reynolds number (re) re = r = a/p v = mean velocity / = (which is kinematic viscosity) re = vr( ...
TRANSCRIPT
Reynolds Number (Re)
Re =
R = A/PV = mean velocity/ = (which is kinematic
viscosity)
Re = VR(/), where
Driving Forces
Resisting Force
Re < 500 Laminar FlowRe > 750 Turbulent Flow
Flow Types
• Laminar:– Water particles move
along paths that do not disrupt the movement of neighboring particles.
– Most resistance is caused by intermolecular viscous forces.
– Resistance is velocity
• Turbulent Flow– Water particles move in all
directions and velocity constantly fluctuates
– Most resistance is generated along channel perimeter; related to channel shape, particle size, and concentration.
– Resistance is square of velocity
Manning EquationDescribes Flow and Resistance
in Open Channels
V = 1.49/n(R2/3)(S1/2)
n = Manning’s roughnessCoefficient
Slope = water surface slope
Assumes English units.
Velocity Distribution in Open Channels
Sediment Transport Terminology
• Entrainment: the processes that initiate the motion of a particle.
• Competence: the size of the largest particle a stream can entrain under any give set of hydraulic conditions.
• Capacity: the maximum amount of sediment that the stream can carry given the current hydraulic conditions.
• Load: amount of sediment that is actually carried by the stream.
• Sediment discharge: time rate of movement through a cross-section (weight/time; tons/day)
Load TypesClassification Based on Mode of Transport
• Suspended Load: Particles transported mainly or entirely in suspension through the supporting action of turbulence.
• Bedload: Sediment which moves by skipping, sliding, and rolling along the channel bed. Remains within a few grain diameters of the channel bed.
Load TypesClassification Based on Bottom
Sediment Characteristics
• Wash Load: Particles so fine that they are not found in appreciable amounts in the channel bed.
• Bed Material Load: Particle sizes that are found in great quantities in the stream bed.– Most bed material load is actually transported in
suspension.
Variations in Suspended Sediment
Concentration with Discharge
Influence of Manning’s n on Resistance and Suspended
Sediment
Methods Used to Describe Entrainment
• Critical Bed Velocity
– Impact or momentum of the water mass on the exposed part of the particle.
– 6th power law: Size of particle entrained increases with the 6th power of the velocity.
– Very difficult to measure
Velocity (V)
Water
Methods Used to Describe Entrainment
• Critical Tractive Force
– Dragging force is exerted on the exposed part of the particle.
– = DS• (Duboy’s Equation)
– Fairly Easy to measure
Depth (D)
Water Surface
Slope (S)
Shield’s Dimensionless Values
• Dimensionless Shear Stress
=
(f - s) d
Dimensionless ReynoldsNumbers
R* = V*d
V * = (gRS)1/2
d = Intermediate grain diameter
R = Hydraulic Radius
- Specific weight of solid and fluid
Dimensionless Shields Plot
Hjulstrom Plot
Tractive Force vs. Critical Velocity?
Stream Power
• = QS• = wdvS• = (dS)v• = S
East Fork River Bedload Trap
Flow
Bedload Trap
