Once the basic prep work of the swale area was complete it was time to build out the French drain.
We decided to run the French drain along the southern edge of the swale (Donna’s side) to avoid the cement piers of Suzie and Morris’s carport.
Look at all that dirt that came out of a 24 x 24” trench! It’s true what they say – soil “fluffs up” to about three times its mass when you dig it out. One of the big challenges with excavation projects is what to do with the dirt afterward because there is always some left over. In this case, some will be put back into the swale area. Some of the rest will be used by neighbors. Any that’s left over, Donna will move to her backyard and we will use it to design with later.
We also wanted to widen the swale area a bit, so we decided to take out an 18” wide cement apron that ran the length of Donna’s driveway. This left us with some large chunks of “urbanite” to deal with. In true permaculture fashion, the problem of disposal became a solution when Donna (the homeowner) and Chip (the installer) rented a jackhammer and created some chunks of an appropriate size to put at the very bottom of the French drain.
We used Brad Lancaster’s book, Rainwater Harvesting for Drylands and Beyond, Volume 2, to guide us through the French drain process, specifically pages 106-113. You can purchase this WONDERFUL resource along with other books I regularly use from my Amazon store.
As directed in the book, we first laid landscape fabric in the trench to hold the aggregate rock. The purpose of this is to stop root penetration into this area. Note that we used a 24” wide bucket on the backhoe instead of a 16” bucket. This left us with an 8” gap to the left of the trench which we had to backfill with soil. Using the 16” bucket would have saved us from having to go back and do this – one of those “lessons learned” things that you run across each time you do a project.
The bottom layer of rock in the trench is the urbanite chunks, followed by some random gravel from Donna’s yard. After that was used up, we still had to order about 3 cubic yards of river rock to fill the 15” deep and 15” wide gravel part of the French drain.
Now it’s time to cut the perforated pipe to length and put in the overflow drains. Donna gets to use more power tools!
Next step is to lay perforated pipe on top of the rock, perforated side down. You want to make sure the area is level along the length. After the pipe is in place, the area around the pipe filled with additional rocks.
The perforated pipe, with its overflow pipes, is connected to the downspout by a solid pipe which drops more than the required 1/2” for every 10 ft.
All that’s left is to fill the space around the pipe with rock, with about 1-2” on top of the pipe, wrap the landscape fabric over the top and cover with dirt.
Voila! The finished swale ready to be planted! Notice the top of the swale is concave. This area will be filled with woodchips so the surface is nearly flat and slightly below the level of the driveways. The concave feature will catch additional water in situations where the French drain fills up and water comes out of the overflows. If there’s still a surplus of water, we’ll be installing a level spillway at the end of the swale nearest the sidewalk. The overflow water will be directed into the street. To keep the woodchips from clogging up the spillway, we’ll plant some bunch grasses (either side oats grama or pink muhly) to hold back any debris while letting the water flow through.
Next up – Planting the swale.