Introduction to Riprap

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What is riprap?

Riprap is large rock used for erosion protection along shore lines, stream banks, under bridges, and wherever there are embankments where soil/sod might erode due to continued contact with water or wind.

The weight of riprap rocks can be from a few pounds to a couple hundred pounds. Typically, riprap rocks are jagged and angular. Think of them as puzzle pieces. This helps them lock together, form stronger bounds and therefore move less preventing erosion.

The best type of riprap comes from granite rock. Granite is ideal because it is heavy and durable. If you use weaker rock for your riprap they will crumble, which defeats the purpose of using the riprap as protection.

Where does the name come from?

It is a rather unusual sounding word isn’t it! According to the boys over at lakeshoreguys.com:

“American English of 1822 the word riprap was connected with a nautical word, rip-rap, which meant a “stretch of rippling water, often caused by underwater elevations.”  The word “rap” also meant “blow, or strike.” People may have started to call it rip rap because the waves constantly blow into or strike the rocks – the same rocks that cause “a stretch of rippling water” when placed in shallow waters.”

[lakeshoreguys.com]

How is riprap made?

Quarries blast small sections of hills or mountains, and then sort the rock, screening it by size.

What is median rock dimension?

A batch of riprap rock consist of various sizes so that void spaces of larger rocks, can be filled in with smaller rocks, giving full coverage. A batch of riprap rock is not all one size of rock.

When choosing what size of riprap to use for your project (different projects may require larger and larger rock), a system called MRD is used: median rock dimension.

Let’s say the average sized rock you want in your riprap application is 12 inches. When you go to get your rock, you’ll get an MRD of 12 inches. 50% of your rocks will be smaller than 12 and the remaining 50% will be larger than 12. Why is this? Because when you use riprap you need a variety of rock sizes: large rocks for the base, and then smaller rocks to fill the voids, this gradation is what helps the rocks lock in together and resist erosion.

For instance, a 12-inch rip rap can include rocks as small as 4 inches, and rocks as large as 21 inches. It is not just rocks that are 12 inches.

It is important to note that the more erosive the location, the larger riprap rocks you’ll need to resist the erosive forces from removing the soil.

Also, the steepness of the slope of the embankment dictates what size riprap you need. 

What are the 3 types of riprap?

Conventional riprap

Commonly used for erosion protection at shorelines, and pipe outlets. It does not look aesthetically pleasing and is hard to walk on, and may cause people to twist their ankles. It is best used in areas where there is no concern for vegetation.  

Soil riprap

Riprap with all voids filled with soil, or it can be buried with a layer of topsoil on top. This is a great option if you want to have vegetation above the riprap. This option is the most natural looking, as you can’t even see it since the rock is under the ground.

Void filled riprap (VFR)

Conventional riprap, with voids filled with cobbles gravels sands, soils. It creates a dense interlocking mass of rock. This is more suitable for areas that have a fairly constant flow of water.  

Design considerations

When it comes to design you want it to look good, but you also want it to do its job effectively, and not at the expense of aesthetics. There are two options:  

Angular rock

• Interlocks better but looks not as natural.

Rounded rock

• More natural looking but can move more, and doesn’t lock as well.

Where should you get your riprap?

At Kalamazoo we’ve got what you need from riprap to boulders and all things in between. We also have the most gorgeous colors in the area. Visit our catalog to see for yourself!