Lecture Twenty Six - Particle transport and erosion of stationary particles Flashcards
How can the way in which a sedimentary structure was formed be determined?
`Distinguish preocesses based on:
- Involvement and nature of interstitial medium between sediment fragments.
–> Water, air, ice or non (or a combination).
- Whether sediments move:
1) Independantly.
–> Driven by motion of medium (e.g. water) - tractional transport.
–> Driven by gravity (with negligable motion from medium) - suspended transport.
2) As large volume of sediment en masse and repidly deposited.
–> Driven by particle interaction (and gravity).
What are the roles of water as an interstitial medium?
Interstitial = filling the pores between grains or particles (water between particles).
Roes of water in sediment transport:
1) Water buoys/suspends light particles (‘Stokes’ Law’).
2) Water acts as a lubricant in som emass flows, reducing the co-efficient of internal friction.
–> Allows mass flows to move more easily over the substrate.
3) Water flows downslope due to its potential energy with only minor disturbance.
4) Relatively high density exerts (extra) force on sediment particles.
–> Drives sediment movement.
In what ways can sediments be transported?
Tractional sediment transport:
Driven by water/air (vs gravity for mass flows).
Particles roll, slide, and/or bounce (salate) down current.
Produces tractional sedimentary structures.
What are the different flow states?
Laminar flow:
Gentle parallel flow paths.
No mixing across flow planes.
No erosion, no bedload transport.
Only already syspended clays remain suspended and are transported.
–> occurs over smooth topography and consistent, low velocity flows.
Turbulent flows:
Flow paths mix and eddy.
Erosion is common.
Course fragments moved.
–> irregular substrates and/or high or variable velocities.
Velocity approaches zero at boundary, creating a laminar (non erosional) layer at the bottom of a water collumn.
What is Reynold’s Numer?
Dimensionless number that is the ratio of inertial fources to viscous forces.
Distinguishing criterion between laminar and turbulent flow.
Increase the substrate irregularity = increased turbulence.
Increase density, velocity and flow depth (aka inertial forces) = increased turbulence.
Increase viscosity = decreased turbulence.
Describe Hjulstrom Diagrams.
To overcome gravity and erode (initiate movement of) particles, a minimum critical water flow velocity must be reached.
Increase particles mass, increase velocity to initialte movement of sediments (with some exceptions).
Hjulstrom diagram = threshold graph for the initiation of grain movement.
Erosional velocity relationships:
Unconsolidated clay/silt = ~20cm/s.
Unconsolidated fine sand = ~20cm/s.
Medium sand-gravel, increasing from 20cm/s to 2-3m/s.
Cohesive clay = ~2m/s
What is Stokes’ Law?
Stokes’ Law = Settling of particles from suspension.
Once suspended, particles are deposited if their current velocity is less than the settling velocity.
The density difference between particles and the medium they are in make a big difference.
What is Frode’s number?
Gravity also plays a role in fluid flow.
Frode number (F subscript R) is the ratio between inertial forces within the flow to the force of gravity.
Determines whether a flow will be tranquil or rapid.
Conclusions:
Increasing water depth = decreasing velocity.
If FR is less than 1 = subcritical turbulence = normal river flow.
If FR is greater than 1 = supercritical turbulence = flood flow, rapids.
Frode number separates lower (FR<1) from upper flow regime (FR>1).
What are the different sedimentary structures formed in different flow environments?
What are the different physical sedimentary structures which can be formed in different flow environments?
Describe the difference between symmetrical and asymmetrical structures formed in sedimentary environments.
Unidirectional current flow:
Asymmetrical ripples and dunes.
Angle of repose (+/- 30 degrees) cross beds (subaerial).
~15-25 (subaqueous).
Any environment with unidirectional water/air currents form asymmetrical structures.
Symmetrical:
Caused by oscillating wave flow - formed due to the up and down motion of the water.
Nearshore ocean or lake environments.
Medium water has no overall, net motion.
