How to Build a Permaculture Swale Garden Bed~ The purpose of a swale is to prevent erosion and to keep water on your land as long as possible. The material is dug out and placed on the downhill side, and the base of the swale ditch is dead level for it’s entire length. Water will collect in the swale, permeating into the ground and providing deep watering for the planting mound that’s created.