Sediment Loading, Hyperconcentrated Sediment Flows, Mud and Debris Flows

1. Sediment Loading, Hyperconcentrated Sediment Flows, Mud and Debris Flows Jim O'Brien FLO-2D Software, Inc.

2. 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

3. Hyperconcentrated Sediment Flows � not an exact science!Using volume conservation model (FLO-2D), not a Lagrangian particle dynamic model. 

4. 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

5. 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

6. Hyperconcentrated Sediment Flows - Basics Concentration by volume: Cv = Volume of Sediment Vol. of Sed. + Vol. of Water

7. Hyperconcentrated Sediment Flows - Basics Specific Weight of the Mixture: ?m = ? + Cv (?s - ?)

8. Hyperconcentrated Sediment Flows - Basics Bulking Factor: BF = 1/(1-Cv)

9. Question: What is the sediment concentration by volume?

17. Evaluating Mud and Debris Potential In small watersheds (< 5 mi2), infrequent floods ~100 yr event generally do not create viscous mudflows. Why?

18. Evaluating Mud and Debris Potential In small watersheds (< 5 mi2), infrequent floods ~100 yr event generally do not create viscous mudflows. Why? There�s too much water for the available sediment supply. Surging occurs with debris frontal waves.

19. Hyperconcentrated Sediment Flows ? = ?y + ? (?v/?y) + C (?v/?y)2

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

24. Viscosity = f (Cv)

26. Yield Stress = f (Cv)

27. To perform a mudflow analysis� Switch on MUD = 1 in CONT.DAT Add sediment concentration to inflow file�INFLOW.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

28. 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

29. 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

30. Typical Fan Apex Mudflow Hydrograph

31. Mudflow hydrograph at fan terminus

32. What�s Missing? Surging in the recessional limb Effects of channel blockage both in the watershed and at bridges and culverts Roll wave phenomena

34. 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

35. Dispersive Shear Stress For dispersive stress to occur, satisfy 3 conditions: High sediment concentration Cv > 0.5 Large velocity gradients ~ > 0.1 s-1 Large sediment particles Ds > 0.05 h

36. Hyperconcentrated Sediment Flows ? = ?y + ? (?v/?y) + C (?v/?y)2

37. Dimensionless Form - Quadratic Model

38. Dispersive vs Turbulent Stress Hyperconcentrated flows are: Primarily Turbulent if Td > 1 h/ds > 70 dispersive stress is small Primarily Dispersive if Td < 1 h/ds< 70 high resistance with particle collisions

39. Classification Dv Td Mud floods > 400 > 1 Mudflows < 30 Granular Flows > 400 < 1 Mud floods � turbulent stress is dominant Mudflows � viscous stress is dominant Granular flows � dispersive is dominant (see handout)

40. 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 Manning�s n value

41. Dispersive Stress � Velocity Profiles

42. Flow resistance in turbulent flows with low sediment concentrations

43. Plot data with high concentrations of non-cohesive particles with previous curves

44. Combine all the data � highlight dispersive stress data

45. Dispersive Stress � Flow Resistance

46. Dispersive Stress � Flow Resistance

47. 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

48. The end�