Blue Galaxy

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Sediment Transport Introduction BAE590
10.01.2004
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sediment - definition and sources

• sediment solid fragments of organic and
  inorganic materials that come from weathering of
  rock (i.e. sand, silt, clay)
• 2 main sources
• landscape erosion
• streambed and banks unstable flow conditions
  (increased volume and frequency of discharge)

American Heritage Dictionary of the English
Language, 4th Edition
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sediment across the landscape
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landscape sources

• Erosion
• interrill splash erosion and shallow overland
  flow.
• rill small concentrated flow (till removal)
  formed during moderate events.
• gully large concentrated flow (no removal)
  formed during exceptional events.
• stream channel due to shear stress on bed and
  banks

Picture - USGS
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bank erosion

• occurs when the ability of the stream to
  transport sediment exceeds the availability of
  sediments within the incoming flow and stability
  thresholds for the material forming the boundary
  of the channel are exceeded
• bank erosion more severe in winter when banks are
  wet
• duration of flow affects erosion (lower, long
  duration flows have greater impact than shorter,
  higher intensity flows)

Fischenich 2001
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landscape transport

• floodplains and hydraulically rough landscapes
  inefficiently transport sediments thus much of
  eroded sediment deposited in landscape and never
  reaches streams

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sediment budget

• accounting for sources and sinks of sediment from
  origin to outlet or POI.
• consider sediment
• weathering and erosion
• transport rate
• storage

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decreased sediments - hungry water

• when stream is deprived of normal sediment load,
  it moves faster and with more energy ? hungry
  water scours and erodes streambanks
• results in widening and incision (degrading) of
  channel
• common below impoundments

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increased sediment loads

• may be stored in floodplain
• channel cannot transport the size and volume of
  sediment contributed by the watershed
• aggradation (filling) of channel
• formation of mid-channel bars
• homogenization of bed features

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channel responses to watershed and anthropogenic
activities

• increases in effective discharge as a result of
  watershed development result in channel widening
  and incision.
• channelized streams have higher discharge and
  sediment-carrying capacity results in scouring
  of bed and bank

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channel response references

• Wolman and Miller (1960) Magnitude and Frequency
  of forces in geomorphic processes. Journal of
  Geology 68 54-74.
• Fischenich, C. (2001) Stability Thresholds for
  Stream Restoration Materials. ERDC TB-EMRRP-SR-29

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Lanes stream balance diagram
Source Ward and Trimble (2004) Environmental
Hydrology
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transport determinants

• particle size
• particle shape
• particle specific gravity
• velocity
• sediment discharge

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Hjulstrom diagram

• note how smaller particles take higher
  velocities to overcome cohesive forces

Source Ward and Trimble (2004) Environmental
Hydrology
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effects of cohesion
ASCE (1975) Sedimentation Engineering
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• total sediment load bed load suspended load

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bedload

• sediment that moves by sliding, rolling, or
  saltating (bouncing) on or very near the bed.

Leopold et al (1992) Fluvial Processes in
Geomorphology
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types of suspended load

• suspended load
• wash load sediment load of a stream which is
  composed of particles sizes smaller than those
  found in appreciable quantities in the shifting
  portions of the stream bed TOO SMALL TO
  DEPOSIT
• suspended load particles which are moved by
  and suspended in the water column, but can settle
  in locations where the travel velocity is low or
  settling depth is small. CAN DEPOSIT UNDER SOME
  CONDITIONS.

Garde and Raju (2000) Mechanics of Sediment
Transport and Alluvial Stream Problems.
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washload

• generated from caving of streambanks of a
  tributary and washes through a reach without
  appreciable deposition.
• simplification - these particles pass through the
  river system relatively unrelated to the
  hydraulic condition in a given reach the wash
  load is independent of the discharge instead,
  depends on erosion/availability of fine materials
  from upstream.
• Einstein recommended that washload include the
  particle size for which 10 of the bed material
  is finer. (Einstein H.A. 1950)

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mobility layers

• active layer the depth of material from bed
  surface to equilibrium depth continually mixed by
  the flow, but it can have a surface of slow
  moving particles that shield the finer particles
  from being entrained by the flow.

USACE HEC6 manual appendix B
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incipient motion

• the flow condition is such that sediment
  particles of given characteristics just start
  moving.
• 3 approaches for describing hydraulic conditions
  at incipient motion
• 1. competency - velocity
• 2. critical shear/tractive force
• 3. stream power

Garde and Raju (2000) Mechanics of Sediment
Transport and Alluvial Stream Problems.
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• Velocity calculate by mannings eq and check
  with critical erosive velocity to make sure it is
  lower than this critical velocity
• Shear stress needs to be greater than critical
  shear to carry the specific particle size
• Stream power combines both into a single
  function to which you could compare with
  reference reach or empirical data

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competency

• conditions for which stream is said to be
  competent to move its sediment (i.e. has reached
  its competent shear stress, stage, or velocity)
• size of the largest particle a stream can move
  under a given set of hydraulic conditions
• of interest to designers - bed velocity at which
  a specific size/weight particle is moved.

Source Federal Interagency Stream Restoration
Working Group (1998)
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• just at incipient motion, all forces balance to
  zero.
• lift occurs when upward hydrodynamic forces
  exceeds particle weight and resistance lift
  forces f(Dp)

upward lift
V0
low pressure
flow
drag
V0
high pressure
resisting force
submerged weight
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flow-transport interaction models - size matters

• f(bed stress, transport rate, grain size (of
  transport and bed surface)
• Wilcock(2001) demonstrated that the bed sand
  content has an important and nonlinear effect on
  gravel transport rate

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design approaches - sediment transport

• threshold channel
• active-bed

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threshold channels

• assumes uniform cross-section and slope, flat
  beds, and negligible bed material transport
• may not be appropriate in channels with
  significant amounts of cohesive materials
• algorithm ? set channel dimensions for selected
  fraction of bed material to be at incipient
  motion for design discharge

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threshold channels

• shear stress tractive stress force per unit
  area in the direction of flow
• f(slope, depth)
• describes lift and drag forces at stream boundary
• critical shear stress force exerted by the
  water at which particles begin to move

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shear stress (general)

• td rgRs gRs
• where
• td average boundary shear stress exerted by
  flow
• water density
• g gravity
• R hydraulic radius (B 2D)
• s friction slope
• g specific weight of water

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shear stress english units

• td gds
• where
• td (lb/ft2) tractive force average shear
  stress in a reach at given depth
• d(ft) depth of flow (assumes RD for wide
  channels)
• s(ft) water slope
• g specific weight of water 62.4 lb/ft3

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particle movement

• determine size moved by fixed tractive force
• ds c tc
• where
• ds grain size, typically median particle (d50)
• c unit conversion factor based on magnitude of
  tractive force
• tc tractive force

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Source Leopold et al (1992) Fluvial Processes
in Geomorphology
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example Seneca Creek, Dawsonville, MD

• given reqd
• dbkf 4 d at single flow condition
• 62.4 lb/ft3 (bankfull)
• s .0007 ft/ft
• c 1 / 3.5
• tractive force at bankful depth
• td gds 62.4 4 .007
• td 0.175 lb/ft2

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example Seneca Creek, Dawsonville, MD

• determine size moved by fixed tractive force at
  bankful depth
• ds c tc 1/3.5 0.175
• ds 0.05 ft

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shields diagram

• a curve of the dimensionless tractive force
  plotted against the grain Reynolds number
  (Ud/v)
• Reynolds number indicates the relative
  significance of the viscous effect compared to
  the inertia effect. flow is 
• laminar if  Re
• transient if  2300
• turbulent if   4000
• shields parameter dimensionless shear stress
  which dictates the beginning of the motion of bed
  material.

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shields diagram
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active bed designs

• can be performed in SAM (USACOE)
• design variables (width, slope, and depth)
  calculated from
• water discharge
• sediment inflow
• bed material composition

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stream power

• controlled by both tractive force and flow
  velocity bankfull velocity shear stress
• useful in evaluating sediment discharge within a
  stream channel and the deposition or erosion of
  sediments

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stream power

• force in a stream?
• water density (r)
• gravitational acceleration (g)
• width (w)
• depth (d)
• slope (S)

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stream power

• where
• Pt total power
• r density of water
• d depth
• w width
• u velocity
• s friction slope

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stream power so what?

• Used to compare with reference or with empirical
  relationships
• Insert cross section talk about how power
  effects boundary conditions