Lecture 5: Fluvial processes and landforms Flashcards
What is the driving force and resisting force in open channel flow? What does the balance of these determine?
Driving force - gravity
Resisting force - friction between water and channel surface
Determines the ability of water to entrain and transport material
How do you calculate mean depth?
area / top width
How do you calculate hydraulic radius?
area / wetted perimeter
What is river discharge the product of?
Water-surface width X average depth of flow X average flow velocity
How do you calculate discharge and what does this show?
Cross-sectional area X velocity
Shows that cross sectional shape varies with discharge
How do you calculate perimeter of stream?
width plus 2Xdepth
Gradient decreases downstream, but what 3 other things increase downstream with increasing discharge?
Width, depth and velocity
Spatial temporal variations in flow velocity may be caused by what 3 factors?
Channel slope, roughness and cross sectional form.
What is the effect of friction on velocities within a channel?
Decreases from centre to bed, with highest rates of decrease closest to bed
Velocity increases with height above bed through what 5 layers?
Viscous sublayer to buffer layer to logarithmic layer to outer layer
What affects thickness of flow in the boundary layer?
friction between fluid and solid boundary
If flow is steady through time, what does not change?
Depth
If flow is uniform in space then what properties must the channel display?
no variation in depth/velocity along a reach of constant slope and cross sectional shape
Shear stress (T) imposed by a velocity can deform a water mass, but where is this felt greatest and how is it calculated?
Felt greatest at bottom as lowest velocity
T = Viscosity (resists shear) X (difference in downstream velocity / difference in distance above bed)
For laminar flow, what does not occur between layers?
Mass transfer
How do you calculate the reynolds number?
(4 X mean velocity X hydraulic radius) / kinematic viscosity
What happens when Re is below 2000? Above 8000?
Below 2000 - laminar flow
Above 8000 - turbulent flow (eddies and mixing)
How can the velocity of turbulent flows be estimated?
The chezy or manning equations
How can the velocity of laminar, turbulent and transitional flows be measured?
Darcy-weisbach equation
What is the hardest parameter to measure that affects velocity?
The roughness
Why do rivers create landforms?
They are primary agents of erosion, transport, deposition
Describe the movement of sediment in the hydrological system.
Headwaters = production
1st order streams = sediment transfer
Deltas, lowslopes = deposition
What do sediment transport and bedform development contribute to one another?
Sediment transport contributes deposition whilst bedform development determines sediment availability, flow geometry and local rates.
What does beform development contribute to turbulent flow?
Resistance
What exchange occurs between sediment transport and turbulent flows?
Sed transport induces resistance and turbulence damping whilst turbulent flows provide unsteady forces and interactions
What are the average annual transport distances of gravel, sand and silt?
- 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
What is suspended load?
Carried in water, during which time has no contact with bed.
What is washload?
Carried in water, not present in bed
For lanes stability constant, Sediment load X median grain size = ?
Discharge X slope
What are the three main types of river channel?
Bedrock, alluvial, semicontrolled
For alluvial channels, what represents the irregularity of channel course?
Sinuousity (ratio of channel length:valley length)
What is the pattern and shape of channels related to?
Quantity and size of sediment being transported and valley floor gradient
Meandering channels are highly sinuous (>1.5) but what acts as controls on them?
Correlations between meander
wavelength, mean radius of curvature
and channel width
Where are the finest particles and lowest velocities found in a meander?
The inside
What characterises the dynamic nature of braided channels?
High rates of fluvial activity, complex flow patterns, large % coarse material transported as bed load
What is often the case in braided channel width?
Erodible banks = large width
Where are braided channels often found, and what is typical of their discharge?
On steep slopes, discharge is variable
