Rip Rap Calculator

What is a Rip Rap Calculator?

A rip rap calculator helps you determine the appropriate rock size (D50) for erosion control using the Isbash equation. It calculates the average rock diameter needed to resist water flow on embankments, streambeds, and open channels. You can also estimate the total volume, weight, and cost of rip rap material for your project.

How to Use the Rip Rap Calculator

The rip rap calculator has two main calculation modes:

1. Find D50 from velocity: Enter the water velocity, select turbulence level, and specify rock specific gravity to get the recommended rock size.
2. Find max velocity from D50: Enter the rock diameter, turbulence level, and specific gravity to find the maximum water velocity the rip rap can withstand.
3. Optionally, enable "Calculate volume and weight" to estimate the total rip rap needed for your project area.

Isbash Equation

The Isbash equation is used to determine the required rip rap rock size:

$$D_{50} = \frac{V^2}{2 \times g \times C^2 \times (S - 1)}$$

Where:

• $D_{50}$ = Average rock diameter (50% of rocks are smaller than this size)
• $V$ = Average water velocity
• $g$ = Acceleration due to gravity (9.806 m/s²)
• $C$ = Isbash constant (1.20 for low turbulence, 0.86 for high turbulence)
• $S$ = Specific gravity of rock (typically 2.50 to 3.00)

For volume and weight calculations:

$$V_{\text{total}} = A \times D \times (1 + W / 100)$$

$$W_{\text{total}} = V_{\text{total}} \times \rho$$

Where $A$ is area, $D$ is depth, $W$ is wastage percentage, and $\rho$ is rock density.

Rip Rap Size Chart

As a quick reference, here are typical rip rap rock sizes for different water velocities: