Rip Rap Calculator

This rip rap calculator will help you determine the size of rip rap rock to use to line an embankment like a dike or an open channel.

In this tool, you will learn how to calculate rip rap rock size using the Isbash equation or with the help of a riprap size chart. We will also briefly discuss the riprap definition and some applications of rip rap. Keep on reading to start learning.

A rip rap or riprap (spelled together) is a manmade structure with piled-up rocks acting as lining along an embankment or a streambed. Riprap can either be loose or filled with mortar to make a grouted riprap. Loose rip rap rock is excellent for water drainage, while adding grout to the riprap creates additional support for the rocks.

We usually use rip raps as retaining walls to prevent the earth beneath it from eroding or being scoured by moving water. Other times, we can also use the concept of piling up rocks to make decorative walls or fences even without the purpose of protecting against hydrostatic pressure due to the presence of water or another similar force.

This calculator focuses on ripraps used for streambeds and embankments exposed to moving water. Keep on reading to learn more.

Estimating the appropriate rip rap rock size is necessary to build your rip rap efficiently. Rocks that are too small can be carried away by the rushing waterwhile using huge rocks can be very costly due to logistics and heavy equipment used to pile the rocks together correctly. Because of that, many experiments have already been done to determine the best rip rap rock size for any given scenario.

One of the easiest and simplest methods available is through the use of the Isbash equation, as shown below:

where:

• $D_{50}$ is the average diameter of 50% of the spherical rocks for the rip rap in meters;
• $V$ is the average channel velocity in meters per second;
• $g$ is the acceleration due to gravity, either $9.806\ \tfrac{\text{m}}{\text{s}^2}$ or $32.17\ \tfrac{{\text{ft}}}{\text{s}^2}$;
• $C$ is the Isbash constant with values equal to 0.86 for highly turbulent flow of water or 1.20 for low turbulence water flow; and
• $S$ is the specific gravity of the rock with values ranging from around 2.50 to 3.00.

With the Isbash method, we only have to determine the water's flow rate approaching the embankment on which we want to install the riprap. Apart from that, we also need to remember the two empirical values for the Isbash constant and the range of values for the rocks' specific gravity.

Let's say we have an open channel we want to line with riprap, and the water flows rather gently (low turbulence) through it at an average of 2.0 m/s. Let's also say that a nearby supplier can provide us with any rock size at a specific gravity of 2.5. Using the Isbash equation, we work out the average diameter of rock we need to be:

We now know that we need rocks with a diameter of around 9.44 cm to keep the surface of our open channel from erosion and scouring without affecting the average water flow rate of 2.0 m/s. Perhaps we can purchase enough rocks to make two to three layers of this rock size for our rip rap.

Knowing the rip rap rock size, you need is one essential factor when building a rip rap. Now that we know how to calculate rip rap rock size let us now learn how much rip rap you need for your project. For that, we have to compute the volume of your rip rap by multiplying your rip rap's area by its depth. It is worth noting that making a rip rap with a depth of at least twice the value of $D_{50}$ is recommended.

Since our supplier may also ask how much rip rap we need in terms of weight, we can obtain it by multiplying the volume we computed by the density of the rock we'd like to use. Riprap rocks have densities that vary from about 1,600 kg/m³ to 2,800 kg/m³ (100 to 175 b/ft³).

Using our rip rap calculator is very intuitive. You can use it to find $D_{50}$ or the maximum water velocity a rip rap with a particular $D_{50}$ can support. Here are the steps to find $D_{50}$:

1. Input the water velocity approaching the rip rap.

2. Select the Isbash constant for your rip rap.

3. Modify the gravitational acceleration if you're using a different rounded-off value or if you want to experiment on other gravitational acceleration values. We have set it by default to 9.806 m/s² (32.17 ft/s²).

4. Enter the specific gravity of the rock available from your local supplier.

On the other hand, to evaluate the water velocity, just skip the first step, perform steps 2 and 3, and fill in the $D_{50}$ field. Doing so will instantly display the maximum water velocity for the entered specifications.

You can also use our rip rap calculator to determine how much rip rap you need in terms of volume and weight. To do it with our calculator, you can follow these steps:

1. Select "rip rap volume" in the first field of our calculator.

2. Enter the rip rap dimensions, like the area to be covered and the rip rap depth. This will instantly find the volume and weight of rip rap rock you need. We have also included a wastage variable to account for the possible wastage of materials during the riprap's construction.

3. Input your preferred wastage for your project.

4. Finally, enter your rock's density in our rip rap calculator. We have it set to a default value of 1,680 kg/m³ (104 lb/ft³).

If you know how much your riprap costs per unit weight, you can input it into our calculator by clicking on the Rip rap cost heading to estimate how much your entire project could cost you for the riprap rocks.

Now that we know how to calculate rip rap rock size, let us take a look at a shortcut version of finding what rip rap rock size you need. As a quick reference, here is a riprap size chart for some of the typical rip rap rock sizes you might find available on the market today for specific velocities and turbulence of the water flow:

Thinking of other uses for rocks and gravel? Check out our gravel driveway calculator to learn about lining your driveway with gravel. If in case you want an even sturdier driveway, you can check our asphalt calculator.