1. Bedload Transport

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• 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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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.
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• 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.

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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
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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) ? u ?b In terms of
u, Power ? u ?b (? u2) (f (u ) ) Power ? ?
u3
Note very small changes in velocity, or bed
roughness, can have significant effects on the
rate of bedload transport.
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Not all energy gets transferred to bedload
grains, need an efficiency factor Bedload
transport rate K Power Bagnolds
Relationship for bedload transport rate
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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.
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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
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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
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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