# Describe the relationship between critical erosion velocity and particle size

### River Processes

Critical erosion velocity→ the minimum stream velocity to entrain and erode a load Use Hjulstrom curve to explain relationship between particle size, velocity . Deposition / Settling Velocity of Particles in Still Water relation to neighboring grains. what is meant by “the beginning of sediment movement” (a.k.a. “ incipient motion”). For large grains on the bed, α is smaller than for the average grain size, and for small . roughly defines a dimensionless critical shear stress of The critical erosion curve shows the MINIMUM velocity required to lift a But below 1mm the relation between particle diameter and velocity is not as well For you paperless earthcachers, I'll try to describe the dimensioning.

What speed must the river be traveling at to erode a particle of size 10mm? Explain why it is more difficult to erode clay and silt.

Name the type of sediment that requires the lowest velocity to be eroded. Name the type of sediment that is likely to be transported at all velocities. Describe and explain the relationship between water velocity and the erosion of clay and sand particles. Explain the variation in water velocity that is required to transport and to deposit sediments of different particle diameter.

What about different densities? SHAPE of load is important too why? What is the largest size of particle it can transport? What is the largest particle it can now erode?

## Channel Processes and Hjulstrom’s Curve

Describe and explain what happens to the load during this phase Answers: As the particle size increases, a river will require more energy in order to erode or to transport its load. However, the above rule does not apply to clay and silt particles which requires high river velocity to erode, as it is extremely cohesive. Hence the mode of transportation is traction as the river drags the particle from its stationary mode.

This is because clay and silt sticky in texture and tend to coagulate amass and stick to one another.

## River Processes

Generally; Larger particles require more velocity to be lifted off the bed Larger particles will be deposited at higher velocities where smaller particles will remain in transport. There are different types of load, including bedloadsolute or dissolved load and suspended load which held in the water. During low flow periods rivers will tend to carry only dissolved and suspended load, and when velocities pick up they will carry bedload as well. Finally, the capacity of a river tends to increase with distance downstream as volumes and velocities increase.

Fill in all the gaps, then press "Check" to check your answers. Use the "Hint" button to get a free letter if an answer is giving you trouble. You can also click on the "[? Note that you will lose points if you ask for hints or clues! The smallest clay particles require velocities of cm per second to be eroded. For the smallest silts it is approximately cm per second. Cobbles are upwards of cm per second. Deposition starts to occur at cm per second for particles of approximately 0.