Sediment Transport by Water

1
Sediment Transport by Water

• Theory
• Processes
• Rainsplash
• overland flow transport
• Rilling and gullying
• Mass movements
• Weathering limited versus supply limited
• The extreme event

2
Theory

• Mechanics of flow
• Stream energy
• Entrainment

3
Mechanics of flow

• Water is subject to two forces
• gravity (Wa g sin ?)
• friction
• Defines ability of water to erode and transport
  sediment

4
Types of flow in open channels
5
Laminar Flow

• Each fluid element moves along a specific path
  with no significant mixing between layers
• Boundary layer in contact with the bed has no
  forward velocity
• Each layer can slip past each other

6
Turbulent flow

• At a critical velocity or depth laminar flow
  becomes unstable and the parallel streamlines are
  destroyed
• Adjacent layers mix, transferring momentum by
  large scale eddies
• Velocity more evenly distributed with depth
• Steeper near bed velocity gradient

7
Reynolds Number (Re)

• Re ? h u/?
• where ? fluid density
• h flow depth
• u fluid viscosity
• ? viscosity
• larger values, larger turbulence

8
Entrainment

• Movement of material depends on its physical
  properties
• grain size shape
• density structual arrangement
• Basic distinction
• cohesive (silt-clay size)
• non-cohesive

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Shear stress

• Causes initial movement
• Shear stress estimate of force exerted on the
  bed by the fluid

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• ?cr ? D
• but doesnt include lift forces
• Lift due to
• eddies
• difference in velocity at top and bottom of grain

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Critical shear stress
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Shields (1936)

• Dimensionless critical shear stress
• Plot against particle Reynolds no. (ratio of
  grain size to thickness of laminar sublayer)

13
Factors producing scatter

• use of average ? or ?
• spatial variability over bed
• channel size
• irregularity of eddies

• degree of exposure
• pivot angles
• imbrication degree of packing
• grain shape
• microtopography

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Erosion

• Entrainment/detatchment
• Transport

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Detatchment vs Transport

• Rainsplash
• Weathering
• Tillage
• Trampling
• Runoff

• Rainsplash
• Overland flow
• Rill flow
• Gully flow

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Rainsplash

• varies with rainfall intensity
• varies with land cover
• varies with slope
• varies with of area which is rilled
• varies with lithology
• crusting?

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Surface Wash

• particles detatched and transported by surface
  flowing water
• force velocity x mass (i.e. Q)
• controls relate to character of materials,
  especially ability to produce rainfall excess

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Resistance to detatchment

• non-uniform
• varies with particle size
• cyclic variation with season
• sand/silt clay ratio
• stoniness

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Rills

• Impermanent channels
• vary in lateral position year to year
• develop once threshold exceeded in a single event

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Gullies

• permanent incised X-sectional form
• develop once threshold exceeded over longer term
  average conditions
• may be discontinuous
• gully / arroyo / donga

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Mass Movement

• possibly only important in extreme events
• directly contribute to load or rills/gullies
• 4 main types
• shallow slides
• slab failure
• rockfalls
• deep seated slides

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Soil Erosion

• Soil loss R K L S P C
• R rainfall erosivity
• K erodibility of soil
• L slope length
• s slope angle
• P coefficient of cultivation methods
• C crop management factor

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Weathering Limited

• When unlimited capacity for
• transport occurs, removal of material is limited
  by the rate at which material is detatched.

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Transport Limited

• When there is an abundant
• supply of material and erosion
• depends on the efficiency of
• forces transporting the
• material away.

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Equilibrium condition

• Removal of material supply of material
• Contionuous range between extremes
• Occurs over different timescales
• Cyclic
• Graded
• Steady-state

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Transport vs Weathering
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Timescales

• Cyclic
• period over which an effective change in basin
  elevation can be measured
• Graded (equilibrium)
• a change in any factor will cause a displacement
  of the equilibrium in a direction which will
  absorb the effect of change
• Steady state
• a measurement can be taken and the system assumed
  to be in a constant condition

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Timescales
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Magnitude-Frequency concept

• Wolman and Miller, 1960
• majority of work carried out by events which
  occur on average 1 or 2 times per year
• basin characteristics adjusted to these events

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• Different in semi-arid channels
• stress-strain reln more complicated
• large spatial variation
• morphology adjusted to extreme events

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Extreme events

• Do majority of work because
• larger particle size
• transmission losses
• poor sorting
• vegetation

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Themes of dryland floods

• Get scour and fill in times of extreme floods but
  channels restore themselves afterwards
• Average sediment yields before a flood are
  exceeded for sometime afterwards
• Work done during a flood is poorly related to
  flow volume or total ppt