Lecture 5: Fluvial processes and landforms

Question 1

What is the driving force and resisting force in open channel flow? What does the balance of these determine?

Answer 1

Driving force - gravity
Resisting force - friction between water and channel surface
Determines the ability of water to entrain and transport material

Question 2

How do you calculate mean depth?

Answer 2

area / top width

Question 3

How do you calculate hydraulic radius?

Answer 3

area / wetted perimeter

Question 4

What is river discharge the product of?

Answer 4

Water-surface width X average depth of flow X average flow velocity

Question 5

How do you calculate discharge and what does this show?

Answer 5

Cross-sectional area X velocity

Shows that cross sectional shape varies with discharge

Question 6

How do you calculate perimeter of stream?

Answer 6

width plus 2Xdepth

Question 7

Gradient decreases downstream, but what 3 other things increase downstream with increasing discharge?

Answer 7

Width, depth and velocity

Question 8

Spatial temporal variations in flow velocity may be caused by what 3 factors?

Answer 8

Channel slope, roughness and cross sectional form.

Question 9

What is the effect of friction on velocities within a channel?

Answer 9

Decreases from centre to bed, with highest rates of decrease closest to bed

Question 10

Velocity increases with height above bed through what 5 layers?

Answer 10

Viscous sublayer to buffer layer to logarithmic layer to outer layer

Question 11

What affects thickness of flow in the boundary layer?

Answer 11

friction between fluid and solid boundary

Question 12

If flow is steady through time, what does not change?

Answer 12

Depth

Question 13

If flow is uniform in space then what properties must the channel display?

Answer 13

no variation in depth/velocity along a reach of constant slope and cross sectional shape

Question 14

Shear stress (T) imposed by a velocity can deform a water mass, but where is this felt greatest and how is it calculated?

Answer 14

Felt greatest at bottom as lowest velocity

T = Viscosity (resists shear) X (difference in downstream velocity / difference in distance above bed)

Question 15

For laminar flow, what does not occur between layers?

Answer 15

Mass transfer

Question 16

How do you calculate the reynolds number?

Answer 16

(4 X mean velocity X hydraulic radius) / kinematic viscosity

Question 17

What happens when Re is below 2000? Above 8000?

Answer 17

Below 2000 - laminar flow

Above 8000 - turbulent flow (eddies and mixing)

Question 18

How can the velocity of turbulent flows be estimated?

Answer 18

The chezy or manning equations

Question 19

How can the velocity of laminar, turbulent and transitional flows be measured?

Answer 19

Darcy-weisbach equation

Question 20

What is the hardest parameter to measure that affects velocity?

Answer 20

The roughness

Question 21

Why do rivers create landforms?

Answer 21

They are primary agents of erosion, transport, deposition

Question 22

Describe the movement of sediment in the hydrological system.

Answer 22

Headwaters = production
1st order streams = sediment transfer
Deltas, lowslopes = deposition

Question 23

What do sediment transport and bedform development contribute to one another?

Answer 23

Sediment transport contributes deposition whilst bedform development determines sediment availability, flow geometry and local rates.

Question 24

What does beform development contribute to turbulent flow?

Answer 24

Resistance

Question 25

What exchange occurs between sediment transport and turbulent flows?

Answer 25

Sed transport induces resistance and turbulence damping whilst turbulent flows provide unsteady forces and interactions

Question 26

What are the average annual transport distances of gravel, sand and silt?

Answer 26

• Gravel 50 –500 m/yr
  • Sand 100-10,000 m/yr
  • Silt-clay many km, or into floodplain, where it
  stays for a long time

Question 27

What is suspended load?

Answer 27

Carried in water, during which time has no contact with bed.

Question 28

What is washload?

Answer 28

Carried in water, not present in bed

Question 29

For lanes stability constant, Sediment load X median grain size = ?

Answer 29

Discharge X slope

Question 30

What are the three main types of river channel?

Answer 30

Bedrock, alluvial, semicontrolled

Question 31

For alluvial channels, what represents the irregularity of channel course?

Answer 31

Sinuousity (ratio of channel length:valley length)

Question 32

What is the pattern and shape of channels related to?

Answer 32

Quantity and size of sediment being transported and valley floor gradient

Question 33

Meandering channels are highly sinuous (>1.5) but what acts as controls on them?

Answer 33

Correlations between meander
wavelength, mean radius of curvature
and channel width

Question 34

Where are the finest particles and lowest velocities found in a meander?

Answer 34

The inside

Question 35

What characterises the dynamic nature of braided channels?

Answer 35

High rates of fluvial activity, complex flow patterns, large % coarse material transported as bed load

Question 36

What is often the case in braided channel width?

Answer 36

Erodible banks = large width

Question 37

Where are braided channels often found, and what is typical of their discharge?

Answer 37

On steep slopes, discharge is variable