I have a drainage problem to solve and I'm considering a French drain. I have read in dozens of places that various drainage systems (including French drains) are usually lined or wrapped in a landscape fabric/filter fabric to prevent sediment from getting in an clogging up the drain. But nobody seems to ask or answer the next logical question: what prevents the sediment from clogging up the fabric instead? Aren't you just pushing the problem up one level?

  • I think that the first approach to consider for a drainage problem should be regrading of the surface. Only if there are insuperable, or at least major, negative factors for regrading should underground drains be considered. There is now a product on the market which is in between a swale and a French drain. These are lengths of porous rubber which conduct water and which are installed in trenches. I think they can be exposed to the surface or shallowly buried. – Jim Stewart Oct 13 '18 at 10:22

Think about how a coffee filter works. You have a paper filter clogged with wet coffee grounds. The water slowly passes through the filter paper and you have a thin film of water between the filter and the funnel. That film from the entire surface of the filter paper flows to the bottom of the funnel. By the time it all aggregates and flows out of the funnel, you get a stream of fast-moving coffee. That's what goes on with the filter fabric and the drain.

The drain channel is sized to move water at a certain rate. There is typically a perforated pipe that offers unimpeded flow to the water once inside, so it is quickly carried away. The water gets into it by traveling through big gaps between large stones, and some of it even travels through the stone gaps.

If the drain system is at the surface of the ground, much of the water will get there by flowing over the ground until it reaches the stones. Some of it gets there by flowing through the soil; flowing much slower, through much smaller paths between dirt grains, over a much bigger area. If the drain is completely buried, like around the footers of your house foundation, all of the water will get there by traveling through the soil.

The filter fabric is porous, and acts like a barrier to separate the soil from the drain. It gets clogged with fine soil, but not really different from the soil the water is passing through to get there.

So you have a lot of water collecting in the soil over a very large area. It moves slowly through the tiny gaps in the soil, but that's a lot of water moving slowly. It passes through the fabric barrier at the perimeter of the drain system, which has a lot of surface area, and the aggregated water becomes the stream flowing through the channel.

If filter fabric is only placed around the drain pipe, the stone in the channel fills with silt until there isn't really a difference between the stone channel and the soil. The water moving through the soil will find its way into the perforations in the pipe, but you're aggregating slow-moving water from a few small holes, so you only get a trickle through the pipe.

  • Wait, don't paper coffee filters get too clogged to function after a few uses? That's exactly what I'm concerned about happening with landscape fabric. – Craig Oct 14 '18 at 2:08
  • On the other hand, your point about effectively increasing the diameter and surface area of the fast-moving drainage channel is well-taken. The more fabric surface area surrounds the channel, the faster water can flow through the fabric into the channel. That part makes a lot of sense. I'm still concerned about the "coffee filter" clogging up though. That's why coffee filters are disposable.... – Craig Oct 14 '18 at 2:20
  • @Craig, coffee filters don't really get "clogged", they just get disgusting. That's why they're disposable. :-) Soil does consist of particles of different sizes. When it is compacted, there isn't movement. If it's disturbed (wetting and drying creating cracks, roots creating channels, organic material decomposing, etc.), moving water can carry the smallest particles through the voids (leaving bigger voids through which bigger particles can be carried). That's what sometimes happens with earthen dams, and how they eventually spring leaks. (cont'd) – fixer1234 Oct 14 '18 at 3:15
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    If that happens around the drain, the finest particles could gravitate to the fabric. Keep in mind, though, that the pores in the fabric aren't holes that match the shape of the soil particles, so that particles the right size plug the pore like a cork in a bottle. They're irregular, and it's difficult for soil particles to totally clog it up. Also, water gets through the fabric slowly, by capillary action, it isn't like a fast-moving stream is driving soil deep into the fabric. Over a very long time, it's possible for portions of the fabric to have reduced flow. (cont'd) – fixer1234 Oct 14 '18 at 3:16
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    If the system is installed right, you should get use from it on the order of decades before this kind of situation develops. If you didn't have fabric around the system, it could be useless in a few years. – fixer1234 Oct 14 '18 at 3:16

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