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Writer's pictureMelissa Irwin

Building Backyard Swales

Updated: Apr 4

Building a Backyard Swale


I completed this project back in the fall and I'm excited to be able to finally share the full story of how we created a micro-swale and began our project to hold and sink runoff in our backyard garden. Let's start off with a quick explanation of swales and what they are: A swale is a berm made of soil that is built on contour to collect and store water as it runs off the surface of the ground. This excess water will pool up in the swale and then in the minutes, hours or even days to follow the rain event, the collected water will sink down into the soil and recharge the underground aquifers. When done strategically, building a backyard swale can be a passive way of watering your gardens, trees, shrubs etc. even to a point where manual watering can become unnecessary. Read on and see how a swale could revolutionize the way you garden in your backyard!


How to build an underground swale in your backyard
Swale filled with large stones before covering with mulch

Research Your Design


So first, for a swale to be effective, you will need a slope. If your property is completely flat, a swale will not work for you, but fortunately, most properties have changes in elevation even if they are slight. Our backyard might appear relatively flat at first glance, but upon further inspection, one would notice that there is a gradual slope down away from the house. A swale built on a steep slope will need to be more significant in size than one built on a shallow slope although any bumps or ripples in a landscape will have some effect at slowing and sinking rainwater.


The next step for planning your swale is to take note of where the buildings on your property and the surrounding properties are located. The water collected in the swale will be stored on the DOWNHILL side of the swale. For this reason, swales should never be placed on the uphill side of a building. Improper placement could result in runoff being directed into a basement or foundation. As a rule of thumb, swales should also be placed at least 10 ft away from any structures.


Take into account the flow of water on your property. It is important to know where your runoff is coming from as well as where you would like it to be stored. With the swale I built on our property, it was my goal to collect as much of the roof runoff as possible and to store it in the perennial and annual garden beds near the house. In order to do this, I needed to know which direction the runoff was flowing. I also needed to be sure the swale was directly above the main garden bed so that the stored water would end up in the soil beneath the plants. If you aren't sure of the direction of your runoff, grab a raincoat and go outside in a torrential downpour. Sometimes direct observation can be a critical component of design.


Once you know where you would like to build your swale, you need to find the contour line that it will be built on. The easiest way to do this is to grab some scrap wood and a level and build an A-frame. Using nails or screws, build an "A" out of the scrap wood. The distance between the legs should be at least a couple of feet apart. If you are building a really long swale, you may want to make a bigger A-frame to save some time. You need to be sure that the legs beneath the cross bar are exactly the same length. Attach a level to the cross bar and make sure the bubble is in the center while the A-frame is standing on a level surface. Start at one end of where you would like your swale to go. Mark the corner with a rock or a flag. Any kind of marker will work. Place one leg of your A-frame on the starting point and move the other leg to different positions until you find the spot where the bubble on the level is in the center. Place another marker by this other leg and then repeat by starting at this new spot. Continue until you have marked the whole length of your swale. The contour may surprise you! I wasn't expecting mine to be shaped exactly how it ended up!


Build Your Swale


This is the part where it became more challenging for me to find information on what I was attempting to build. Most guides and tutorials I found online and in books were referring to large scale swales. Even the ones described as "small" or "backyard" were bigger than what I was planning. It is also much more common to find information on building open swales. These would be planted with grass or some other shallow rooted ground cover and would fill up like a pond during rain events. We have very limited space in the area where our swale is now located so having a grass covered ditch seemed inconvenient. We were also only attempting to capture a limited amount of rainwater. Large scale was simply not necessary.




Trench and berm for backyard swale
Swale dug on contour

What we came up with was a mixture of several techniques and so far it seems to be working well! Once the hot days of summer arrive, I will update this post on whether or not the swale has cut down on how much we need to water the garden. First, I dug a trench on contour. There is a second rainwater capturing system that I am installing, so I made sure the corner or my swale was fed by this other small drain. I dug the trench approximately 2 ft deep and 18 inches wide. We are fortunate to have very good soil in our yard but of course, the spot where I dug the trench was full of clay. Go figure! We piled the soil from the trench on the downhill side, following the same contour. This component of the swale is called the berm. Next, I filled the trench most of the way with rocks. We have an abundance of rocks from another project we have going on, removing an old cement foundation at the end of the yard. It was nice to find a way to use some of them! The rocks are there to help hold the trench open so that it can fill with water despite being covered by the next layer which is mulch. We also have an abundance of mulch from an old tree we had cut down this fall. It was great to find uses for all of these free resources! The mulch that now covers the swale can hold up to 10 times its weight in water! I like to think of this as a water battery, storing water for the dry days of Summer. The mulch also serves as a walking path for us to access the garden beds and to have a barrier between the perennials and the lawn.



Underground swale filled with stones before covering with mulch
The swale is filled with rocks to leave space for runoff and maintain the structure

As a finishing touch, we placed a newly built cold frame right on top of the berm. My hope is that the swale will help to keep the cold frame moist from underneath, combatting the fact that covered growing systems such as this are always protected from rain and dew. This may not be the outcome since the cold frame is on top of the berm rather than on the downhill side, however it was worth a try!


When Spring arrives, we will plant perennials around the cold frame and on top of the berm to hold the soil in place. These will need to be shallow rooted plants so that they do not undermine the structure of the berm. On the downhill side of the swale is the main vegetable garden which will hopefully be the recipient of all the stored rainwater! It can take several years for groundwater to be recharged to its deepest level. We hope to see positive effects for years to come! Stay tuned for updates!!




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