Lecture 5 Flashcards
What is the bed load
Coarser material down at the base of the river
What force resists the motion on bed forms
Gravity
What can the drag component on the bed also be known as
The shear stress force
What is the force that moves grain up into the flow
The list component
Why do we normally ignore the lift force
Smaller than the shear stress force and really hard to measure
What is the force acting parallel to the bed surface
Shear stress/drag FD
What else can cause the particle to lift up into the flow
Slight pressure difference
What is the pivot angle
Angle around the point of contact with the next downstream grain, angle at which it will get out of the bed
What is entrainment
First initial movement into the river stream
What is sediment transport
Sediment moving across the bed
What can flow entrainment also be seen as
The threshold needed to get to in order to move a grain
What is the name of the diagram used to determine when sediments move
Hjulstrom curve
What is the draw back of the hjulstrom curve
Does not consider the forces just velocity
Who came up with the first model for particle entrainment
Shields 1936
What was Sheilds looking at with his flume experiments
Force balance on the grain
Why does the grain size determine the force stopping motion
Bigger the grain heavier it is
What was main factors shields was looking at in ratio
Shear stress and grain size
What did sheilds find about the number between the forces
Was always the same number
What is the ratio according to shields
0.06
What relationship is there between the shear stress and grain size
Linear
What is critical shear stress
At the point of entrainment
What is pW
Water density
What is pS
Sediment density
What kind of entrainment is proposed by shields
Size selective entrainment
What is the problem with shields equation
For rocks that stick out or for different sizes the entrainment value will be different
What sort of grain sizes was shields using
All the same size
What happens with gravel in most gravel beds
Beds become organised
What is it called when gravel beds become organised
Pavement/armour layer
What is coarser is gravel bed rivers surface or subsurface
Surface, small grains fall through to the sub surface
What is petrusion
When large gravel a stick up into the flow
What is the name for when small grains stop and are then carryed on in transport
Winnowing
What is it called when small particles are out of the way of the flow
Hiding
What is equal mobility an expression of
Relitive size
What is important in terms of the particle and relation to others around it
Size of the particle relative to those around it
When happens when larger particles trap other ones
No particle moves until the biggest one is moved, equal mobility
What is the consensus now which sheilds work
Was important, but there are now more accurate studies that better show entrainment
What did Powell and ashworth show about shear stress
Not one fixed number for critical dimensionless shear stress, between 0.0096 and 0.067
What did Andrews say is the most important thing to consider about grain Size
Relative grain size is more important
What value does tbh e pretending grain have in Andrews equation
Di
What value does the surrounding particles have in Andrews equation
D50
If there is perfect equal mobility what would rhe shear stress component equal
-1
What is the equation for entrainment according to Andrews
Critical dimensionless shear stress= 0.0834(Di/D50) -0.872
If there is no overlap between shear stress and shear stress required for movement what will happen
Nothing, no movement of particles
How do we get entrainment in a river system
Shear stress needs to over lap with shear stress required for entrainment
What is transport
The volume of sediment moved by each transport event
What is qb
Bed load discharge, about of sediment in transport
What cancels out level values of shear stress
Bed load pulses
What does Reid et al 1985 show us about shear stress
Not a simple relationship between that and pulses
What did Gomez and church find out
Looked at 12 sediment transport equations and found that none were particularly accurate best was bagnold
For hydraulically smooth boundaries what drag dominates
Viscous drag
For hydraulically rough boundaries what drag dominates
Form drag
Is there any flow seperation around a grain in laminar flow
No
is there any flow separation around a grain in rough flow
yes
What does the level of effective drag depend on
Depends on the shape and position of the grain on the bed
Which sediment grains move close to the bed
more than 0.1mm
Why do larger grains near the bed move slowly
Intermittent collision with the bed
What is the name for the larger particles that are in contact with the bed while moving
bed load
What is necessary to continue movement of the bed load
Upward dispersive force that must be exactly balanced by the weight of the moving grains (Bagnold 66 and 73)
What is Saltation
Jumping, the dominant mode of bed load transport, with rolling and sliding only at the threshold of entrainment
How does jumping occur
as particles are forced over a bed with increasing velocity, they often are forced in to the air through contact with other rocks
What does the mean height of the bed-load zone control
roughness height
Who came up with the idea of shear stress and shear stress needed for entrinmant graph
Grass 1970
what is the sediment transport rate
the amount of sediment that can be moved past a given width of flow in a given time
What does sediment transport rate control
the formation of bedforms such as ripples and dunes
How can the bed-load transport rate be expressed
ib=Wub, ib is the bed load transport rate, W is the immersed weight of bed load grains and ub is the mean bed load grain
How do grains stay in transport motion
The fluid must exert a mean downstream force to maintain steady motion
