[Note that this article is a transcript of the video embedded above.]

In February of 2017, one of the largest spillways in the world, the one at Oroville Dam in northern California, was severely damaged during releases from heavy rain. You might remember this. I made a video about it, and then another one about the impressive feat of rebuilding the structure. In the forensic report following the incident, one of the contributing causes identified in the failure was the drainage system below the spillway. Rather than being installed below the concrete, each drain protruded into it, reducing the thickness of the concrete and making it more prone to cracking. But why do you need drains below a spillway in the first place? Put simply: water doesn’t just flow on the surface of earth. It also flows through the soil and rock below it. Water that gets underneath a structure creates pressure that can lift and move it. That’s especially true when the water is flowing. Dam Engineers deal with the challenge in two ways: make concrete structures like spillways massive (so gravity holds them in place) and use drains to relieve that pressure, giving the water a way out.

Even though we depend on it to live, water is the enemy of all kinds of structures. Pressure is far from the only problem it causes. Most of us have come face to face with it in some way or other. Water causes some soils to expand and contract. It freezes, promotes rot, erodes, and corrodes, wreaking all kinds of havoc on the things we build. On the surface, water is relatively easy to manage through channels and curbs and slopes. Below the ground, things get much more challenging. Subsurface drainage is a really interesting challenge, and it applies to everything from simple landscaping at your house to the biggest structures on Earth, and there are a lot of things that can go wrong if they’re not designed correctly. I’m Grady, and this is Practical Engineering. Today, we’re talking about French Drains.

The idea of a subsurface drain is really pretty simple. And I built a model here in the garage to show you how they work. This is just an acrylic box with a hole at the bottom. I filled the box with sand to simulate soil. And I left a small area of gravel in front of the hole. A few strategically-placed dye tablets will help with the visualization. When I turn on the rainfall simulator, watch what happens. Water percolating into the subsurface continues flowing within the sand. It moves toward the gravel, eventually flowing into the holes between the stones and out of the model. (Don’t pay attention to those dye traces on the left. Turns out there was a small leak in the box that was acting as a… secondary outlet to my drain). When the rain is over, the subsurface water continues to flow until the soil is mostly dries out.

This is a very simple model of what’s often referred to as a French drain. It’s not from France but named after an American farmer, lawyer, politician, and inventor Henry French whose 1846 book on Farm Drainage cataloged and described many of the practices being used around the world. Funny enough, he was explicit that he didn’t invent these drains, claiming “no great praise of originality in what is here offered to the public.” Still, I have to admit, after reading his book, I understand why he became the namesake of the drains he made famous. The man had a way with words:

“The art of removing superfluous water from land must be as ancient as the art of cultivation; and from the time when Noah and his family anxiously watched the subsiding of the waters into their appropriate channels to the present, men must have felt the ill effects of too much water, and adopted means, more or less effective, to remove it.”

Well before we worried about draining subsurface water to protect buildings and structures, farmers were doing it in one way or another to keep their fields from sogginess that affects the growth of crops and bogs down agricultural equipment. In fact, “tile drain” is another common term for subsurface drains because clay tiles were used to hold the drains open. And there are plenty of fields still drained using clay tiles today. But French pointed out that rocks sometimes work just as well:

“Providence has so liberally supplied the greater part of New England with stones, that it seems to the most inexperienced person to be a work of supererogation, almost, to manufacture tiles or any other draining material for our farms.”

He was mostly right, and gravel-filled trenches are used all over the place for simple and non-critical applications. The problem with rocks is that they clog up. You can kind of see how sand migrated into the spaces between the gravel in my demo. Since it’s sand, it’s not really a problem, but if this were a finer-grained soil, it would eventually reduce the drain’s ability to transport water, slowing down the drainage process. Tiles provided the benefit of holding open a clear space for water to flow. Over time, perforated or slotted pipes began to replace tiles for use in drains. You’ve probably seen these before; there are a hundred different styles and materials. Rather than flowing in through the joints between the tiles, the water just comes into the holes in the pipe. But which way should the holes face? Turns out it’s a debate as old as pipes themselves among engineers and contractors, and there are strong opinions on both sides.

If the holes are on the top, water has to fill the gravel to the top of the pipe before it can get in and be carried away. If the holes are on the bottom, the flow path isn’t smooth, so the water flows slower and is less likely to wash away any soil or debris that gets inside. From my research, it seems like most of the manufacturers recommend holes down so the gravel envelope doesn’t have to be completely saturated before water can enter the pipe. I think, in practice, it’s really not too important, and actually, a lot of perforated pipes you can buy for drainage have holes all the way around so you don’t even have to think about it. That’s the best kind of decision, in my book. But, if it seems counterintuitive to you to orient the holes downward, I can demonstrate it in my model.

With a pipe in the middle of the gravel layer, I can turn on the rain again. Just like before, water makes its way through the soil toward the drain, and eventually out of the model. Let’s watch that sped up. When the rain is off, the soil continues draining out until it’s no longer saturated. Hopefully it’s clear how beneficial this is. Without that drain, water will eventually dry out of the soil by flowing away or evaporating over time. But getting it out quickly, with a drain, gives it less opportunity to apply pressure to basement walls, freeze against a structure creating long-term movement, swell the soils, or cause rot and corrosion.

I’m using sand in my model to speed up these simulations, so this envelope of small gravel with a pipe inside is working pretty well to keep the soil in place. But, somewhat inconveniently, most places we want to drain aren’t overlain by playground sand. They have finer-grained soils, including silt and clay. These small stones are holding back the sand, but tinier particles would just flow right through the cracks. That can lead to erosion over time as water dislodges and carries soil particles away through the drain. Watch what happens when I try my French Drain model with large stones between the sand and the outlet. You can see the turbid water coming through the drain, indicating that soil particles are making their way out. And if you watch closely on the right side, you can see where they’re coming from. Eventually, enough sand washes through the rocks to create a sinkhole, and the rest of the water bursts through. Made a HECK of a mess (pardon my French drain). I’ve talked about internal erosion and sinkholes in a previous video, so check that one out if you want more details. This erosion can also result in clogging if the soil particles move into the gravel and pipe. In fact, clogging is the biggest problem with subsurface drains, so properly designed ones usually have some kind of filter.

The design you’re probably most familiar with if you’ve seen or installed a french drain yourself uses geotextile fabric. These are permeable sheets that have a wide variety of applications: separating different layers of soil or rock, protecting against erosion, adding reinforcement to backfill, and filtering soil particles out of flowing water. A typical french drain design uses geotextile fabric around the gravel envelope to keep the fines from migrating in. It’s sometimes known as a pipe-within-a-pipe. But geotextile has some limitations. It’s easy to damage during installation. It’s pretty much impossible to repair or replace once it’s in place. And it also gets clogged up. It’s just a thin mesh of fibers, after all, so once soil particles get stuck, they can quickly lead to a decrease in permeability and efficiency. But there is another option for filtration, and it’s most commonly used on dams.

It is hard to overstate the importance of properly filtered drains for dams. If you don’t believe me, take it from the Federal Emergency Management Agency in their 360-page report, Filters for Embankment Dams: Best Practices for Design and Construction. If that’s not enough, try the Bureau of Reclamation in their 400-page report, Drainage for Dams and Associated Structures. A civil engineer could spend an entire career just thinking about subsurface drains, and for good reason. Lots of high-profile dam failures have directly resulted from a lack of drains or ones that weren’t designed well, including the Oroville Spillway incident I mentioned. For embankment dams that are built from compacted soil, any movement of those soil particles can spell demise. And if you think about all the ways that water is terrible for structures, you can imagine how hard it is to design a structure whose literal job is to hold it back. That’s why they use filters of a different design. You can see it in bold right here in this FEMA status report: “It’s the policy of the National Dam Safety Review Board that geotextiles should not be used in locations that are critical to the safety of the dam.”

Instead, they use sand. Just like the gravel in my demonstration lets the water through while holding back the sand particles, sand can hold back smaller particles of silt and clay, acting as a filter. But it’s a little more complicated than that. Every soil consists of a variety of sizes of particles. I can show that pretty easily, again using sand as an example. I have a collection of sieves with different sizes of holes, each one finer than the one above. I put my sand in at the top. Then give it a little shake (a little razzle-dazzle). And when I open it back up, the sand is all sorted out. If you weigh out the fraction that got caught in each sieve and plot that on a graph, you get something like this: a grain size distribution curve, also called the soil’s gradation. Soils can have a wide variety of gradations. And it’s super important to understand in this case, because before you can design a filter, you have to know what you’re trying to filter out. Once you know the base soil’s grain size distribution, there are a number of engineering methods to find a material that will both allow water to flow while still holding the soil back. And in a lot of cases, that just happens to end up being some variation on the sand we’re used to using in concrete and sandboxes and demonstrations about french drains.

Actually, for dams, you often can get either the filtration you need or the capacity to let water through, but not both in the same material. So lots of dams use two-stage filters. The first stage filters the base soil material. The second stage filters the first stage, but lets water flow more freely. And then, you put a perforated pipe in the middle to get the water out of the drain as quickly as possible. So they look basically identical to the demonstration I built: sand, then gravel, then pipe.

As for dealing with the water once it’s out of the ground, there are really just two options. The easiest is to simply release it by gravity to the surface at some low point. But if you don’t have a low point on the surface nearby, the other alternative is to pump it. If you have a basement at your house, there’s a good chance you have a sump, which is just a low spot for drainage to collect, and if you have a sump, it’s a REALLY GOOD idea to have a sump pump, to move that water out and somewhere outside your house.

Of course, there’s a lot more to this. Dams have all kinds of drainage features depending on their design. Concrete dams often include a gallery or tunnel with vertical drains into the foundation. Embankment dams often feature a large internal drain called a chimney filter to keep water moving through cracks or pores from carrying soil along with it. And it’s not just dams. Plenty of structures, like retaining walls, rely on good subsurface drainage for protection against all the bad things that water does, not to mention their widespread use in agriculture. There are lots of interesting designs and maybe even more proprietary products on the market all trying to accomplish those two main tasks: get the water out without getting the soil out too. In the end, it’s all the same engineering whether you’re trying to protect a multi-million dollar structure or just keep your basement dry. I think Mr. French put it best:

“Indeed, the importance of this subject of drainage, seems all at once to have found universal acknowledgement throughout our country, not only from agriculturists, but from philosophers and men of general science.”

I don’t think anyone could reasonably call me a philosopher, but I do love drains, and I hope you agree that, from dams to fields to foundations of houses, they are pretty important.

French drains are one of those topics that be hard to sell in a pitch meeting, right? No studio executive would be like, “Yes, this is a million dollar idea!” But the thing I love about this channel is that it’s created a passionate community around seemingly mundane things like subsurface drains. TV used to be like that too: something for everyone. I loved the old History and Discovery channel shows. Now it’s all converged into reality shows and reruns, and I’ve found that pretty much everything I watch these days is done by passionate independent producers. If you feel the same way, I have a recommendation for you: The Getaway by my friend Sam at Wendover Productions.

It’s a gameshow with a hilarious premise, which is that all of the contestants (who are all big YouTubers, by the way) are snitches, but each one thinks they’re the only one. And it just leads to all these very funny situations where everyone is trying to secretly sabotage the contests. Plus the behind-the-scene cuts to the producers trying to keep all the confusion under control are wonderful. It’s such a great twist on a game show, and it’s one of those creative experiments that only works because it’s independently produced. The chaos of it is what makes it great, and that’s why it’s only available on Nebula.

I talk about Nebula a lot. It’s a streaming service built by and for independent creators, and it’s growing super fast. After the major overhaul of the home page, making it easier to find new stuff to love, we’ve leaned into producing really good original content, like The Getaway; basically allowing your favorite creators to make bigger budget videos without the fear of having it flop on YouTube’s algorithm. That means you get more creative, interesting, and thoughtful videos. My videos go live on Nebula before they come out here, and right now, a subscription is 40% off at the link in the description.

Plus if you already have a subscription, now you gift one to a friend. We have annual gift cards now. Give someone you love a year’s worth of thoughtful videos, podcasts, and classes from their favorite creators. It’s 40 percent off either way at nebula.tv/practicalengineering for yourself or gift.nebula.tv/practical-engineering for a friend. Thank you for watching, and let me know what you think!