Capturing water in the landscape is often the easiest and most efficient way to store it. Swales can help us do that. Learn how to choose the best site for a swale and how to build one.
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I’ve written about swales before: If you missed it, see What is a swale and why you need one.
Today I’m going to talk about the technical details of a swale: how to choose the best site for a swale and how to build one.
A swale is a shallow trench dug out (dead level) along the land’s contour, with a berm on the downhill side. Swales 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, they can be more efficient at storing rainwater than rain barrels (when used appropriately) and require no engineering or mechanical skills to build.
Constructing a Swale in 7 Steps
Step 1: Observe Water on the 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 sits and slowly soaks deep into the soil, producing 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?
Some observations to make about your property:
- Identify springs, creeks, ponds, wetlands, wells, cisterns
- Identify seasonal flows or standing water
- Identify where water is wasted
- Observe drainage patterns
- Locate steep slopes
- Determine how much water is needed if irrigation is the goal
- Determine quality of sources/water pollution
- Determine annual rainfall
- Calculate roofwater catchment area (if applicable). For example, my roof area is 1,200 square feet.
Step 2: Identify the Ideal Site for a Swale
Rules for siting a 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.
Swales may not be appropriate for areas with a high water table or on extremely steep slopes.
Multiple swales can be constructed if there is space and a need for them. A good rule of thumb is 18 feet apart–farther apart in flatter areas and perhaps closer together on a slope.
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 simple, homemade A-frame level to mark the contour lines.
Step 4: Dig the trench along the marked contour line.
Swale dimensions depend on a lot of variables. If you’re digging this swale by hand with a shovel, I bet you’re going to make the smallest swale you can manage! A backhoe will make the job easier if you have access. We dug ours by hand and it took about a month working weekends.
The standard range of size for a swale trench includes a depth of between 6 inches and 1.5 feet deep; and a width of between 18 inches and 2 feet wide. The length will depend on how large your space is, and whether you’re catching roof water or not.
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 in on the downhill side.
Step 6: Test and Adjust the Swale.
First, 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; or (last ditch effort!) dig another swale 18 feet or so below that one along the contour. Multiple swales are connected by a spillway, which is a notch in the berm where overflow can be directed. Spillways are usually lined with rocks to prevent erosion as the water spills from one swale into the next.
Step 7: Plant the Berm.
Plant the berm as soon as you know the swale is level and is the right size for the job.
I especially like strawberries (3-foot-deep roots), asparagus (6-feet-deep roots), or rhubarb (10-feet-deep roots) because their roots are guaranteed to capture the water from the swale and slow it down. Fruit trees have shallow roots that reach wide, holding the topsoil intact, but be sure to plant them with deep-rooted herbaceous plants to better stabilize the berm as a whole. Adding some berry-producing nitrogen-fixing shrubs would make an excellent combination. Perennial sunflowers are also a good choice.
I like the edible options, but your swale berm might also serve as a windbreak, privacy fence, native habitat or whatever else you can dream up! A swale berm could make a nice hedgerow.
Swales can be aesthetically-pleasing, if necessary, for the residential yard. Fill the trench with gravel and you have a walkable pathway. Just be sure to lay down weed blocker fabric on all sides before dumping in the gravel! Give the berm a rock border, and you have a raised bed. I topped my walkable trench with 4 inches of wood chips because I didn’t like cleaning fall leaves off of gravel.
If your swale is located in an underused area and does not need to be walkable, then you might consider seeding clover seed in the trench. At the very least, put a layer of straw in the trench 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.
There are two ways to go about digging a swale for roof water catchment: A precise, calculated kind of way, and a “wing it” kind of way.
The “Wing It” Way to Catch Roof Water in a Swale
If you’d like to wing it, be sure you’ve already chosen an appropriate site (water drains away from your house and your neighbor’s house) and you’ve done an infiltration test (see above) to make sure that the collected water will infiltrate the soil and drain well. 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 how it performs, by making the swale trench deeper, wider, or longer.
The “Geek” Way to Catch Roof Water in a Swale
If you’re a numbers kind of person, here is the way that I like to calculate for a roof catchment swale:
#1: Start with this equation: Square Feet of Rooftop X .1 = Volume in cubic feet your swale needs to hold in a 1.0″ rain event.
For my roof: Half of my 1200-square-foot roof drains to the front yard (600 feet). The volume of water my front yard swale needs to hold is 60 Cubic Feet.
#2: Next figure out how deep, wide, and long to make the swale trench to hold that volume of water. Since the trench of my swale is going to be a walkable path, I want it to be a minimum of two feet wide.
This equation will help you find the right proportions (in feet):
Cubic feet = Length x Width x Height
I wanted to find out how long to make my swale, given that it needs to hold 60 cubic feet of water, be a minimum of two feet wide, and just one foot deep (to reduce the amount of digging in the deep layers of hard clay):
60 cubic feet = ? Length x 2 feet wide x 1 foot deep
Length = 30 feet long
Problem Solving for Swales that Catch Roof Water
What if you don’t have the space for a 30-foot-long swale? I don’t! Let’s play with the numbers. I can make the trench wider for better absorption. Let’s adjust the width of my trench from 2 to 2.5 feet wide.
60 cubic feet = ? Length x 2.5 feet wide x 1 foot deep
Length = 24 feet long
In our case, we didn’t have space for a 24-foot-long swale either. So we dug the swale trench as close to the ideal size as we could, mounding the soil from the trench on the downhill side for the berm.
Then we observed it during a hard rain. It didn’t overflow, but building in redundancy ensured that the system was robust and could handle a 100-year rain event.
We directed the overflow into a rain garden to accommodate any excess water due to the shorter-than-ideal length of swale. You can also direct a spillway into another swale, but we didn’t have space for a second swale in our yard.
Once your system is solid and doesn’t overflow, you can proceed to plant the berm and fill in the trench if it will be a walkable path.
Here’s a picture of our front yard before we added the water catchment system and gardens:
After we added the water catchment system and perennial plantings to slow the water, spread it, store it, and lock in moisture, it now looks like this:
This is the effect that storing water in the ground can have on soil health and moisture retention. It will absolutely work more efficiently than any above-ground rain catchment system. Nature always 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
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, will amaze you. It demonstrates how swales can turn desert into productive gardens.
Are you digging a swale in your yard? Will it catch roof water?