Flow processes, sediment entrainment, transport, and deposition

Question 1

Flow and gravity processes are responsible for what three actions of sediment?

Answer 1

Entrainment, transportation, deposition

Question 2

Name the three generators of fluid flow

Answer 2

Gravitation potential of Earth, gravitational potential of the Moon and Sun, and surface pressure changes

Question 3

Give an example of a flow process generated by the gravitational potential of the Earth

Answer 3

The unidirectional flow of water in rivers

Question 4

Give an example of a flow process generated by the gravitational potential of the Moon and the Sun

Answer 4

Unidirectional flow of water as tides. Reversed every six hours.

Question 5

Give three examples of a flow process generated by surface pressure changes

Answer 5

Unidirectional flow of air from high to low pressure. Storm surges. Oscillatory flow in water

Question 6

Describe turbulent fluid flow

Answer 6

Chaotic particle movement, movement averaging in one vector

Question 7

Describe laminar fluid flow

Answer 7

All particles move in a vector

Question 8

What affects the Froude number?

Answer 8

Flow velocity, gravity, and flow depth

Question 9

What does a Froude number above 1 indicate?

Answer 9

Supercritical, laminar flow

Question 10

What does a Froude number below 1 indicate?

Answer 10

Subcritical, turbulent flow

Question 11

Describe a typical supercritical flow

Answer 11

Fast and shallow

Question 12

Describe a typical subcritical flow

Answer 12

Slow and deep

Question 13

Describe how a sediment is entrained by a flow

Answer 13

The viscosity imparts a shear stress of the grain, which initiates drag and lift

Question 14

What is described by Stoke’s Law

Answer 14

The threshold for entrainment, transportation, and deposition by fluid flow

Question 15

With U as the settling velocity, what is the result of U<flow velocity?

Answer 15

The grain will be entrained and transported

Question 16

With U as the settling velocity, what is the result of U>flow velocity?

Answer 16

The grain will be deposited

Question 17

What does the Hjulström curve show?

Answer 17

Deposition velocity increased with grain size. Entrainment velocity is higher with coarse and medium grains due to friction.

Question 18

Describe a suspended load

Answer 18

Smaller particles at higher velocities, where the particles are in suspension above the bed.

Question 19

Describe a bed load

Answer 19

Larger particles roll and salute (jump)

Question 20

Describe the process that results in upward fining in a bed

Answer 20

Flow velocity decreases passing through different thresholds of deposition for different grain fractions, depositing larger grains first.

Question 21

What is a bedform?

Answer 21

The features that develop at the interface of a moveable sediment surface and flowing fluid

Question 22

Give an example of bedforms and what they can be used for

Answer 22

Ripples and dunes. Can be used to reconstruct flow depth and velocity (Froude number)

Question 23

How do bedforms become sedimentary structures?

Answer 23

When they are buried and preserved

Question 24

What bedforms are produced from a unidirectional flow?

Answer 24

Asymmetric ripples and dunes

Question 25

What characterises asymmetric ripples and dunes?

Answer 25

Low angle stoss that faces up flow and erodes and a steeper lee that faces down flow and is deposited

Question 26

Which is more likely to be persevered, the stoss or the lee?

Answer 26

Lee

Question 27

What are the parameters (amplitude and wavelength) of ripples?

Answer 27

<10cm amplitude and <60cm wavelength

Question 28

What happens to ripple size when grain size increases?

Answer 28

Increases

Question 29

What are the parameters (amplitude and wavelength) of dunes?

Answer 29

> 10cm amplitude and >60cm wavelength

Question 30

What dictates dune size?

Answer 30

Depth of flow (deeper water result in larger dunes)

Question 31

What causes changes in dune morphology?

Answer 31

Increasing velocity

Question 32

Why can wind generated dunes be much bigger than water generated dunes?

Answer 32

Because of the potential depth of air compared to potential depth of water

Question 33

What can wipe out bedforms entirely? And what is this known as?

Answer 33

As the flow approaches supercriticality (laminar flow), the bedforms can be wiped out. Known as upper stage plane beds.

Question 34

How are waves (oscillation) produced in water?

Answer 34

Wind exerts a shear stress on the water

Question 35

Why are oscillation flows only formed in shallow water?

Answer 35

This is where the oscillation cells interact with the seafloor. Only depths of up to half a wavelength are affected.

Question 36

What do oscillating flows produce?

Answer 36

Symmetrical ripples

Question 37

Describe a symmetrical ripple

Answer 37

They have no stoss or lee and do not migrate in any direction (do not prograde). Instead they grow upwards (aggrade).

Question 38

What do cross sections of symmetrical ripples show?

Answer 38

Wave ripple cross lamination. Presents as overlapping laminae that dip equally in both directions.

Question 39

How are hummocks and swales produced?

Answer 39

Oscillatory flows are transferred to greater depths during storms. Density flows are caused by increase in sediment suspension concentrations This flow sinks through gravity, producing hummocks and swales