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Fluvial basics (Flow (Flow regimes (Perennial - continual flow all year…
Fluvial basics
Flow
Water is a Newtonian fluid, it has very little internal strength so when you apply stress it has a linear strain response. You can form water into things aka putting water into a bucket. If you put sand into a bucket, if you tip it upside down then it will hold its shape vaguely from a while, meaning it’s a pseudoplatic.
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Discharge is the volume of water that passes a particular point of a channel cross section every second (m3s-1)
Flow frequency and magnitude (Wolman and Miller, 1960)
Frequency occurs on a range of time scales, expressed as the average recurrence interval eg. a 1 in 100 year flood.
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Sediment movement
Calibre
Calibre refers to the size of material that is available to be carried by a river, often illustrated by the Wentworth scale.
Hjulstrom diagram
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This is due to sediment having to undergo certain levels of energy to entrain and transport, if the energy drops then the sediment is deposited.
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Sediment supply
Supply limited - sediment supply/volume is less than the capacity of the channel to carry the sediment so all the sediment has been transported already and there is no more left, excess energy will erode the bed instead.
Transport limited - the sediment supply or size of grains is too much for the river to transport, transport limited can be split into two types:
Capacity limited - the sediment supply is greater than the channel capacity often causing aggregation.
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Forces
Shear stress
Tractive force - the force of the water along its boundary. Bedload and sediment movement is a product of shear stress.
Stream power
Total - A measure of the rate of work done by flowing water in overcoming bed and internal flow resistance, and sediment transport.
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Fluid resistance
Froudes
As velocity increases, the surface morphology of the water changes. It is the critical velocity between subcritical and supercritical flow.
Reynolds
The critical velocity between laminar and turbulent flow. It measures the ratio between inertial and viscous forces within the flow
Fluid resistance is produced by the dynamics of water flow. It occurs through internal and surface resistanace
Flow
Turbulent flow
Most natural water flows are turbulent, with curved, swirling motions (turbulent eddies) superimposed on the mean downstream motion. In turbulent flows, the magnitude and direction of the flowvelocity vectors at any point in the flow change with time over intervals of fractions of a second to many seconds.
Laminar flow
Laminar flow may be regarded as thin (molecular scale) laminae sliding past each other, with velocity constant at a given point in the flow.
The range of flow types reflect the relationship between flow depth, velocity and substrate conditions.
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