Saturday, November 1, 2014

The French Drain

This is not a French drain but a picture of a lateral meeting a main tile.

The French drain, also called a sub-drain, is so called because the person who is supposed to have invented it, an American, was named French. First used for agricultural purposes in New England before the development of perforated plastic pipe, the system invented by Mr. French was probably somewhat different from what we call a French drain today. Unlike a surface drain system which collects and removes surface moisture, this type drain system collects and removes subsurface moisture.

A sub-drain, by modern definition is basically a trench filled with gravel with a perforated pipe at the bottom. The pipe and gravel are wrapped in a protective “geotextile” fabric. Moisture accumulating in the trench percolates down and enters the pipe which transports the moisture to some point of discharge. This page will discuss when and where a French drain should be installed. I will also provide step-by-step details on how the work should proceed. Just for fun, I have added highlights from a French drain job which I found both challenging and rewarding. Before I do that, I want to talk a bit about a corollary in nature which can help explain the basic principals at work:

A few years back I invested in a small farm in the San Simon Valley of Arizona. Originally developed back in the 1950’s, the farm used furrow irrigation. The property was carefully leveled to a uniform gradient prior to installation of the irrigation system. The USDA maps describe the soils as clayey loam. The first year, I planted wheat and oat seed on about 80 acres and pumped out over 15 million gallons from my ground-water well to the irrigation furrows. Although I harvested a crop, the yields were far below normal. Later I was told by locals that I had not irrigated enough!

After studying some maps and old aerial photos taken prior to development of the farm, I discovered that there had been three buried streambeds cutting diagonally across the trend of my irrigation furrows. Grading and leveling had covered them over but they were still there. After walking the property and observing the soil closely, I was able to locate the buried streambeds visually based upon the increased frequency of rounded cobbles in the overburden soil. The sub soil in these zones contained a high percentage of sands and gravels. It became evident to me that a significant proportion of the 15 million gallons of irrigation water had “percolated” into sands and gravels of the buried streambeds without any benefit to my thirsty crops."

I have a few French drains installed but I don't fully understand how they work compared to tiled drains with gravel.

Do you have any French drains?  They are not French but started by an American named French!



  1. French drains + a mulch finish tool + grandpa who liked to jump in a tractor and start working ground at random = no more French drain.

  2. Yes they require different management!