French Drains

French drains and trails, really? Trails often have to cross seeps or aquifers/springs, perched water tables, and tiny ephemeral streams that all seem too small for wooden solutions, or wood is not desired. Sometimes these wet spots appear or come into being after cutting a trail, which can cause water to begin to “see daylight” more-so than it would have if left untouched. There are also instances where roadside drains and ditches, or even irrigation runoff cross trails. If any of these cause wetness, and possibly mud depending on the soil content, for an extended period of time then perhaps its time to start addressing the issue by first doing what most scientists do when confronted with a matter for study: ask questions? My first question is typically, “Is it acceptable to be wet a few days, weeks, or seconds?” Then, “How wide is too wide?” Of course if its a new trail the answer is typically that wet is not acceptable, and/or anything more than one can reasonably hop across is unacceptable. There is also a chance that either of these may be quickly tolerated depending on how individuals answer the first question.

If having a wet spot is  not acceptable, kind of like a spot or stain on an otherwise nice shirt (on a visible part like the shoulder), then I start with possible solutions for the stain, e.g. turnpike, causeway, paving, stepping stones, bog bridge, drainage lens, or perhaps a french drain. If ignored it could become “ring around the collar,” the trail, that is, may creep around the issue or become a wide mess.

When is a French drain “the” solution? I’d call it “a” solution, sometimes it is “the” solution. This is when I might employ it: in an urban or landscaping setting, if it can tie into nearby drainage or a lower area for storage/ponding a safe distance away, if the terrain is relatively flat (<5%), if it can be dug deep enough (easily) to bury the water deep enough(especially if adding tile), if its presence can be disguised, and/or the seep/spring is narrow and not fast enough to overwhelm the pipe with large rain events.

Very wet areas that are prone to freeze or too steep may not be the best candidate for french drains as the trenches or pipes can freeze or clog, as well as blow out and fail if too steep.

French drains and swales can be used to move water away from a trail or into another catchment before it crosses the trail, or used to tie together a large wet spot into a few concentrated channels for exit– typically in a herringbone like arrangement if needed.


Hopefully nothing this extensive is needed on a trail, just one or two lines, otherwise other solutions might be better served. Sometimes a sheet drain may be the call for seeps.

I usually start with surface drains and  swales to confine water to a small foot print that is easy to cross in stride (sub 16 inches). If that doesn’t work, depending on the flow volume, I move to French drains, culverts, and/or drainage lenses. The last is essentially an open french drain, or one in which water skirts under or through.

My golf course days and time at the Karsten (Ping) Turfgrass Lab in Tucson aren’t the forest and wilderness of most trail settings, but I learned a thing or two about eliminating wet spots, or moving them, quite often with gravel and drain pipe, or as we often said, tile (a meme that still holds to this day). I worked on several courses, and most had a French drain or two, if not an entire network of tile draining nearly every square inch (at least at tee boxes and greens). I saw, repaired/replaced, and added a number of post hoc drain networks into original drain lines. In the end the goal was to make the drains disappear into or under the landscape, along with the wet spot. That said, if a French drain is the solution then the following are some pointers for French drains.

  1. “Subsurface drainage problems are generally correctable only to the extent that large soil pore spaces can be increased to allow for better water movement. Use of soil drainage tiles are only effective to the extent that the soil will allow water to flow through it to the drain tile, and water in the drain tile can flow downhill to an outlet.” source
  2. If the drainage ditch can’t be dug at 1-3% slope when complete then another method should be considered. If the volume and velocity for a french drain network is too great (sometimes exacerbated by steep slopes) the drain pipe my erupt or surface, and the gravel may be gouged out of the drainage ditch.
  3. Do you need pipe? To some people a French drain has no pipe, or only uses certain types of pipe. Pipes run the risk of being crushed, clogging, working their way to the surface (if not covered appropriately) or out of grade, and maybe not bridging* with the envelope(surrounding soil/gravel). Gravel, sin pipe, can clog as well, but this depends on the particle size abutting the gravel.
    1. *Bridging: “In most soils, the wide distribution of particle sizes from clay through sands and small gravel form natural bridges of particles over the entry points of water into the tile. Soils, however, which are of a relatively uniform particle size, are unable to form these natural bridges and are considered to be unstable when saturated with water; the condition which exists when water is about to flow into the tile line. These soils have been identified as very fine sands, loamy fine sands, fine sands, and silts. Clays would be expected to cause even greater problems, however, because of the plate-like nature of clay particles. Due to strong cohesive  forces between the particles, clays react similar to bridging soils.” source
    2. The criteria are based on engineering principles which rely on the largest 15% of …particles “bridging” with the smallest 15% of the gravel particles. Smaller voids are produced, and they prevent migration of …particles into the gravel yet maintain adequate permeability. The…particle diameter below which 85% of the soil particles (by weight) are smaller [will bridge with gravel if it’s] …particle diameter below which 15% of the gravel particles (by weight) are smaller [than the enveloping soil]. For bridging to occur, the …gravel must be less than or equal to eight times the [neighboring soil]…To maintain adequate permeability across the …gravel interface, the …gravel shall be greater than or equal to five times the [neighboring soil]. The gravel shall have a uniformity coefficient** (Gravel D90/Gravel D15) of less than or equal to 3.0. Furthermore, any gravel selected shall have 100% passing a ½” (12 mm) sieve and not more than 10% passing a No. 10 (2 mm) sieve, including not more than 5% passing a No. 18 (1 mm) sieve.” source
    3. Whether you need a pipe or not, still hasn’t been answered. I’d prefer not to bring pipe into the equation on trails with large root potential, but it depends on the setting. From a volume standpoint, assuming there is good bridging, pipes can handle more volume and drain faster than rock alone. Flow and removal rates can be determined by measuring it on the surface or doing some calculations. (more) You might ask if a culvert, pipe or rock, might do the trick. A french drain isn’t much different, just invisible. Of course some culverts disappear into the landscape as well.
  1. See this document which covers a great deal of ground worth considering, a few points which I repeat.
  2. Do you need geotextile or another “envelope” to slow the clogging potential of gravel and/or the pipe? see the same two sources above, and this
  3. **“The size of particles above which 60% of the particles lie and the particle size below which  10% of the  particles lie is determined. **The uniformity ratio or coefficient is calculated as D60/D10. Where the ratio is less than five, an envelope should be considered. If the ratio is  greater than five, there is no need for an envelope.” or: “The ratio of (1) the diameter of a grain (particle) of a size that is barely too large to pass through a sieve that allows 60 percent of the material (by weight) to pass through to (2) the diameter of a grain (particle) of a size that is barely too large to pass through a sieve that allows 10 percent of the material (by weight) to pass through. The resulting ratio is a measure of the degree of uniformity in a granular material.”
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