How To Install a Trench Drain

DEAR TIM: I've got a severe erosion problem where storm water runs off my driveway. The water is creating an ugly ditch. What's a great way to collect this water and stop the erosion. Whatever I do it needs to withstand the weight of a car or light truck as it's possible that tires could drive over this area. What's to stop the erosion from happening further away from the current location? Nina N., Cary, NC

DEAR NINA: I've solved problems like yours at customers' houses and even my own home. I'll never forget the first trench drain I created about thirty-five years ago at the second home I owned. I formed and poured the actual drain using concrete. I obtained the top cast-steel grates from a metal salvage yard. Fortunately there are modern plastic and composite products that allow you to install a trench drain in hours, not days like it took me!

This trench drain will collect water that’s rushing off the driveway. More concrete needs to be added between the drain body and the wood form. Photo Credit: Tim Carter

My college geology degree helps shed an interesting perspective on the overland flow of water. That's what's happening in your yard. I feel you need to really understand the mechanics of the water moving so you can permanently fix your problem. Currently, you're on track to make your own miniature Grand Canyon!

When it rains hard, the water flowing on hard surfaces like driveways, patios and sidewalks can't soak into the soil. As more water falls and moves across the hard pavements, it accumulates more potential energy.

Remember when you were in your high school or college physics class and the teacher put that really easy formula up on the blackboard (yes, we used to have black chalkboards!)? It was Force = mass X acceleration.

Translated to English, this means that the water's ability to erode gets greater as more water crosses your driveway faster. If you have a steep driveway and it's really raining hard, the force of the water leaving the driveway might be able to move rocks as big as basketballs. I recently watched a video of flash flood water out west moving a boulder the size of a small car down a pretty flat stream!

The first thing you need to study is where the water leaves the driveway. Does the water flow evenly across the entire driveway as it hits the soil, or does it get concentrated, because of the shape of the driveway, to a small area near the driveway?

If the water flows across the driveway pretty evenly, then you'll want to put in a trench drain. These are long drains that may be 4, 6 or even 12 inches wide at the top and can be as long as 10, 20 or even 30 feet! They work the same way a gutter collects water at the bottom of a roof.

If the water from your driveway is heavily concentrated to one location, you may need to install a larger rectangular or square area or field drain. These can be round, square or rectangular. You can find them as small as 1 by 1 foot, and as large as 18 inches by 30 inches or so. You size the drain to handle the flow of water in the worst storm event.

I recently installed a unique trench drain at my own home that was made from a cast polymer material. It has a strong steel grate that will take auto and light truck traffic. The top of the drain is only 5 inches across and each section of the drain is about 3-feet long. You can connect as many lengths of this drain together as you want. The system comes with end caps and outlets to fit standard-sized plastic drain pipes.

If you plan to have traffic drive over your drain, you really should install the drain on a poured concrete base. I dug a hole for my drain and made sure I had 4 inches of concrete under my drain. I placed my drain assembly in the fresh concrete so there were no voids under the drain. I then surrounded the drain with 4 inches of concrete to lock it into place. You can put more concrete around the sides if you feel it needs it.

I made sure the drain was about 1 inch lower than the outer edges of the concrete so the water would flow down into the trench drain. You don't want to make the concrete level because then the water may decide to not flow into the drain. Be sure that you have the drain low enough to capture the water.

When the water exits the trench drain through the pipe that connects to the trench drain, it will have MORE force than the water entering the drain. The trench drain and the pipe concentrate the water. Many people overlook this.

To prevent erosion where the drain pipe ends, you need to create an area of large rocks that will absorb the energy of the water and disperse this energy. As the water splashes against the rocks as it exits the pipe, the rocks act like shock absorbers.

This rock area doesn't have to look like an ugly gravel pit. Use some imagination and obtain some larger decorative rocks to create a nice hardscape feature in your yard. Keep in mind you want the rocks in this area to be different sizes and shapes so the water energy is spread out. This allows the water to get into the top soil and soak naturally back into the ground like it used to before your home and driveway existed.

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