Putting In French Drains

[Fossil Farm]

The 1 inch in a hundred feet is more for field tile for a plow or wheel machine in flat ground. I've seen good operators with targets do just as well as ones using laser systems. But again, that is just a minimum to get water moving. You do not want too much slope in the tile or else you do run the risk of causing too much turbulence ( I'm sure there is a name for it ) and causing tile damage without a breather in the line. Yes a few inches per hundred for the french drain would be better, just suggesting the minimum slope to move the free water out of the area.

 

[DJ54]

2% fall is the norm for culverts and basic drainage. Total fall over the entire drain is the largest factor. You have plenty of that, and may even want to go diagonal if possible to reduce that much fall. If not possible, or feasible, then time will tell I guess.

Don't know that I would put the B3, or 8's as we call them here, on the bottom. If it's only 24" deep, it's not that tough to dress up the bottom of the ditch. Dress it up, and lay the pipe, then the gravel. A true French Drain is just all gravel anyway. We'd use 4" perforated, just to keep a hole open for water to flow. And keep in mind, just perforated pipe in a roll only has 5' of crush strength. Anything deeper, we used the smooth bore, in 20' joints.

I might suggest undercutting the outlet end, and back 18" to 24" or so from the end. Add some larger stone, or pour a splash pad, and bring it up on the sides, higher then the outlet. Water coming from the end of a pipe like that will eventually wash back under the pipe. Depending on how high you are above that stream bed, it could wash back, and let the end of the pipe eventually turn straight down.