Debris Flow Modeling and Troubleshooting with

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Debris Flow Modeling and Troubleshooting with
Jim OBrien FLO-2D Software, Inc.
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Approach
Start with water flooding or rainfall/runoff Obtai
n concurrence on water volume and peak
flows Assess sediment supply (fan apex)
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Based on 2 rain gage records (long) Average July
31, 24 - hour rainfall
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Soldier Canyon Model

• Rainfall runoff
• Hyperconcentrated sediment flows
• Combined hydrologic and hydraulic model rain on
  the flood
• Details buildings, culvert bridge, variable
  n-values, minor infiltration

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Modeling Approach

• Simulate the July 31, 2006 rainfall using the
  NEXRAD data on the entire basin
• Create a flood hydrograph at the basin mouth
  Mt. Lemmon Road culvert
• Bulk the hydrograph with sediment
• Route the hyperconcentrated sediment flow over
  the alluvial fan applying appropriate fluid
  matrix parameters
• Delineate the flood hazard

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Bulking the Hydrograph

• Sediment Loading at the fan apex or point of
  concentration

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Route Hyperconcentrated Sediment Flow

• Assign sediment concentrations to inflow
  hydrograph
• Match total volume of sediment estimated by
  PCRFCD
• Select fluid matrix parameters for a low viscous
  and yield stress, high dispersive stress

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Bulk the Water Hydrograph at Mt. Lemmon Road
Culvert
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Sediment Loading Condition

• Supply limited or transport capacity limited?
• PCRFCD estimated basin sediment scour/deposition
  from LiDAR data
• Add sediment entering water courses
• Match with FLO-2D inflow to alluvial fan

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Sediment Bulking

• Supply Limited
• Water volume is known
• Sediment supply is fixed
• Adjust sediment concentration to match sediment
  supply volume

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Sediment Bulking

• Supply Unlimited
• Water volume is known
• Sediment volume is unknown
• Adjust sediment concentration to achieve an
  average of 25-35 by volume

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Predicted Flood Peak Discharge
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Mudflow or Dispersive Granular Flow?
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Flood Hazard Delineation July 31, 2006 Flood
Event
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100-year Flood Bulking
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Flood Hazard Delineation 100-year Flood Event
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Troubleshooting

• No assigned sediment concentration
• Minimum bulking of 10 concentration by volume
  for mudflows
• Realistic viscosity and yield stress parameters
  avoid making up numbers
• Maximum concentration by volume 55
• Higher concentration will cause surging
• Dont use excessive n-values to cause flow
  cessation

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Range of Viscosity
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What Should I Check?

• Sediment volume, concentration by volume in
  SUMMARY.OUT
• Area of Inundation
• Maximum velocities (MAXPLOT or Mapper)
• Final flow depths and velocities for flow
  cessation

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Example Barnard Creek, Utah

• Urbanized alluvial fan
• Very steep slopes (streets 10)
• Small debris basin at fan apex
• Street flow conveyances
• Numerous buildings
• Mudflow, water flood and rain

Mud
Water
No Streets
No Buildings
Rain Mud
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The End.