Lecture #4 Flashcards
3 characteristics of discharge
- Channel shape characteristics determines how discharge is allocated
- imposed by watershed
- Independant variable
What does the shape of the cross section affect?
Width of channel
-square vs. irregular channel
What does resistance characteristics affect?
depth and velocity
What is the resistance to flow based on?
Characteristics of the channel itself
What is the main cause of resistance in the river channel?
The roughness of the bed itself
What is the velocity at the bed?
0
What is the momentum extraction?
The flow has momentum that is being transferred to the boulder or other object
What is the driving force the same as?
Bed shear stress
-Gradient of the channel and the wright of the water acting down the gradient of the channel
Where can shear stress act on in the water?
SS is anywhere and there is a shear stress acting on it
Where is shear stress 0?
At the surface
-it varies linearly in between
What is the vertical velocity gradient?
rate of change of velocity which elevation above bed
What happens to the driving force when you increase velocity?
it increases as well
Which area has the highest rate of displacement?
Highest at the bed
Steepest gradient
As elevation increases, what does that do to the velocity?
Velocity increases too but the rate if increase decreases upwards
What are the 2 different states of flow?
Turbulent
Laminar
What are Turbulent and Laminar flow defined by?
Reynolds #
-w x v x d/viscosity
What flow do you get when Reynolds number is greater than 5000?
Turbulent flow
What flow do you get when Reynolds number is less than 2000
Laminar
Why is velocity lowered in turbulent waters?
Slow and fast moving water mixes together reducing the overall velocity
Where would you find subcritical conditions?
Deep and slow moving water
Where would you find supercritical conditions?
Shallow and fast water
What number do we use to define subcritical and supercritical conditions?
Froud #
- v/sqrt(g x d)
What flow conditions do we get if Frouds # is greater than 1?
supercritical
What flow conditions do we get if Frouds # is less than one?
Subcritical
What are the 2 methods of transferring sediment material within an Alluvial channel?
Bed load
Suspended Load
What characteristic of a particle determines the resisting force?
Particle size
What moves the particles down the channel?
Driving force
-we use bed shear stress normally
What is the Critical Bed Shear Stress?
Stress at which particles begin to move
What happens to critical shear stress as particle size increases?
Critical shear stress needs to increase as well in order to move the particles
