Reynolds Number (Re) Re = R = A/P V = mean velocity  /  =  (which is kinematic viscosity) Re = VR(  /  ), where Driving Forces Resisting Force Re < 500 Laminar Flow Re > 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 Equation Describes Flow and Resistance in Open Channels V = 1.49/n(R 2/3 )(S 1/2 ) n = Manning’s roughness Coefficient 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 Types Classification 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 Types Classification 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 Reynolds Numbers R * = V*dV*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

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