Changing channel characteristics Flashcards
What is the river like in the upper course?
Channel is narrow and uneven.
Presence of deposited boulders.
Both banks are being eroded - gives channel a rectangular shape.
What is the river like in the middle course?
Meanders present.
Channel becomes asymmetrical on river bends.
Symmetrical on straight stretches.
What is the river like in the lower course?
River widens and deepens.
Banks of deposition and eyots can disrupt shape of the channel cross section, leading to braided channels.
Levees are also present (can be man made).
What is the wetted perimeter?
The total length of the river bed and banks in cross section that are in contact with the water in the channel.
Should decrease.
Where is the largest wetted perimeter found?
Upper course, where the channel is narrow and uneven due to the presence of large boulders.
How is friction related to the wetted perimeter?
When there is a large wetted perimeter in relation to the amount of water in the river, there is more friction.
What does friction do to the river?
Friction (from contact with the bed and bank) results in energy loss - velocity decrease.
Where is the river most efficient?
Middle and lower courses, where the channel is larger and smoother.
The wetted perimeter is proportionally smaller than the volume of water flowing in the channel.
Therefore there is less friction to reduce the velocity.
What is the formula for the hydraulic radius?
Cross-sectional area of the channel DIVIDED by the wetter perimeter.
What does a high hydraulic radius mean?
How efficient the river is as the moving water loses less energy in overcoming friction.
Should increase in efficiency and power from source to mouth.
Where is the highest amount of friction located in a river channel?
Flood level and below bankfull.
When is the maximum efficiency achieved?
Bankfull, as there is low friction.
What does Bradshaw’s model show?
Describes the river changes from source to mouth.
Triangle widens - variable increases.
How does velocity vary from source to mouth, and what impacts it?
Increases.
Gradient has an impact BUT rivers are steep at the source - lots of friction - velocity decrease.
Influenced by channel shape.
Wider shallower channels have a bigger wetter perimeter (more friction) so flow slower than narrow deep channels.
Asymmetric channels - higher velocity.
How does discharge vary from source to mouth and what is it measured in?
The amount of water passing a certain point per second.
CA x velocity.
Cumecs (cumecs metres per second).
Increases from source to mouth with velocity/water amount.
How are the width and depth of a river measured?
Water surface width/measuring from bank to bank.
Generally increases.
Depth can be measured with metre stick.
OR area/width.
What are the first 3 stream orders?
- NO tributaries, original stream source.
- Two streams join one river - confluence.
- Two streams orders two rivers - confluence.
12 stream orders in total.
How can the CSA be measured/what does it show?
Water surface area from bank to bank in metres2.
Average depth x width.
Increase - more water from tributaries.
What is channel roughness/how can it be measured?
Measure of how rough the bed is.
Manning’s Roughness coefficient.
How does the gradient vary from source to mouth?
Decreases as the river meanders across the land, rather than eroding it.
Changes from vertical erosion upstream, to lateral erosion downstream.
How does load vary from source to mouth?
Decreases.
Smoother/rounded stones - attrition.
Quantity, capacity and competence all vary.
Shown by the Hjulstrom curve.
