as well as provide mulch and organic matter for it.