Lecture 6 - Sedimentary notes PDF

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Lecture 6: Bed-load transport(10 mins late)Particle weight in a fluid:Horizontal drag force:- Drag Force = particle cross-sectional area x critical boundary stress x inclination- Critical stress depends on the size of both particles, the density difference, angle of slope andgravity- To make the str...

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Lecture 6: Bed-load transport (10 mins late) Particle weight in a fluid:

Horizontal drag force:

- Drag Force = particle cross-sectional area x critical boundary stress x inclination - Critical stress depends on the size of both particles, the density difference, angle of slope and gravity

- To make the stress non dimensional by dividing by D(pp - p)g

- Ocr is the non-dimensional form of the critical boundary stress to generate motion, and we can see its dependence on slope and roughness

- Increases beta, reduces the stress - When beta and alpha equal each other, stress is zero - When D is smaller, it decreases alpha which decreases stress — less stress needed to move a smaller particle

- Make it non dimensional to get a wide range of particle sizes - Ar = non dimensional particle size, properties on the material not the flow

- For large particles (D> 400 microns) the stress becomes constant (around 0.047)

Boundary stress and roughness: - The stress acting due to the flow of a stream against a rough boundary depends on the boundary roughness and the dynamic pressure of the flow

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f is called the Darcy-Weisbach friction coefficient — measure of roughness Quantity in brackets is the dynamic pressure of flow Stress and pressure will have the same units If behind someone — receive less dynamic pressure — e.g. cycling peloton Can describe the stress acting against the boundary Can rearrange to:

- Now we can relate the threshold for bedload transport to flow velocity Bed load transport rate: - Bed load transport can occur in laminar, as well as turbulent flow - Find out the rate at which the bed load is transported - Rate of transport is directly proportional to the stress that the stream is exerting on the material, above the critical stress where it can move - Volumetric rate of bed load = Q has units m^2/s

- Rate depends on the difference in stresses Boundary Stress in Channelised Flow: - For flow in a uniform channel, we can estimate the boundary stress from observable properties of the channel and flow conditions - Can find the stress by observations in the field - Flows down the slope due to gravity, depth of liquid, density of fluid - Different ways measuring the stress...