Well, these swales with 'flat bottom' (drenches or ditches) in permaculture are quite amazing water harvesting features on contour of a land. This means water in swales slowly soak into the surrounding landscape and helps passively irrigate plants and hydrate the earth.
The soil dug out from swales is placed (not compacted any way) on the downhill slope where long term (perennial) and annual (short term) plants are planted to stabilize the soil. This is also great way to establish a food forest.
Swales also help slow down running water on the down slope and prevent erosion of a top soil. A level sill spillway is set up 100mm higher than a bottom of a swale to discharge excess water in the swale. Any excess water in the swale can be directed into another swale, a dam or where ever you need it to go.
I am excited about this passive irrigation system and thought you might be interested in it too. So here is a great video from Jack Spirko on Large Scale Swales in permaculture design system and another video from David Spicer on A Backflood Swale with 2 Levelsill Spillways .
Did this post help you to understand more of swales?
Heli & Allan
Did you like this video? If so, please be sociable and share :-)