The swales I built last year were done with a mini-excavator. I dug 2 foot wide trenches about 18 inches deep on contour, and then placed the material downslope in a berm. I then proceeded to fill the trench with organic material. This is not a bad way to build a swale, but it was expensive finding enough organic material to fill the trench. Also, I had some erosion of the steep edges. For my new swales, I did not want to purchase organic material, and I did not want erosion.
Old swale filled with organic material
I decided to build much larger and wider swales to allow for a slope in and out of my contour ditches, and then I would seed the entire structure.
How to Build Swales 2.0
1. Mark your contour lines. I used a laser level to find my lines. I marked the lines with flags then I painted with turf marking paint to connect the flags. Finally I removed the flags before I started digging.
2. Dig a 20 inch trench the width of the bucket on the contour line, placing the soil evenly downslope in a berm, leaving a space for the shallower trench that you will dig next to the berm. I used a 25 inch wide bucket. It is important that you watch the bucket and dig at the same level throughout your swale.
3. Dig an 8 inch deep trench above and below the main trench, depositing the material evenly on the downslope berm.
Digging out a swale
4. Clean out the main trench to 20 inches deep again, depositing the material on the downslope berm.
5. Run the teeth of the bucket back and forth one time through each shallow trench to smooth out the sharp edges. If you prefer, you can shave the edges by back dragging a tractor bucket. We did this successfully, but ended up with better quality, but a little more handwork just using the excavator. Also, we got the tractor stuck in the trench.
Using the bucket to shave the edges of the swale
6. Run the teeth back and forth one time through the upper edge to smooth out that sharp edge.
7. Now you are ready for the hand work. Rake the edges and berm so it is even. Make sure the bottom of the trench is level. Some material will end up in the bottom of the trench as you rake the edges to a 45 degree angle to prevent erosion. I ended up with a 15 inch trench. The width of the entire trench was about 75 inches and the berm was also about 75 inches. So I ended up with a 12 foot wide swale system.
Swale
8. Make your spillways. You can easily compact an area on the berm every 100 feet with the bucket of a tractor to give you a level spillway in the event of a large rain event.
9. Swales are tree growing systems, so don’t forget to plant your trees. Plant your trees on the downslope berm. I have found that midway up the berm is a good place to plant the trees.
Swale planted with trees
10. Seed the entire system. I used clover as a groundcover mixed with daikon radish, calendula, dill, white yarrow, bee balm, & chicory. This gives me some nitrogen fixation, nutrient accumulation, predator insect attracting, and aeration.
11. Add straw to the entire system. This will get those seeds to germinate, and help the trees with mulch.
Swale planted and seeded with straw
12. Initially, make sure the trees are getting an inch of water per week for the first couple of months.