Mobile Bed Sediment Transport

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Mobile Bed Sediment Transport

• Jim O'Brien
• FLO-2D Software, Inc.

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Sediment Transport Considerations

• For large river flood events (100-year) the
  effect of scour/deposition on the maximum water
  surface is negligible
• For small flood events 2 yr to 10 yr or
  alluvial fan flooding - avulsion, blockage,
  conveyance loss associated with scour/deposition
  is important

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Sediment Transport

• Uncoupled sediment transport
• FLO-2D calculates flow hydraulics, then estimates
  sediment transport
• The sediment is nonuniformly distributed on the
  channel cross section. Uniformly on floodplain.
• Assumes changes in channel geometry or floodplain
  topography for a given timestep are relatively
  small and do not significantly effect the flow
  hydraulics

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Sediment Transport Concepts

• ? Storage (scour/deposition) for a channel or
  floodplain element
• Sediment supply in sediment transport capacity
  out
• Generally 5 or more timesteps (1-10 seconds) are
  required to change the bed elevation by 0.10 ft

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Sediment Transport Equations

• Choice of nine sediment transport equations
  determine sediment transport capacity
• Zeller-Fullerton
• Yang
• Engelund Hansen
• Ackers White
• Laursen
• Tofaletti
• Woo-MPM
• MPM-Smart
• Karim-Kennedy
• Each formula was based on unique river
  conditions. Research equation applicability to
  each project.

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Sediment Transport Equations

• Zeller-Fullerton Multiple regression sediment
  transport equation for a range of channel bed and
  alluvial floodplain conditions.
• A computer generated solution of the Meyer-Peter,
  Muller bed-load equation combined with Einsteins
  suspended load to generate a bed material load
• Assumes all sediment sizes are available for
  transport (no armoring). The original Einstein
  method is assumed to work best when the bedload
  constitutes a significant portion of the total
  load

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Sediment Transport Equations

• Yangs Total sediment concentration is a
  function of the potential energy dissipation per
  unit weight of water (stream power f(velocity
  and slope))
• Sediment concentration is a series of
  dimensionless regression relationships.
• Based on field flume data with sediment
  particles ranging from 0.137 mm to 1.71 mm and
  flows depths from 0.037 ft to 49.9 ft. Mostly
  limited to medium to coarse sands and flow depths
  less than 3 ft
• Can be applied to sand and gravel

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Sediment Transport Equations

• Engelund-Hansen Method Bagnolds stream power
  concept was applied with the similarity principle
  to derive a sediment transport function.
• Uses energy slope, velocity, bed shear stress,
  median particle diameter, specific weight of
  sediment and water, and gravitational
  acceleration
• Can be used in both dune bed forms and upper
  regime (plane bed) D50 gt 0.15 mm

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Sediment Transport Equations

• Ackers-White Method Expressed sediment
  transport based on Bagnolds stream power
  concept. Only a portion of the bed shear stress
  is effective in moving coarse sediment. The
  total bed shear stress contributes to the
  suspended fine sediment transport.
• Dimensionless parameters include a mobility
  number, representative sediment number and
  sediment transport function.
• The various coefficients were determined from
  laboratory data for Di gt 0.04 mm and Froude
  numbers lt 0.8. The condition for coarse sediment
  incipient motion agrees well with Sheilds
  criteria. The Ackers-White approach tends to
  overestimate the fine sand transport.

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Sediment Transport Equations

• Laursens Transport Function Had good agreement
  with field data from small rivers. For larger
  rivers the correlation between measured data and
  predicted sediment transport was poor (Graf,
  1971).
• Involves relationship between the flow hydraulics
  and sediment discharge. The bed shear stress
  arises from the Manning-Strickler formula. Based
  on flume data for lt Di 0.2 mm.
• Expresses the effectiveness of the turbulence in
  mixing suspended sediments. The critical
  tractive force in the sediment concentration
  equation is given by the Shields diagram.

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Sediment Transport Equations

• Toffaleti Procedure to calculate the total
  sediment load by estimating the unmeasured load.
  
• Following the Einstein approach, the bed material
  load sum of the bedload discharge and the
  suspended load in three separate zones.
• Bedload concentration from his empirical equation
  for the lower-zone suspended load discharge and
  then computed the bedload.
• Simons and Senturk (1976) reported that
  Toffaletis eqn compared well with 339 river and
  282 laboratory data sets.

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Sediment Transport Equations

• MPM-Woo Relationship For steep sloped, sand bed
  channels. Woo et al. equation (1988) to account
  for the variation in fluid properties due to high
  sediment concentration. Mussetter, et al. (1994)
  linked Woos relationship with the
  Meyer-Peter-Mueller bed-load equation.
• Multiple regression relationship computes the bed
  material load as a function of velocity, depth,
  slope, sediment size and Cvf Applicable for
  velocities lt 20 fps (6 mps), a bed slope lt 0.04,
  a D50 lt 4.0 mm, and a Cvf lt 60,000 ppm.
• Estimates high bed material load in channels for
  which the other sediment transport equations are
  not applicable.

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Sediment Transport Equations

• MPM-Smart Relationship For steep channels
  ranging from 3 to 20. Smart (1984) modified
  the MPM equation (1988) to account for
  deficiencies in roughness values in steep
  channels.
• Used for sediment sizes greater than 0.4 mm.
• Modified to account for the affects of nonuniform
  sediment distributions.
• Will generate sediment transport rates that
  approach those of Englund-Hansen on steep slopes.

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Sediment Transport Equations

• Karim-Kennedy Fsimplified Karim-Kennedy
  equation (F. Karim, 1998). Nonlinear multiple
  regression relationship based on velocity, bed
  form, sediment size, and friction factor for a
  large data set. Use for large rivers with
  non-uniform sand/gravel conditions.
• Sediment sizes 0.08 mm to 0.4 mm (river) and 0.18
  mm to 29 mm (flume) and up to 50,000 ppm
  concentration.
• Slope range 0.0008 to 0.0243.
• Will yield similar results to Laursens and
  Toffaletis equations.

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SEDTRANS.OUT
MAXIMUM SEDIMENT TRANSPORT CAPACITY (CFS OR CMS)
FOR GRID ELEMENT 1961 (1 OF 8 DIRECTIONS FOR
FLOODPLAIN FLOW) TIME(HRS) ZELLER-
YANG ENGLUND- ACKERS- LAURSEN
TOFFALETI MPM-WOO FULLERTON
HANSEN WHITE 0.10 0.000
0.000 0.000 0.000
0.000 0.000 0.000
0.20 0.000 0.000 0.000
0.000 0.000 0.000
0.000 0.30 0.041
0.261 0.186 0.283
0.083 0.071 1.292
0.40 0.172 1.565 0.970
2.567 0.569 0.246
2.820 0.50 0.328
2.495 1.904 5.952
0.953 0.385 4.458
0.60 0.548 4.086 3.439
12.569 1.471 0.725
6.447 0.70 0.599
4.319 3.781 14.099
1.563 0.638 7.510
Profiles
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Sediment Routing by Size Fraction

• Sediment Diameter (mm) Percent Finer
• 0.074 0.058
• 0.149 0.099
• 0.297 0.156
• 0.590 0.230
• 1.19 0.336
• 2.38 0.492
• 4.76 0.693
• 9.53 0.808
• 19.05 0.913
• 38.10 1.000

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Bed Armoring

• The armoring process occurs when the upper bed
  layers become coarser as the finer sediment is
  transported out of the bed. An armor layer
  occurs when coarse sediment covers the bed and
  protects the finer sediment below.

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Bed Armoring

• The FLO-2D model tracks the sediment size
  distribution and volumes in an exchange layer.
• Exchange layer - three times the D90 grain size
  of the bed material (Yang, 1996).
• When the exchange layer is reduced to 33 of the
  original volume, it is replenished from the
  initial bed material.
• Potential armoring is automatically assessed if
  sediment routing by size fractions is invoked.
  No switches.

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Bed Armoring (cont.)

• The potential armor layer is evaluated on a
  timestep basis for each channel element by
  assessing the volume of each size fraction in the
  exchange layer.

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Sediment Scour and Deposition

• For each timestep, the sediment transport
  capacity is compared to the sediment
  inflow/outflow in a floodplain or channel
  element.
• The sediment deposition/scour then effects the
  hydraulics for the next time steps in terms of
  slope changesmoderating effect.

McCoy
Whitewater
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Sediment Transport Control

• In CONT.DAT
• Set ISED 1
• Set IMUD 0
• Set XCONC 0.
• In CHAN.DAT
• Set ISEDN 1 (line 1 for channel sediment
  transport)
• Create SED.DAT

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SED.DAT file

• Line 1 is the hyperconcentrated sediment flow
  parameters
• 1 SEDCHAR M, VA, VB, YSA, YSB, SGSM, XKX
• NOTE IF ISED IS EQUAL TO 0, IGNORE REST OF
  FILE
• Line 2 lists parameters for sediment routing.
• 2 SEDCHAR C, ISEDEQG, ISEDSIZEFRAC, DFIFTY,
  SGRAD, SGST, DRYSPWT, CVFG, ISEDSUPPLY,
  ISEDISPLAY
• NOTE IF ISEDSIZEFRAC 1, LINE 3 IS FOLLOWED
  BY SEVERAL LINE 4s (one for each size fraction).
  THE COMBINED LINES 3 AND 4 ARE A SEDIMENT GROUP.
  THE FLOODPLAIN SEDIMENT IS ENTERED AS THE FIRST
  SEDIMENT GROUP. SUCCESSIVE GROUPS CAN REPRESENT
  CHANNEL REACHES
• Line 3 has the sediment routing by size fraction
  control parameters.
• 3 SEDCHAR Z, ISEDEQI, BEDTHICK, CVFI
• NOTE LINE 4 IS REPEATED FOR EACH SIZE
  FRACTION AND EACH GROUP MUST HAVE THE SAME NUMBER
  OF SIZE FRACTIONS (IDENTICAL SEDIAMs)
• Line 4 lists the sediment routing by size
  fraction sediment size distribution.
• 4 SEDCHAR P, SEDIAM, SEDPERCENT
• NOTE IF IDEBRV IS EQUAL TO 0 IN THE
  CONT.DAT FILE, IGNORE LINE 5
• Line 5 contains debris basin parameters.
• 5 SEDCHAR D, JDEBNOD, DEBRISV
• Line 6 represents the optional scour depth
  limitation for channel and floodplain grid
  elements.
• 6 SEDCHAR E, SCOURDEP
• Line 7 contains the list of rigid bed nodes.
• 7 SEDCHAR R, ICRETN(N), N 1, number of
  rigid bed nodes

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Whats coming next? RiverFLO-2D Workshop