Using stones and plants to prevent soil erosion probably began with the second human-seeded crop at the base of a hill. The first crop got washed out! That’s why we’re about to share our best tips for how to place rocks on a slope to stop erosion.
Regardless of the degree of slope, you can generally stop or slow down erosion using different types of rock and plants. We are going to provide some helpful pointers to make you more successful.
Then let’s get digging!
Can You Put Stones On a Slope?
Yes. But before dragging boulders up a hill, look at this Erosion Control Report from Southern California Watershed Recovery and NRCS. It’s our favorite resource for backyard erosion control. And the report teaches us that much of what you can do with rocks gets determined by the slope angle.
- Slope Under 33% (3 to 1). You can design and use almost any erosion control method, including rocks, gravel, and riprap, according to NRCS.
- Slope Ranging From 33% – 50%. You can use trees and riprap, according to NRCS.
- Slope Over 50%. Any rock you use has to be in the form of retaining walls to hold terraced fields. Or, at the very least, partially sunk into the ground to prevent sliding or rolling.
Note! Riprap rock is famous for controlling erosion on shorelines, levees, culvert channels, et cetera. Anywhere water can flow fast and hard. Riprap generally ranges in size from 4 inches to 30 inches. Check out this excellent riprap rock guide by Ayres Associates for much more information about using riprap to stop or slow erosion.
Do Rocks On a Slope Stop Erosion?
Yes. Rocks on a slope prevent erosion, as long as you place them correctly! Generally, erosion gets caused by big rains or fast snow melt combined with little or no ground cover. Cultivation, fires, or non-productive soil (or loose soil) can cause a lack of ground cover. Placing rocks strategically on the slopes can help slow and divert water flow.
I am always surprised by the amounts of heavy rainfall reported in southern and eastern US states. Ten or twelve inches in 24 hours is outside my experience.
The point is that it does not take much to erode the soil. A few years ago, we had 2 inches of rain in 6 hours, a clogged downpipe (see our article Creative Gutter and Downspout Drainage Ideas), and a freshly worked and seeded garden. Quite extraordinary just how much dirt a 5% slope can deposit on your lawn!
Preventing Erosion On Slopes
Rocks alone will not always solve your erosion problems. Depending on the slope and type of soil, you may need to use a combination of ground cover, hedges, trees, rocks, and grading to prevent erosion. I would spend a little more time looking at the natural hillsides around my home than trying to match Mother Nature.
Ground Cover that Prevents Erosion
Learning how to place rocks on a slope to stop erosion isn’t the only way to slow erosion! You can also use plants. If you wish for your slope to remain semi-natural, here are some of the more popular stabilizing plants you can use.
- Grasses. Buffalo Grass and Kentucky Blue Grass have excellent root structures that hold soil in place during wind and water runoff. For more grass selections compatible with growing zones, please visit Southland Organics and their guide on using seed and grass for erosion control.
- Legumes. Alfalfa and red clover also have tremendous root systems. Stay away from sweet clovers. Some of it will grow 6 feet tall.
- Broadleaves. Variegated Snow on the Mountain (Bishop’s weed or goutweed) because it lives in our yard and is impervious to everything – including water, weed whacker, rototiller, and fire. For more suggestions, check out this excellent groundcover for slopes guide.
- Shrubs & Trees. Something hardy that requires little or no care. Such as caragana, lilac, or juniper shrubs. They have root structures requiring a backhoe to remove and grow low to the ground to slow down rushing water.
Another note! The degree of slope and choice of ground cover may require a goat for vegetation control. (Goats have famous agility, which makes them ideal slope lawnmowers!)
Rock Placement In Drainage Channels
It is a rare hillside that has uniform water flow. There are almost always slopes within the slope. In other words – you might have to stand out in the rain and note where the water runs before planning your rock placement.
One of the most successful erosion control methods is the creation of dedicated channels. The channels help with water running off. Finding one, two, or three naturally occurring channels allows you to help nature a bit.
With any luck, it will only require small amounts of widening, deepening, and re-sloping to direct the water where you want it.
Important Note! You are not trying to make a grain chute here. The idea is to slow down the water – even in a dedicated channel. Leave a few bumps and gentle curves.
Line the channel with landscaping fabric and pin it to the ground cover area with river rock or riprap. You can also put rocks in the dedicated channel without the cloth. Skipping the landscape cloth provides a better opportunity to dig rocks into the soil to keep them in place on steeper slopes.
Never forget the following. Water runs through the gaps in your rock bed. Without landscaping fabric, the water may undercut your rocks and allow them to sink or move downhill. You can also have a bigger weed problem sooner that will clog or slow down drainage.
Rock Placement to Prevent Slope Erosion
Many (if not most) terrain slopes are too inconsiderate to run water into two or three convenient locations. So – you may need to learn how to place rocks on a slope to stop erosion with some strategic rock placement to slow down or divert water flow.
Cautionary note! The upside of planting good ground cover is that it will hold your soil in place. The downside is that it could become so root-bound that it cannot adequately absorb anything more than a heavy dew – allowing water to whistle down the slope even faster.
Most slopes are not smooth or consistent areas. They tend to be whatever the last ice age and mother nature make of them. Spend some time watching the rain and runoff. Then plan to get wet and mark areas that will benefit from judicious rock placement.
The purpose of rock placement is not to stop the water. You can’t. Individual rocks, riprap, and gravel collections will slow down, divert, and spread water flow to help prevent erosion.
Rock Placement at the Base of the Slope
Regardless of which system you use to get water to the bottom of the slope without topsoil, you have to do something with it when it gets there. A French drain or blind drain installed perpendicular to the garden slope can get used to absorb or carry the runoff away.
Building a rock retaining wall or placing large stones at the foot of your drain channels will dissipate the force of water coming down the hill. Strategic stone placement can help protect the flatter part of your yard.
Install your French drain between the base of the slope and your retaining wall. Water will be absorbed into the French drain and carried away from your yard.
(See French drain construction at wikihow.com/Build-a-French-Drain.)
French drain (or curtain drain) designs and ideas are limited only by people’s imaginations. There are hundreds of drainage ideas on the internet. Almost all of them use perforated pipes and lots of rocks.
The French drain systems could have a retaining wall of rock, brick, or concrete. The idea of the wall is to stop the water long enough for it to absorb into the drain.
Terraced Erosion Control and Gardening
Building a terraced slope is probably the ultimate in erosion control and land use. Terracing has been used worldwide for centuries to get the best usage out of slopes. It also requires a lot of time and effort.
Historically, building terraces is a matter of making a section of hillside wide enough and flat enough to allow building a rock retaining wall and then flattening the area behind the wall adequately to use as a garden or field area.
The garden area is gently sloped downhill to allow for gentle drainage. Quite often, the bottom of the walls will have gaps built into them to get rid of excess water. Most terrace rock walls follow the contours of the hillside. These designs are less work and make for a softer, more natural look.
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Learning how to place rocks on a slope to stop erosion is a useful skill – rocks are a great addition to your erosion-prevention arsenal!
They do not rot. If placed correctly, they very rarely move. They do not wear away – at least for several millennia. And if you live in a rocky area, they are easy and inexpensive to acquire. (Any farm with rocks will have a pile of instant riprap in the corner of a field.)
A cubic foot of rock weighs an average of around 165 pounds. (Here’s a ton more info about how much stones and rocks weigh.) Their heavy-duty nature means you aren’t throwing a few into the wheelbarrow and running them up the hill.
So – make no mistake about using rocks to manage erosion! Landscaping with garden rocks – especially on a slope – is hard, heavy, and labor-intensive. It usually requires motorized equipment and significant time investment. But the result can be tremendously effective in preventing soil erosion. And the results can also be surprisingly attractive!
If you have questions or tips about how to place rocks on a slope to stop erosion or how to use rocks to manage erosion, please share them.
We love hearing from fellow homesteaders, rockery gardeners, and erosion control enthusiasts!
Thanks again for reading.
Have a great day!