Sedimentology and Palaeontology Lecture 15: Sediment Erosion and Transport

Question 1

Define Erosion

Answer 1

Removal of of Regolith from bedrock

Question 2

What are the three kinds of erosion by gravity

Answer 2

Landslides + Slumping
Surface creep
Collapse

Question 3

What are the four erosion products by water and gravity

Answer 3

Sheet wash
Rills
Gullies
Stream channels

E.g. Red deer river canyon, Alberta

Question 4

What is the product of erosion by wind water and gravity

Answer 4

Yardangs

A mushroom shaped structure erected through rocks having the lower parts of rock eroded away or sandblast

Question 5

Erosion by ice ± water ± gravity

Question 6

What are the controls on the rate of erosion

Answer 6

Climate
Relief
Bedrock

Question 7

What are the transport mechanisms of transporting sediment

Answer 7

Suspension load
* Silt and clay in turbulent
suspension

Bed load/traction
* Sand and gravel (water) or sand (air) that is rolling,
sliding or saltating

Solution load
* Dissolved ions in surface or
groundwater

Question 8

Transport mechanism is a function of which two things

Answer 8

Fluid properties
* Density
* Viscosity
* Velocity
* Turbulence

Grain properties
* Size
* Shape
* Density

Question 9

Describe and explain the two types of fluid dynamics

Answer 9

Laminar flow
All grains/particles flow in the same direction

No mixing or hydrodynamic sorting of grains

Turbulent flows
Grains and particles are flowing in random directions

Mixing and hydrodynamic sorting of grains

Question 10

What is the Reynolds number equation and what does it determine

Answer 10

Re determines if a flow is
turbulent or laminar
* Laminar: Re < 500
* Turbulent: Re > 2000

                   v L p Re =         ------------------
                      µ

v = velocity of flow
L = depth of channel (or pipe diameter)
p = density of fluid
µ = viscosity of fluid

Question 11

Define Entrainment in a sedimentological context

Answer 11

The critical fluid velocity necessary to overcome gravity and friction to move a grain

Critical fluid velocity
* Proportional to grain size for
non-cohesive sand and
larger grains
* Non-linear for cohesive clay
to silt

Question 12

What are the two components in fluid flow

Answer 12

Drag = lateral force due to friction between grain and fluid

Lift = vertical force due to Bernoulli effect

Question 13

Describe the Bernoulli effect

Answer 13

As flow depth (or pipe diameter) decreases the velocity must increase

In order that the same volume of fluid may pass through

Question 14

Why is the Bernoulli effect important in sedimentology

Answer 14

Air travels faster over the top of a wing because it has to travel further

Air is travelling slower under the wing and so the pressure is therefore greater and generate lift

If you are not going fast enough then you cannot generate enough lift

Question 15

What is Stokes law

Answer 15

Stoke’s Law defines the settling velocity of a grain/ at which speed a grain is deposited

Stokes law says that the heaviest and largest grains are deposited first

             Δpd²g  W =      ---------------
                18µ

Δp = density difference between fluid and grain
d = diameter of grain
g = gravity
µ =Viscosity of fluid

Question 16

Describe how grains behave in water

Answer 16

• Need energy to generate lift to entrain grains
• Less energy needed to keep grains in suspension
  once they have been entrained
• Lift must exceed settling velocity

Question 17

Describe the settling velocities and entrainment of mud as a sediment

Answer 17

Fine grain size means very low settling velocities and should be easily entrained

But electrostatic charges mean that clay particles stick together into silt-sized particles by flocculation

Question 18

Describe how grains would behave when being transported/carried by air

Answer 18

Air has lower viscosity and density than water

• Low viscosity and high difference in density between grain and fluid

Grain impacts would have less cushioning and therefore more abrasion of particles.

Traction carpet of saltating sand

Question 19

Describe desert sands

Answer 19

Clays winnowed

• • gravel stays put
• = very well sorted
• Impacts of saltating
  grains
• = well rounded
• = texturally supermature