Sediment Loading, Hyperconcentrated Sediment Flows, Mud and Debris Flows

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Sediment Loading, Hyperconcentrated Sediment
Flows, Mud and Debris Flows

• Jim O'Brien
• FLO-2D Software, Inc.

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Factors Contributing to Mudflows

• Watershed Conditions
• Drainage and channel development
• Sediment availability (channel storage, hillslope
  failure)
• Exposed slopes (fires, logging, vegetation)
• Debris (logs, boulders, trash)
• Channel roughness and constrictions
• Volcanic eruptions

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Hyperconcentrated Sediment Flows not an exact
science!Using volume conservation model
(FLO-2D), not a Lagrangian particle dynamic
model.
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Evaluating Mud and Debris Potential

• Objective Investigate sediment supply and
  balance sediment volume and water hydrograph
  volume
• Mudflow Potential
• Nature of sediment transport (flood or mudflow) -
  channel and fan deposits (natural boulder levees,
  lobate deposits)
• Loose debris and boulders in channel
• Estimate available sediment volume (channel
  storage, bank failure, landslide, overland
  sediment yield)
• Compute a sediment volume and average sediment
  concentration for the design flood event. For
  mudflows average 30 to 35 concentration by
  volume

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Five Primary Sources of Sediment

• Landslides
• Hillslope sloughing or failure
• Channel bank failure
• Channel bed scour
• Overland sediment yield
• Objective To balance sediment supply with the
  sediment volume computed in the output files

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Hyperconcentrated Sediment Flows - Basics

• Concentration by volume
• Cv Volume of Sediment
• Vol. of Sed. Vol. of Water

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Hyperconcentrated Sediment Flows - Basics

• Specific Weight of the Mixture
• ?m ? Cv (?s - ?)

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Hyperconcentrated Sediment Flows - Basics

• Bulking Factor BF 1/(1-Cv)

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Question What is the sediment concentration by
volume?
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What is the sediment concentration Cv?
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Classic Mudflows
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Granular dispersive flooding
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Dont be fooled by surface washing
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Evaluating Mud and Debris Potential

• In small watersheds (lt 5 mi2), infrequent floods
  100 yr event generally do not create viscous
  mudflows.
• Why?

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Evaluating Mud and Debris Potential

• In small watersheds (lt 5 mi2), infrequent floods
  100 yr event generally do not create viscous
  mudflows. Why?
• Theres too much water for the available sediment
  supply. Surging occurs with debris frontal
  waves.

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Hyperconcentrated Sediment Flows

• ? ?y ? (?v/?y) C (?v/?y)2

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Select mudflow parameters

• Select viscosity and yield stress parameters to
  match the field conditions (e.g. high viscosity
  and low yield stress).

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Viscosity f (Cv)
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Yield Stress f (Cv)
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To perform a mudflow analysis

• Switch on MUD 1 in CONT.DAT
• Add sediment concentration to inflow
  fileINFLOW.DAT
• Add line 1 in SED.DAT, coefficients and exponents
  for viscosity and yields in Table 9
• M 0.000602 33.1 .00172 29.5 2.74 0.0
• Turn off ISED and XCONC in CONT.DAT
• Flow is treated as a fluid continuum
• Simulate flow cessation and remobilization

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Construct a Sediment Concentration Hydrograph

• Average sediment concentration range 30-35 by
  volume
• Bulk the frontal wave 45-53
• Hydrograph peak discharge 40-45
• Result
• Maximize peak discharge moving over the fan
• Slow the flow down for higher depths
• Fluid motion to inundation a large area
• Peak discharge catches the frontal wave

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Select mudflow parameters

• Select viscosity and yield stress parameters to
  match the field conditions (e.g. high viscosity
  and low yield stress)
• Compute the viscosity and yield stress for one
  sediment concentration by volume
• Try several different sets of data for a one
  inflow hydrograph and sediment concentration

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Typical Fan Apex Mudflow Hydrograph
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Mudflow hydrograph at fan terminus
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Whats Missing?

• Surging in the recessional limb
• Effects of channel blockage both in the watershed
  and at bridges and culverts
• Roll wave phenomena

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Select mudflow parameters
Laminar flow resistance parameters Table 7, page
53
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Dispersive Stress

• The different fluid shear stresses with high
  sediment concentrations are
• Cohesion between fine particles
• Viscous interaction between particles and
  surrounding fluid
• Inertial impact between sediment particles
  dispersive stress
• Turbulence

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Dispersive Shear Stress

• For dispersive stress to occur, satisfy 3
  conditions
• High sediment concentration Cv gt 0.5
• Large velocity gradients gt 0.1 s-1
• Large sediment particles Ds gt 0.05 h

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Hyperconcentrated Sediment Flows

• ? ?y ? (?v/?y) C (?v/?y)2

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Dimensionless Form - Quadratic Model
? 1 ( 1 Td ) ai Dv
Excess Shear Stress (applied vs. viscous)
? (? - ?y) / (µm du/dy) If ? 1, Bingham
fluid
Dispersive vs. Viscous Ratio
Dv ?s ? ds2 (du/dy) / µm
Turbulent vs. Dispersive Ratio
Td ?m lm2 / (ai ?s ? ds2 ) where lm ?h
If ? 0.4, ai 0.01, ?m/?s 1.5
Td 0.1/ ?2 (h/ds)2
Bagnolds linear concentration term
? (Cv max /Cv total)0.333 1. -1.
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Dispersive vs Turbulent Stress
Td 0.1/ ?2 (h/ds)2
h/ds relative submergence

• Hyperconcentrated flows are
• Primarily Turbulent if Td gt 1
• h/ds gt 70 dispersive stress is small
• Primarily Dispersive if Td lt 1
• h/dslt 70 high resistance with particle
  collisions

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Classification

• Dv Td
• Mud floods gt 400 gt 1
• Mudflows lt 30
• Granular Flows gt 400 lt 1
• Mud floods turbulent stress is dominant
• Mudflows viscous stress is dominant
• Granular flows dispersive is dominant
• (see handout)

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Task Relate the turbulent and dispersive stress
in practical terms

• Define a relationship that would permit the C
  coefficient in the shear stess to be evaluated
• Resistance to flow as defined by Mannings n value

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Dispersive Stress Velocity Profiles
Low Concentrations
High Concentrations
Higher Cv
Dry Sand
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Flow resistance in turbulent flows with low
sediment concentrations
The range of resistance defined by Darcy-Weisbach
f
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Plot data with high concentrations of
non-cohesive particles with previous curves
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Combine all the data highlight dispersive
stress data
(8/f)0.5 0.2 h/ds
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Dispersive Stress Flow Resistance
Relationship between Mannings n and Darcy
Weisbachs f
n 0.00926 R1/6 f 0.5 or f 1.16.66 n2
R-1/3
Turbulent Friction Factor
(8/f)0.5 5.75 log (? h/ds)
Dispersive Friction Factor
(8/f)0.5 0.2 h/ds
Where remember
h/ds (Td ?/0.1)0.5
and Td 1 (turbulent/dispersive stress
ratio)
? (Cv max /Cv total)0.333 1. -1.
Bagnolds linear concentration term
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Dispersive Stress Flow Resistance
Cv 0.56 Maximum packing factor for spheres
Cv 0.615 Bagnolds Cv max maximum sediment
concentration
Cv 0.74 Natural ground
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Plotting nd/nt f (Cv)

• ntd nt b e mCv
• where b 0.054 and m 6.09
• At high concentrations, dispersive stress
  transfers more momentum flow to the boundary by
  particle contact flow resistance
• Hardwired in the model

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The end
Introduction to RiverFLO2D Show Mudflow Movies