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
