September 30, 2011

I’ve got several places in my yard and along my driveway that erode during periods of heavy rain. I also have issues with the gravel road I drive to get to my house. What can I do to stop or minimize erosion around my home? I feel like I never stop repairing the damage. It’s really a headache. There has to be an effective way to deal with the water.

My geology degree taught me long ago that water is the ultimate equalizer. Who would ever think that solid granite would succumb to a clear liquid that we drink and bathe with? Water, when you swim in it, seems so soft and unassuming.

But if you watch the news on television after hurricanes and tropical storms dump trillions of gallons of water in a period of a few hours, you’ll quickly see that water is a wolf in sheep’s clothing. When lots of water moves quickly down a stream it possesses enormous energy and power.

Water is highly efficient at eroding the soil around your home because of the simple formula many of us discovered when we took high school physics: Force equals mass times acceleration.

Put in terms you and I can understand, the force that water can exert on something is a function of its weight and how fast it’s accelerating. You already know that water is fairly heavy, assuming you’ve carried a five-gallon bucket of it around. We can’t control water’s weight, but we can often control the speed at which it travels.

Actually, we can do something about the mass part of the equation. We know that it doesn’t take much water mass to move a small grain of sand, which is nothing more than a piece of rock. If you increase the size of the rock, it takes more water to move it.

To help visualize all of this, let’s consider my own driveway. It’s very steep at one part, and I have chronic erosion on one side because a lot of water is concentrated there. The sand and one-inch angular gravel is constantly washed out of the ground and deposited farther down the drive.

I’m going to solve this dilemma by putting in angular rock that’s the size of a baseball. Based on the water flow I’ve seen in the heaviest rain, I’m pretty sure this size material will resist the force of that amount of water. That’s the trick: You need to match up the size of the rock to the force of the moving water.

It’s also important to use angular stone or rock if you can source it. This material interlocks and provides more friction when something tries to move it. Rounded gravel reacts like ball bearings. The rounded surface of a rock allows it to move far more easily if the water gets the rock moving.

Depending upon what your landscaping plan looks like and how you can integrate larger rocks into it, you can slow down the water in many cases by having it crash into larger rocks that are in the drainage channel where the water flows. The water expends some of its energy on the rocks instead of on the soil in the channel. If you slow down the water, it has less force.

One of the worst things you can do is to create a smooth paved channel for the water to flow in or down. This causes the water to accelerate to its maximum speed, depending on the steepness of the channel.

When it gets to the bottom of the channel, the water is looking for something to waste its energy on. If the channel ends in the grass or at some soil, the water will chew away at it. You can often see evidence of this wherever a drainage pipe exits a hillside. The water shooting out of the smooth pipe causes significant erosion unless there is something there to absorb all that energy.

Tim Carter is a columnist for Tribune Media Services. He can be contacted at www.askthebuilder.com.

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