A Visual Explanation of How French Drains Work

In this Practical Engineering video, Grady Hillhouse explains the purpose and design of French drains—subsurface drainage systems used in everything from home landscaping to massive dam structures.

Using models, history, and real-world examples, he shows how proper drainage prevents erosion, reduces pressure, and protects the integrity of buildings and infrastructure.

KEY HIGHLIGHTS:

• French drains are subsurface drainage systems designed to redirect water away from structures and prevent damage.
• Proper drainage reduces pressure beneath structures like dams and spillways, helping to maintain their integrity.
• Water below the surface can cause erosion, soil expansion, and structural cracking if not properly managed.
• Models and real-world examples illustrate how French drains facilitate water flow through soil and gravel layers.
• Effective drainage systems are crucial for both large infrastructure projects and residential landscaping to prevent water-related issues

TRANSCRIPT:

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.

Water Is the enemy of structures

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.

Demonstrating a French drain

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 mostly dries out.