Lecture 4 Flashcards
What is the law for fine suspended sediment
Stokes law, only for fine gravels and clay
How does turbulence influence travel within the flow
Can cause higher retention depending on sweep burst ect, also caused shear stress to pick up grains
What is the name of the process of a particle falling to the bed
Settling, particle settling time
What is the stokes law equation
Vg=((o-p)gDsquared)/18
What is the name for the Greek symbol used to represent sediment density
Sigma o
What is the name of the Greek symbol used to represent water density
Rho p
What is the symbol used to show viscosity
u
What is the settling velocity a function of
Particle size square
What is the common value we use for sediment density
The value of quartz 2650kg per cubic meter
What is the normally value of water density
A thousand kg for cubic meter
What is the value of gravity
9.81
What is fluid viscosity
The ability for the liquid to move eg lava vs water
Why might some particles behave differently to other particles and fall faster
Cohesion with other particles, flocculation
How does a particle remain in suspension
If the positive verticals fluctuations are greater than the negative ones and Settling velocity
What material can stay in the flow for a long time
Very fine material
What happens if the positive v fluctuations are not greater then Vg and negative v
Particle will fall out of suspension
Where is the concentration of larger sediments
Near the bed
What does stokes law only hold for
Smooth spheres falling through a still fluid at low Re
In what law is fall velocity proportional to root of Diamater
Impact law
With small particals what is unique in the way they move in the flow
They move through the liquid as if the flow is laminar
With larger particles how do they behave in the water
They have more mass so they create turbulent Wakes
At what particle size of most common for stokes law
Less than 0.01cm
What is it called when particles join together
Aggregation, flocculation
At what sizes particle is viscous drag dominant
Less than 0.1mm
What size partical is form drag dominant
Larger than 0.1mm
When is the viscosity of the water changed
When there is a huge amount of suspended sediment
What happens with sediment density in the case of the Rio bermejo
The denser bermejo sinks under the Paraguay
At what sizes do particles start becoming more common at the bottom of the river
0.062-0.125 mm
What particle size is only found at the bottom of the river depth
Over 0.25 mm
What is the highest concentration of upper river sediment by size of grain
Less than 0.002 mm
What is the method to calculate sediment concentration at depth
Rouse equation
What is k in the rouse equation
Von karman constant
What is the rouse equation based on
Th fall velocity of the different particles
What is the section of the rouse number called that is fall velocity divided by shear velocity x Von Karmans constant
Rouse coefficient
If the rouse number shows a low value, what does this mean?
Fine particles and high turbulence and velocity, uniform distribution of suspended sediment
What do high rouse numbers show
Coarse particles, low velocity and turbulence, strong vertical sediment distribution
What are some other factors that control suspended load
Precipitation intensity, run off amount, site area sampled, differences in transport rates, amount of sediment already in the channel and the depletion is easily erodible debris
Who created the idea of hysteresis graphs with river sediment suspended load
Williams 1989
If there is an even level of sediment with discharge what will the graph look like
Straight
What is the name of the straight type of graph seen in Williams 1989
Class 1
What creates curving in a Williams class 1 graph
C/Q reach the same peak at the same time but are spaced differently
What causes a Williams 1989 class 2 graph and what is it called
Clockwise loop, caused by when concentration change rises faster on the rising limb then discharge
How is a counter clockwise loop created (class 3)
Discharge (Q) rises faster than C with Q dropping on the way back
What conditions are associated with a class 1 graph
Uninterrupted sediment supply throughout the flood, but increased and decreased at the same rate
What conditions create a class 2 clockwise loop
Either a depletion of available sediment before discharge has peaked or flushing out of sediment as discharge builds, common in the early part of a storm
What conditions can cause a class 5 figure 8 loop
Ice break up or another peak which causes the hydrograph to show two distinct peaks between sediment and discharge
What causes grains to accelerate towards the bed
Gravity
When does a grain reach terminal settling velocity
When the fluid drag on the accelerating grain becomes equal to the weight of the grain
What are the separation forces detached from each side of the falling grain
Karmen Vortex
What do rotating grains experience that is different to a non oscillating grain
Magnus effect
If the stokes number is less than 1 what path do particles follow
Follow the motion of turbulence eddies
What is the vertical velocity profile sometimes called
Law of the wall
What conditions can cause a type 3 graph
Flood waves move faster than sediment flux, high soil erodability throughout flood event
What happens as a large amount of sediment is added to river systems
Can enhance turbulence especially near the bed for example shear stress
Why is turbulence enhanced under thicker flows
As there is more viscosity it is harder for water to move past each other and more resistance is created
What happens when more sediment is put into an already turbulence enhanced flow setting
Turbulence is dampened and there is a chance that a laminar plug flow can occur at the top of the flow
Who showed the idea of turbulence enhanced flow and turbulence plug flows
Bass et al 2009
What causes the turbulence to damped in high sediment systems
Greatly increased viscosity
At a dune level, what features are present in an Baas turbulent enhanced flow
More turbulence in the seperation zone, maximum turbulence at reattachment point, packages of low velocity upwards shed, low turbulence near surface due to plug development
At what area of the dune are boils created
Reattachment point, area of highest shear stress
When can boils mic suspended sediment
All the way from the bed to the surface
What is the name of the bursting structure that can be created by dunes
Dune generated vortex m, horseshoe vortex
What is the name of the zone where the boil is present on the waters surface
Upwelling zone
What is the symbol for settling velocity
Vg
What is the symbol for sediment transport rate
i
What are the 4 categories of grain shape
disc, sphere, tabulate. rod
what is the predominate size of sediment grains in river systems
1/2 - 1/4 of a mm
Why will a grain sink
if the weight if the grain exceeds buoyancy force
What is the immersed weight of a sediment grain
gravity force minus the buoyancy force
What is the resistance to motion acting on sediment particles
drag
What type of drag is caused by fluid pressure
form drag
What is the name given where the drag on an accelerating grain becomes equal to the immersed weight of the grain
terminal settling velocity
