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8GEOG landscapes and water
Channel Forms
23
Geography
Undergraduate 1
04/05/2010

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Term
fluvial geomorphology
Definition

the study of processes and landforms related to rivers meaning "of, relating to, or inhabiting a river or stream" and "produced by the action of a river or stream"

 

Term
what is a channel?
Definition

a feature created by running water with identifiable banks

 

focus on channels formed by rivers in sediments deposited by the river (alluvial channels)

Term
what are alluvial channels (what is alluvium)?
Definition

channels formed by rivers in sediments deposited by the river

 

alluvium - sediment deposited by flowing streams

Term
what is a bedrock channel?
Definition

channels cut into solid rock, rather than loose sediment

 

where rivers carve into rock, slice mountain ranges etc

Term
what is bankfull discharge?
Definition

the discharge that just fills the channel, in which the water is at the level of the adjacent floodplain

 

considered teh channel-forming discharge

 

the channel form is a reflection of this levle of flow, not higher floods or lower flows

 

has a recurrence interval of 1.5 years, a flow level that is reached at least 2 out of 3 years

Term
what shapes do alluvial channels take?
Definition

1. meandering: the channel bends this way and that, it is sinuous

2. braided: the channel consists of multiple threads, speparated by many small transitory islands

commonly found in rivers that carry lots of gravel and have large variations in discharge

Term
what is the sinuosity ratio?
Definition

the length of a river (down the middle of the channel) divided by the straightline distance commonly thought of as the vally length

 

Sinuosity = Lriver/Lvalley

 

the more bends in the channel, the greater the sinuosity; if the channel is straight, the sinuosity ratio is one

Term
what is the sinuosity ratio of meandering rivers?
Definition
>1.3
Term
if a channel has a sinuosity ratio between 1 and 1.3 what is it considered?
Definition
sinuous
Term
What does Luna Leopold say about the distinctions between river forms?
Definition
[image]there's no real distinction, they're all meandering
Term
why is there such similarity in form of rivers over different scales?
Definition
the length of the meander, L (the straight line distance between a pair of bends (right and left)) is proportional to the width of the channel, w, such that on average L=11w
Term
what is the radius of curvature
Definition

a way to measure the tightness of the river bends

 

r, the radius of a circle that scribes the bend

 

in general, the meander length L is 4.7 times the radius of curvature, L=4.7r

Term
what is a pool?
Definition

a deep spot on a channel bottom, often on the side of the channel, not in the middle

 

flows tend to be fastest here

Term
what is a riffle?
Definition
a shallow steep section on a channel bottom,
Term
what is a thalweg?
Definition
the part of the channel with the fastest flow and carrying the most water
Term
what causes riffles?
Definition
bars - deposits built up of coarse sediment on the channel bed
Term
how do pools, riffles, bars, and thalwegs relate to eachother?
Definition
[image]
Term
how does the length of a bar relate to the length of the channel?
Definition
the length of each bar unit (or alternately, pool spacing) is about 3-5 channel widths
Term
what is hydraulic geometry?
Definition

the relationships between discharge and widgth, depth, velocity, and slope

 

relationships are considered separaely for the changes at-a-point (at one cross section of a river) and for changes downstream

Term
describe at-a-point hydraulic geometry relationships
Definition

as discharge increases:

*channel widdth increases, but does not change much

*channel depth increases. depth increase is much more significant in accommodating extra flow than width changes

*velocity increases

*channel slope (steepmeness of water surface) does not change much with discharge

Term
describe downstream hydraulic geometry relationships
Definition

as discharge increases downstream:

*width incrases significantly downstream, increasing proportionally to the square root of discharge

*depth increases significantly downstream, just as depth changes with discharge at a point

*velocity is nearly constant, or only slightly increasing, along the length of a river

*channel slope declines over the length of a river

Term
what have we seen in the hydraulic geometry relationships found in many rivers by Leopold and Maddock?
Definition

at a particular point in a river, higher discharge is accomodated mostly by the flow gettting deeper and faster, with relatively little adjustment in width or slope

 

increasing width and depth accomodates the increase in discharge downstream through a river

 

velocity does not change much downstream, mostly due to the declining slope

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