Water flowing down a hill that may erode that hill is not really termed a “problem”. It is termed an unused “resource”. If you can slow that water down and get it to hydrate the soil where it is at, then you prevent erosion and conserve the water for the plants. One way to accomplish this and tap that unused resource is to use a swale.
The definition of swale in Wikipedia is as follows: “A swale is a low tract of land, especially one that is moist or marshy. This term can refer to a natural landscape feature or a human-created one. Artificial swales are often designed to manage water runoff, filter pollutants, and increase rainwater infiltration.
Swales A Permaculture Practice
The swale concept has also been popularized as a rainwater harvesting and soil conservation strategy by Bill Mollison, Geoff Lawton and other advocates of permaculture. In this context it usually refers to a water-harvesting ditch on contour. Another term used is contour bund.”
Swales are used to slow and capture runoff giving the water time to soak into the soil, thereby hydrating the soil. This type of swale is created by digging a ditch on contour and piling the dirt on the downhill side of the ditch to create a berm.
In arid climates, vegetation (existing or planted) along the swale can benefit from the concentration of runoff. Trees and shrubs along the swale can provide shade which decreases water evaporation.
Swales are best when used on slope grades of 3 to 15%. Much less grade than 3% and the water is moving slow enough to to hydrate the soil already. Much more than 15% and it would be to steep to slow the water down and store which could cause the swale to fill and overrun and possibly wash away from the quick moving water. As a general rule, the steeper the slope the closer together and deeper your swales need to be.
Swales are most useful in reforestation of degraded, mostly-bare, arid or semi-arid hillsides, to direct water to trees. They also help immensely on soil with shallow depth to bedrock, or relatively impermeable and compacted soils because these type soils need the water slowed down to help hydrate the soil. A swale would usually not be needed in an established forest because the fertile ground and leaf litter will allow the water to soak into the soil.
For the Plants – Swales A Permaculture Practice
Perennials trees, shrubs, herbaceous plants and vines do best in swales. The varied root structures help hold the swale together and a swale will allow the soil to hydrate which helps the plants with deep roots. Annuals, hay, and pasture land don’t benefit as much from swales. They usually have shallow root systems so deep hydration is of little benefit.
You need to plan for what happens if your swale fills up. The recommended way of dealing with a full swale is to have a spillway channel in your swale that will divert overflow to a ‘safe’ area. While the berm in a swale is purposely not packed down so that plants will grow in them better, the spill way is packed down to allow water flow without erosion.
After building the swale, you will want to plant into the berm that is on the downhill side. You will want to plant a centerpiece tree, some shrubs and groundcover. You will want to have plants with varying levels of roots, both deep down and near surface roots. These varying root systems will help stabilize and hold your swale in place. After the plants are in, you will want to put mulch down on the swale and the berm to conserve moisture.
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Another type of swale is a diversion ditch. This is where you are trying to slow the water down and move it to or away from something. This could be away from a house or to a pond. The diversion ditch is usually at a 1% angle. This means that you have 1” of drop for every 100” of horizontal distance. Or it could be 1-foot of drop for every 100-foot of horizontal distance. At 1% angle you slow the water down enough that it will not allow wash away to occur.
Swales A Permaculture Practice
While actually not my swales because they are not on my property, they are swales that I helped dig and level. The pictures on this post are of a project I worked on at Elisha’s Spring Farm in WV in the spring of 2015.
We used a two bottom plow to dig a trench along a contour line. Orange marking flags and the laser level (discussed below) were used to mark the contour line. We actually did two passes with the plow to make the swale wider. After the two bottom plow went through, we used a lot of manpower with a lot of Rogue Hoes to shape the berm on the downhill side.
We used the Spectra Precision LL100N-1 Laser Level laser level to level out the ditch of the swale. This laser level has one stationary tripod base piece and one mobile receiver. A receiver is moved to a reference point for your initial level and all other measurements will be done based on your initial level. Then you move the receiver to other areas and move it up or down to figure out what actions must be done to get that spot level. Actions could be to dig down a little further or to add a little soil to build the area up slightly. The model I bought from Amazon is shown below. I will be doing a product review on this model in the future.
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After the swales were put in and shaped, chestnut trees were put in about every 6 to 8 feet. Then an annual cover crop with some perennials mixed in were thrown down on top of the soil to help hold the berms together. After the plants were in, we put up an electric fence to keep the cows out of the swales.
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Writing this blog post, Swales A Permaculture Practice, reminded me of all the fun I had last year working with permaculture.