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Suspended Load

Suspended Load. Bed Load. 1. Bedload Transport transport rate of sediment moving near or in contact with bed particles roll or hop (saltate), with grain-to-grain contact 2. Suspended-Sediment Transport fluid conditions suspending particles particles supported by turbulence.

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Suspended Load

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  1. Suspended Load Bed Load • 1. Bedload Transport • transport rate of sediment moving near or in contact with bed • particles roll or hop (saltate), with grain-to-grain contact • 2. Suspended-Sediment Transport • fluid conditions suspending particles • particles supported by turbulence

  2. Bedload Transport • Definition: • 1. Bagnold - the particles which are supported by inter-granular collisions as opposed to fluid drag. • 2. The particles moving in a band up to some height above the bed. • 3. Pragmatic - those particles that can be caught in a bedload sampler.

  3. Major early body of work done by: • H.A. Einstein (1950s) • Meyer-Peter and Muller (1948) • Bagnold (1940 - 1950s) • As with initiation of motion, bedload transport can be treated: • Empirically • Balance of forces • Dimensional arguments incorporating both physics and empirical findings.

  4. Bagnold - Concept of bedload sediment transport is related to the rate of transfer of energy (work) done by the fluid on the moving grains. • Work = transfer of energy across a system boundary (e.g., from a shaft to a fluid) • Power = rate at which energy transfer is done, or Work/Time

  5. On the seabed, Work can be defined in terms of the shear stress. • Transfer of energy from bottom boundary layer fluid to seabed particles b • Rate of transfer (Power)  ū b • In terms of u*, • Power  ū b= ( u*2) (f (u*) ) • Power   u*3 Note: very small changes in velocity, or bed roughness, can have significant effects on the rate of bedload transport.

  6. Not all energy gets transferred to bedload grains, • need an “efficiency factor” • Bedload transport rate = K · Power • Bagnold’s Relationship for bedload transport rate:

  7. Need to experimentally evaluate K • First work focused on: K = f ( D, relative roughness) • Inman, Coastal dunes: • where C = 1.5 in uniform sand, 1.8 in naturally sorted sand, and 2.5 in poorly sorted sand • D is diameter in m • Need to consider Flow as well as seabed parameters. • - Marine Environment - Sternberg & Kachel, 1971.

  8. Sternberg & Kachel, 1971 • Measured ripple migration rates with stereo-cameras in Puget Sound. • Evaluated K as a function of: D & flow conditions • Found: Applicable for: D between 0.2 and 2 mm steady to accelerating flow limited amounts of suspended sediment

  9. 1. Given flow conditions, find u* and then b 2. Given D and u* use a threshold curve to find cr 3. Find K graphically, or through the curve-fit equations. 4. Calculate j Sternberg and Kachel, 1971

  10. Tidal Bedload Transport Example: The changes in tidal current velocity measured at 1 m above the bed during a complete tidal cycle in the North Sea. As a result of the u 3 relationship, appreciable differences occur between the amounts of sediment that can be transported in each tidal direction Open University, 1989

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