What is riprap? Simply put, it is a permanent layer of large, angular, natural stone, cobbles, or even boulders typically used to armor, stabilize, and protect soil surface against soil erosion. We usually see riprap edging in areas of concentrated flow or wave energy.
Typically placed along the graded ditch, channel, and shoreline banks over geotextile. Placed to prevent erosional undercutting, riprap edging can also be used with other mixed size rocks. In that case, you can construct retention berms for sediment traps and check dams protecting high volume culvert inlets.
Purpose and function
As we said before, riprap is used to stabilize areas on a construction site with high erosive power. How does it do it? By increasing surface roughness and slowing the velocity of runoff. Applicable areas include inlets and outlets of storm pipes and culverts, bridges, slope drains, storm drains, and other areas with concentrated runoff. Here is a quick summary of its purpose and uses:
- Protect slopes, channels, and areas subject to water erosion by wave action
- Protect soil from erosion due to concentrated runoff
- Stabilize slopes that are unstable due to seepage
- Slow the velocity of concentrated runoff and increase the potential for infiltration
Advantages and disadvantages
Let’s talk about the pros and cons of using riprap edging. As we said before it is an easy-to-use method for decreasing velocity and protecting slopes from erosion. The biggest advantage is that this method is simple to install and maintain.
Mowing and maintenance is easier
Imagine what you might be doing without a decorative border like riprap. Rock along your shoreline that starts in the water comes up the slope, and then ends up in your yard. You end up with large-sized rocks in your rock garden.
That would make mowing and maintenance of your yard much harder. You cannot get your mower into all the hundreds of small voids between each crushed stone.
Increase protection against high water
With a decorative border like riprap, you keep your shoreline from washing away. It increases the protection against heavy waves and high water. Put the edging just two feet farther landward from your shoreline structures, and you get much more protection.
And all you need is a single piece of riprap fabric.
Keeps dirt out of your yard
Without a border of some kind, all fresh topsoil can run downhill and into your rip rap. Tiny bits of debris can puncture and weaken your fabric and dirt can become weedy patches.
The riprap border creates a small barrier that will hold back dirt and debris in the case of heavy rain.
Keeps grass clippings out
Here is another advantage. Without a border along your shoreline, you might be forced to weed-whip every week. And all that whipping will shoot grass clippings all over your fresh and new clean riprap. With a border like rock wall, you keep grass clippings out.
Do you want to add some curb appeal to your home and yard? Well, riprap edging can help you with that. You will get a smooth edge along the landward side of your riprap.
The edging also frames your shoreline and adds some curves. In the end, it makes your entire yard job look more professional and your property more inviting.
With everything in life, there are advantages and disadvantages. In this case, the pros vastly outweigh the cons. Still, we have to mention some possible problems.
High price tag
There is no getting around this. You are using much more landscape rock, much more fabric, and you are installing edging. That will cost more.
Can suffer damage
Ice ridges are part of the life for many lakeshore homeowners in the northern part of the country. Yes, you might do anything you can to protect your shoreline from Mother Nature. But an ice ridge can cause damage to your rip rap.
Does not provide the habitat enhancement
Compared to vegetative practices, riprap edging does not provide habitat enhancement. But hey, you have to make a choice in life. You cannot have everything.
When you plan a riprap installation, it is important that you consider several factors. Here are some planning considerations before you think of riprap edging.
- It should be placed as soon as possible after site disturbance begins and before additional water is concentrated in the drainage system
- Use well-graded riprap rather than uniform riprap, it will form a dense and flexible cover that can adapt well to different situations
- Use appropriate size rock for the discharge or flow velocity. When you are in doubt, go for larger, not smaller rocks. Water can easily carry away small rocks
- Always go for angular and variably sized rocks. Use the largest for the flow rate of the discharge, but also include enough small and sharp-edged rocks that will lock the large rock
- Riprap has to be placed on a proper filter material of sand, gravel, or fabric that will prevent soil from piping through the stones
- Stones should be of good quality and placed to a proper thickness
As we said before, graded riprap consists of a variety of stone sizes. That is why you need to use a special method to specify the size range of the mixture of stones.
You can do it by specifying a diameter of stone in a mixture for which some percentage, will be smaller. For example, when we say d85, we refer to a mixture where 85% of the stones by weight should be smaller than the diameter specified.
The design is based on the median size of natural stone in the mixture.
How to install
Now, let’s talk about a step-by-step guide on how to install riprap edging. How to install a riprap shoreline that will stand the test of time and resist erosion?
Start by deciding on the proper grade. That should be no steeper than a 3:1 slope, horizontal to vertical. Grade the shoreline prior to fabric and riprap installation.
Next is compaction. Without it, a proper grade will be destroyed. Compact the grade prior to fabric and riprap installation.
Now, about the fabric, it should be thoroughly stapled to the ground or shoreline. The best is to use a 6-inch steel staple throughout the entire sheet of fabric. The fabric should also be free of seams wherever possible. If there is no way to avoid a seam, the fabric must be overlapped by several feet and shingled in the proper direction.
Now, it is time to decide the proper type and size of riprap. The common option is 6 to 12 inches in diameter. Another option is the 6 to 18 inches. The steeper the shoreline, the larger the riprap.
Use a rock bucket during the installation. It will keep the new shoreline free of dirt and other contaminants. The rock bucket is basically a bucket with small openings in the sides and bottom. Use it to scoop up the landscape rock, allowing the dirt to fall into the bucket. This way, only the clean riprap remains.
When you install the decorative border or splash aprons as many builders call them, it is important for the same piece of fabric beneath the riprap to extend landward far enough and cover the entire area beneath the decorative rock or mulch.