Sediment Transport Flashcards
Waves, currents, gravity and wind
What is competence in sed. transport?
An assessment of the largest particle a medium can transport.
How does viscosity effect turbulence?
An increase in viscosity reduces turbulence. Low turbulence results in less transport and reduction in erosion and sediment entrainment.
How does velocity change through a body of material?
Velocity is highest at top of unit, because of higher frictional forces at bottom. Shear stress = viscosity * shear rate
What is the Reynolds number and what does it represent?
The Reynolds number is used to determine whether a fluids flow is laminar or turbulent (dimensionless).
Re = water depth/ kinematic viscosity = (densityvelocitychannel diameter)/(viscosity)
Re<2000 = laminar
Re>2000 = turbulent
What is an eddy and how does it form?
An eddy is the swirling of a fluid and the reverse current created in a turbulent flow regime. Moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object.
Where does most sediment transport take place?
Within the boundary layers, layers between the bottom surface and part of the column where velocity increases.
What is the Froude number and what does it represent?
A dimensionless number defined as a ratio of flow inertia to the external flow. AKA. the tendency of a moving fluid to continue moving despite the gravitational forces acting upon it.
What are the 2 forces which lead to entrainment?
- Fluid Drag: Horizontal force in direction of flow.
- Fluid lift: Vertical force raising particles into the water column.
What are the 3 types of sediment loads?
- Dissolved load: Fraction in solution as ions.
- Suspended load: fine portion which is kept in suspension by interactions with water.
- Bed load: portion that cannot be kept in constant suspension. Moves along bed of water channel.
explain the difference between saltation vs traction.
Saltation: Bouncing of material along base of channel, or repetitive pick-up and dropping.
Traction: Rolling and sliding of material along base of channel.
What does the Hjulstrom diagram represent?
The relationship between the streams velocity/energy and the size of class which it can transport.
Describe Stokes’ Law.
Describes 3 types of forces experienced by a falling object:
a. Gravity
b. Buoyancy
c. Drag: pushing upward
What is the settling rate?
Settling rate describes the point where drag and buoyancy balance gravity. Larger particles induce sufficient turbulence.
What factors are required for deposition to occur?
a. velocity below the threshold velocity for a given grain size.
b. change in stream profile - changes in gradient; bed roughness; stream confinement; stationary water.
What is flocculation?
A process of contact and adhesion whereby the particles of a dispersion form a larger particle.
