It’s not unusual to be confused about permaculture swale spillways. It can take a bit to get your head around this concept, especially if you’re doing an online PDC (Permaculture Design Course).
Unless you’ve seen spillways in action in real life, it can be tricky to visualize exactly what is happening. I have included a great video below that shows Geoff Lawton building a swale with spillway in miniature. It shows the principles of permaculture swale spillways really well.
The first thing to mention is that spillways aren’t necessarily at the end of the swale. They can be anywhere along the swale because your swale is on contour. “On contour” means it is level, right along the whole swale.
We’ve talked about swales on contour before, and you can read more about this here: Two Cool Things You Can Do With Permaculture Swales.
Your swale will hold water until you make a spillway. If you don’t make a spillway, the water will do one of three things.
- The water will go around the end, mainly if we forget to build it up around the end. We don’t want it to go around the end. If you have water flowing out of the end, it can cause an erosion issue because it’s not a level spillway.
- The water will sit in the swale. This only happens if your swale doesn’t receive a lot of water. On our property, falls of 12” in a couple of hours aren’t unusual. No swale will hold this amount of water.
- The water will go over the mound. It doesn’t have anywhere else to go.
As you start building your swale, you survey your contour line. While you’re doing that, think about where you want the water to go.
- You might want it to drop into another swale, on a lower point of your property.
- You can direct the water to a ridge. Ridges are often dehydrated, and a spillway helps you direct water to that ridge, to rehydrate it.
- You can direct water to dams on your property.
We don’t usually build a swale with that exact purpose in mind, but it’s something to think about while you’re building it. Mark out your swale on contour and decide where your spillway will be. As your digging, pile up the dug-up earth to create the mound.
When you reach your spillway point, stop piling the dirt up into a mound. Your spillway should be on the original soil, at original surface height.
Carry on digging and mounding after your spillway, as you did before. Continue on until your next spillway, or the end of your swale.
Now, if you decide down the track that you need to put it another spillway or move your existing spillway, all you need to is take away the mound.
Just keep in mind: Don’t disturb the original soil of your spillway. Carefully move the mound, taking care not to touch the natural earth.