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Flow Energy PE = mgh H1 H2 Datum • PE + KE = constant between any two points • PE (loss) = KE (gain) • Rivers are non-conservative; some energy is lost from the system and can not do mechanical work Available Energy is Used To: • Overcome Viscous shear and turbulence (internal friction within the fluid); • Overcome friction at the channel boundaries; • Erode sediment from the channel boundaries; • Transport the sediment once it has been eroded. Flow Energy PE = mgh H1 H2 Datum • Rate of potential energy loss along the channel can be determined by the slope of the water surface – commonly referred to as the energygrade line. Flow Types Laminar vs. Turbulent • 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 to 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 to square of velocity Velocity Distribution in Open Channels Modified from Wolman, 1955; Figure from Ritter et al., 2002 Flow Types Steady/Uniform Flow • Uniform/non-uniform: velocity is constant/variable with position; • Steady/unsteady: velocity is constant/variable with time at any given location Manning Equation Describes Flow and Resistance in Open Channels V = 1.49(R2/3)(S1/2)/n n = Manning’s roughness Coefficient Slope = water surface slope Assumes English units; remove 1.49 from equation for metric Influence of Manning’s n on Resistance and Suspended Sediment 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. Types of Sediment Load Chernicoff S., Fox, HA., and Venkatakrishnan, R., 1997, Essentials of Geology, Worth Publishers, Fig. 13-13, pg. 261 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. Methods Used to Describe Entrainment • Critical Bed Velocity Velocity (V) Water – 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 Methods Used to Describe Entrainment • Critical Tractive Force Depth (D) Slope (S) – Dragging force is exerted on the exposed part of the particle. – = γDS • (Duboy’s Equation) – Fairly Easy to measure From Knighton, 1998, Fluvial Forms and Processes, Arnold, Figure 4.4a, page 108. Shield’s Dimensionless Values • Dimensionless Shear Stress = Dimensionless Reynolds Numbers (γf - γs) d γ - Specific weight of solid and fluid R* = V*d V * = (gRS)1/2 d = Intermediate grain diameter R = Hydraulic Radius Dimensionless Shields Plot Hjulstrom Plot Stream Power • • • • = γ QS = γ wdvS /w = ((γ dS)v)/w /w = v East Fork River Bedload Trap Flow Bedload Trap