Flow processes, sediment entrainment, transport, and deposition Flashcards
Flow and gravity processes are responsible for what three actions of sediment?
Entrainment, transportation, deposition
Name the three generators of fluid flow
Gravitation potential of Earth, gravitational potential of the Moon and Sun, and surface pressure changes
Give an example of a flow process generated by the gravitational potential of the Earth
The unidirectional flow of water in rivers
Give an example of a flow process generated by the gravitational potential of the Moon and the Sun
Unidirectional flow of water as tides. Reversed every six hours.
Give three examples of a flow process generated by surface pressure changes
Unidirectional flow of air from high to low pressure. Storm surges. Oscillatory flow in water
Describe turbulent fluid flow
Chaotic particle movement, movement averaging in one vector
Describe laminar fluid flow
All particles move in a vector
What affects the Froude number?
Flow velocity, gravity, and flow depth
What does a Froude number above 1 indicate?
Supercritical, laminar flow
What does a Froude number below 1 indicate?
Subcritical, turbulent flow
Describe a typical supercritical flow
Fast and shallow
Describe a typical subcritical flow
Slow and deep
Describe how a sediment is entrained by a flow
The viscosity imparts a shear stress of the grain, which initiates drag and lift
What is described by Stoke’s Law
The threshold for entrainment, transportation, and deposition by fluid flow
With U as the settling velocity, what is the result of U<flow velocity?
The grain will be entrained and transported
With U as the settling velocity, what is the result of U>flow velocity?
The grain will be deposited
What does the Hjulström curve show?
Deposition velocity increased with grain size. Entrainment velocity is higher with coarse and medium grains due to friction.
Describe a suspended load
Smaller particles at higher velocities, where the particles are in suspension above the bed.
Describe a bed load
Larger particles roll and salute (jump)
Describe the process that results in upward fining in a bed
Flow velocity decreases passing through different thresholds of deposition for different grain fractions, depositing larger grains first.
What is a bedform?
The features that develop at the interface of a moveable sediment surface and flowing fluid
Give an example of bedforms and what they can be used for
Ripples and dunes. Can be used to reconstruct flow depth and velocity (Froude number)
How do bedforms become sedimentary structures?
When they are buried and preserved
What bedforms are produced from a unidirectional flow?
Asymmetric ripples and dunes
What characterises asymmetric ripples and dunes?
Low angle stoss that faces up flow and erodes and a steeper lee that faces down flow and is deposited
Which is more likely to be persevered, the stoss or the lee?
Lee
What are the parameters (amplitude and wavelength) of ripples?
<10cm amplitude and <60cm wavelength
What happens to ripple size when grain size increases?
Increases
What are the parameters (amplitude and wavelength) of dunes?
> 10cm amplitude and >60cm wavelength
What dictates dune size?
Depth of flow (deeper water result in larger dunes)
What causes changes in dune morphology?
Increasing velocity
Why can wind generated dunes be much bigger than water generated dunes?
Because of the potential depth of air compared to potential depth of water
What can wipe out bedforms entirely? And what is this known as?
As the flow approaches supercriticality (laminar flow), the bedforms can be wiped out. Known as upper stage plane beds.
How are waves (oscillation) produced in water?
Wind exerts a shear stress on the water
Why are oscillation flows only formed in shallow water?
This is where the oscillation cells interact with the seafloor. Only depths of up to half a wavelength are affected.
What do oscillating flows produce?
Symmetrical ripples
Describe a symmetrical ripple
They have no stoss or lee and do not migrate in any direction (do not prograde). Instead they grow upwards (aggrade).
What do cross sections of symmetrical ripples show?
Wave ripple cross lamination. Presents as overlapping laminae that dip equally in both directions.
How are hummocks and swales produced?
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
