Question

In wind erosion, the amount of soil transported varies

• A. directly as the cube of the threshold wind velocity and inversely as the square of the mean soil particle diameter
• B. directly as the cube of the actual wind velocity and inversely as the square of the mean soil particle diameter
• C. directly as the cube of the difference in actual and threshold wind velocity and the square of the mean soil particle diameter
• D. directly as the cube of the difference in actual and threshold wind velocity and the square root of the mean soil particle diameter

Hint:

Think about which velocity term actually drives particle movement once the threshold is crossed.

Answer 1

The Correct Option is D

Soil transport by wind erosion depends on how far the wind speed exceeds the threshold speed needed to start moving particles, and on the size of the particles themselves.

1. Cube of threshold wind velocity, inverse square of particle diameter: The threshold velocity alone does not tell us the driving force once the wind is already blowing above it, so this option misses the actual excess velocity that does the work.
2. Cube of actual wind velocity, inverse square of particle diameter: Using only the actual velocity ignores the fact that no transport happens until wind speed crosses the threshold, so this is not the correct dependence.
3. Cube of the velocity difference, and the square of particle diameter: This gets the excess velocity term right but pairs it with the square of the particle diameter, which does not match the accepted relation for particle size.
4. Cube of the velocity difference, and the square root of particle diameter: The soil transport rate rises with the cube of the difference between actual and threshold wind velocity, since this excess velocity supplies the energy for saltation and creep, and it varies with the square root of the mean particle diameter, reflecting how easily grains of a given size get moved.

The relation that matches the physics of wind erosion transport is option (D).