Suspended Load Bed Load 1 Bedload Transport transport

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

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.

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

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

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.

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

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.

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

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

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