I'm planning some swales on a very slightly sloping field above an area I plan to make into a food forest. I would like to concentrate the run-off from this field onto the food forest area. The collection area is perhaps four times as large as the food forest. Should I make swales on perfect contour, or should I angle them slightly toward the food forest? Our average rainfall is about 28 inches per year, and I would like the food forest to survive without artificial irrigation once established.
If I was the one planting the food forest I would incorporate the swales into it. Put a swale in that overflows into the next and so on and plant around them. Or if the spot you are planning the forest for is flat maybe just overflow your swales one into the next and have the final one overflow towards the forest. I would say stay on contour and just have the overflow designed to pour over once there is a certain depth of water present in it. This way the ground will saturate and with time the lower ground where the forest is will too. I feel you would ultimately be better off staying on contour. I could be wrong but from my understanding of it the level ground below the slope should saturate eventually too as the swales and forest mature.
I kind of like the idea of a slight slope (1-2%)going towards the food forest area, then going level once you get there. It seems like a really efficient way to concentrate that rainfall without causing erosion or taking away from the other area. It seems like a net gain to me! You should get more than enough moisture in there to get things growing well even with your sometimes harsh summer weather. Will you be planting trees along the swales throughout or are you just going to concentrate your efforts on the food forest?
If the plan is to incrimentally expand the food forest into the other area I would just make the whole thing pretty much level. Maybe incorporate some wider shallow areas, creating some pocket ponds or vernal pool type areas.
based on the pictures, there doesn't seem to be a slope. angling them off contour would be counter productive as you would not capture the full amount of rainfall. maybe you could incorporate a trench/swale that all the swales flowed over into and angle that toward your forest.
great peice of land. lots of potential!
Thank you. The slope is very slight. I've decided to make the swales on contour and have them overflow into the food forest area, as you suggest. This should solve our problems with flood water rushing between our house and shop, which are downhill from the future food forest area. My plan is for the food forest to absorb as much runoff as possible, and with luck have only a trickle passing between the buildings next time it floods.