Long Profile Flashcards
What is long profile
The height of the channel floor from source to mouth
What is the channel cross profile like in the upper course
Narrow and uneven, because of the presence of deposited boulders.
Rectangular shapes banks.
What is the channel cross profile like in the middle course
It starts to meander, the channel becomes asymmetrical on the river bends but mainly smooth and symmetrical on the straight stretches
What is the channel cross profile like in the lower course
It widens and deepens, banks of deposition and eyots (islands of deposition) can disrupt the shape of the channel cross section, leading to a braided channel. Levees can also form.
What does the shape of the channel influence
The velocity of the river
What is wetted perimeter
The total length of the river bed and banks in cross section that are in contact with the water in the channel
What is the wetted perimeter like in the upper course
The channel is narrow and uneven due to the presence of large boulders so there is a large wetted perimeter
When is there more friction
When there is a large wetted perimeter in relation to the amount of water in the river
What does friction result in
Energy loss and the velocity of the river slows
When are rivers most efficient
In the middle and lower course as the channels are larger and smoother; the wetted perimeter is proportionately smaller than the volume of water flowing through the channel (less friction to reduce velocity)
Why is velocity slower in the upper course
Because so much energy is expended over coming friction on the uneven channel bed in the upper course, whereas in the lower course there is little to disrupt water flow
What is hydraulic radius
The channel shape
How is hydraulic radius calculated
Cross sectional area of the channel / wetted perimeter
A high hydraulic radius =
More efficient river (because the moving water loses proportionately less energy in overcoming friction than when the ratio between the cross sectional area and the wetted perimeter is low).
How efficient are larger channels
More efficient; area increases to a greater degree than wetted perimeter
