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.

What is rip rap? The riprap definition

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.

How do we calculate rip rap rock size?

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 water, while using huge rocks can be very costly due to logistics and heavy equipment used to pile the rocks together correctly. With that, a lot of 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. The Isbash equation is straightforward and the simple-to-follow formula is shown below:

D50 = V² / (2 * g * C² * (S - 1)),

where:

  • D50 - Average diameter of 50% of the spherical rocks for the rip rap in meters;
  • V - Average channel velocity in meters per second;
  • g - Acceleration due to gravity, either 9.806 m/s² or 32.17 ft/s²;
  • C - Isbash constant with values equal to 0.86 for highly turbulent flow of water or 1.20 for low turbulence water flow; and
  • S - 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 we want to install the riprap on. Apart from that, we also need to keep in mind the two empirical values for the Isbash constant and the range of values for the rocks' specific gravity.

Sample calculation of rip rap rock size

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 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:

D50 = V² / (2 * g * C² * (S - 1))

D50 = (2.0 m/s)² / (2 * 9.806 m/s² * 1.2² * (2.5 - 1))

D50 = (4.0 m²/s²) / (19.612 m/s² * 1.44 * 1.5))

D50 = 0.09442 m = 9.44 cm

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.

How much rip rap do I need?

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 it is recommended to make a rip rap with a depth of at least twice the value of D50.

Since we may be also asked by our supplier 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).

How to use our rip rap calculator?

Using our rip rap calculator is very intuitive. You can use it to find D50 or the maximum water velocity a rip rap with a particular D50 can support. Here are the steps to find D50:

  1. Input the water velocity approaching the rip rap.
  2. Select the Isbash constant for your rip rap.
  3. 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 D50 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 just in case.
  3. You can input your preferred wastage for your project.
  4. You can also enter your rock's density in the advanced mode of 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 to estimate how much your entire project could cost you for the riprap rocks.

Riprap size chart

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:

Water velocity
D50 at low turbulence
D50 at high turbulence
(m/s)
(ft/s)
(cm)
(in)
(cm)
(in)
1
3.28
2.4
0.9
4.6
1.8
2
6.56
9.4
3.7
18.4
7.2
3
9.84
21.2
8.4
41.4
16.3
4
13.12
37.8
14.9
73.5
28.9
5
16.40
59.0
23.2
114.9
45.2

Want to learn more?

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.

FAQ

How do I install a riprap?

Riprap installation can be done even when water is present. However, it would be best to install a riprap without water so that you can pile the rocks properly. Pile the rocks on top of each other making sure they're stable on their own. Fill the gaps with smaller rocks for more stability. Fill the gaps with grout if extra strength is needed.

What is a rip rap for?

The stream of water can quickly scour the bottom of the river due to the absence of underwater vegetation or rocks. To help avoid scouring and soil erosion at the bottom and sides of a river, we can cover them with layers of rocks, and we call these layers of rocks the riprap.

Why does rock size matter in building a rip rap?

To effectively prevent soil erosion with a riprap, we need to choose the right rock size that does not get easily moved by the approaching water. Small rocks could get easily eroded by the stream of water, undermining their purpose. In contrast, huge rocks can affect the stream's natural flow rate, leading to water stagnation or difficulty in water distribution.

What is a D50 riprap?

D50 riprap is a riprap with 50% of its rocks' diameters smaller or, at times, larger than its recommended rock size. The other 50% of the rocks have the D50 diameter. On the other hand, a D90 riprap has 90% of its rocks either smaller or larger than the recommended D90 diameter. Only 10% of them are around the D90 diameter.

How much rip rap do I need?

  1. Determine the area and thickness of the riprap you want to build. Thicker ripraps need more layers of rocks.
  2. Multiply the area by the thickness of your riprap to find its total volume.
  3. Multiply this volume by your desired rock's density. Finding the weight is optional but sometimes, suppliers will ask you for it rather than the volume.

How many square feet does a ton of riprap cover?

For a depth of 1 foot, a ton of riprap can cover around 20 square feet of rocks. Then imagine taking the rocks in half of this area and piling it over the other half making a 2 foot deep riprap. For that, your 1 ton can then cover 10 square feet of riprap rocks.

Kenneth Alambra
I want to find the...
average rock diameter (D₅₀)
Rip rap specifications
Water velocity (V)
ft/s
Isbash constant (C)
Highly turbulent (0.86)
Specific gravity (S)
Average rock diameter (D₅₀)
in
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