A permaculture swale is a tool for capturing and storing water in a garden. Learn how to build a swale in the residential landscape.
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I’ve written about swales before: If you missed it, see What is a swale and why you need one.
This article is about the technical details of a permaculture swale: How to choose the best site and how to build it.
A swale is a shallow trench dug out (dead level) along the land’s contour, with a berm on the downhill side.
They can be important features in an earth-friendly yard because they help slow, store, and spread stormwater so as not to overwhelm the sewer systems.
For the gardener, a swale can be more efficient at storing rainwater than rain barrels (when used appropriately) and requires no engineering or mechanical skills to build.
Constructing a Swale in 7 Steps
Step 1: Observe water on your site.
The goal of managing rainwater is to hold it as high in the landscape as possible for maximum efficiency and absorption.
As water flows across a typical surface or landscape, most of it will run off quickly without having a chance to be absorbed by the soil. With a swale or other earthwork such as a rain garden, the water flows into the trench, where it slowly soaks deep into the soil. This produces little to no runoff.
Collect information about your yard or property of interest. Where is the water coming from and to where is it going?
Write down the following observations.
- Identify where water is wasted.
- Observe drainage patterns
- Locate steep slopes
- Determine annual rainfall
- Calculate roof water catchment area (if applicable). For example, my roof area is 1,200 square feet.
Alternatively, follow my instructions for 6 maps to draw for the permaculture farm design.
Step 2: Identify the ideal site.
Rules for siting a permaculture swale:
- 10 feet away from a building (water must drain away from building)
- 18 feet away from the edge of a steep slope or septic drain field
- Uphill from a low spot that doesn’t drain well
- An infiltration test demonstrates an infiltration rate of at least 1 inch per hour.
This may not be an appropriate strategy for areas with a high water table or on extremely steep slopes.
Step 3: Mark the contour line.
Use a contour level and utility flags to mark the contour line every 6 feet. This video will show you how to use a homemade A-frame level to mark the contour lines.
Step 4: Dig a trench along the marked contour line.
- Typical trench depth: 6 inches to 1.5 feet deep
- Typical trench width: 18 inches to 2 feet wide
- Length: Varies by your needs, size of the space, and how much water you may catch
Step 5: Mound the soil from the trench on the downhill side to create a berm.
Take the soil dug from the trench and place it on the downhill side.
Step 6: Test and adjust the swale.
Work the A-frame level along the bottom of the trench to test whether it is level. Fix uneven spots. Next, observe the swale during a heavy rain event. How did it do?
If it overflowed, then do one of the following: Make the swale deeper, wider, or longer.
If that isn’t possible, then consider digging another swale 18 feet or so below that one.
Multiple swales are connected by a spillway, which is a notch in the berm where overflow can be directed. Lined it with rocks to prevent erosion as the water spills from one swale into the next.
Step 7: Plant the berm.
Plant the berm with perennials.
(To name a few of my favorite ideas.)
Would you like to learn more about using earthworks like swales to reduce maintenance and increase yield in your productive landscape?
You’ll find loads of information just like this in my book, The Suburban Micro-Farm.
Swales can be aesthetically pleasing, if necessary, for the residential yard.
Fill the trench with gravel or wood chips and you have a walkable pathway.
Give the berm a rock border, and you have a raised bed.
If your swale does not need to be walkable, consider seeding clover in the trench or adding a layer of straw to reduce evaporation.
Using a Swale for Roof Water Catchment
Catching roof water is a little more serious than capturing water that naturally runs across the landscape because of the high volume of water coming down at one time.
Be sure you’ve already chosen an appropriate site:
- Water drains away from your house and your neighbor’s house.
- An infiltration test demonstrates a healthy infiltration rate for proper drainage.
Dig a swale trench around 12 inches deep, 2 ½-feet wide, and 20 feet long (or so). Construct a spillway about as wide as your trench as a starting point. Direct your gutter into the swale, then observe during a heavy rain event.
Preciseness isn’t usually necessary because as long as you’re not going to flood the house, you can make adjustments after observing during a heavy rain. You can always make the swale trench deeper, wider, or longer before planting.
Build in Redundancy
Always think about where the water will go if the swale overflows. Have a redundancy built in that can handle a 100-year rain event.
When we built our front yard swale, we directed the overflow into a rain garden to accommodate any excess water.
You can also direct a spillway into another swale.
Here’s a picture of our front yard before we added a swale and gardens:
The swale system and perennial plantings work to slow the water, spread it, store it, and lock in moisture. The front yard now looks like this:
Storing water in the ground will benefit soil health and moisture retention. Nature does it best!
For more details and pictures of swale-building, see:
- Gaia’s Garden by Toby Hemenway
- Rainwater Harvesting for Drylands and Beyond, Volume 2 by Brad Lancaster
- The Suburban Micro-Farm: Modern Solutions for Busy People by Amy Stross (shameless plug!)
You might question whether storing water in the ground using swales actually works as well as I’m prosthelytizing. This 5-minute video, Greening the Desert, demonstrates how swales can turn desert into productive gardens.
Need more ideas for growing a permaculture garden?
Are you digging a swale in your yard? Will it catch roof water?