1. Sediment Transport

2. Flowing water transports sediment as:
Bedload – particles roll, slide, or bounce along bottom
Suspended load – particles carried above bottom by fluid turbulence and grain collisions (“dispersive pressure”)
Bedload transport movie linked to image

3. What is the significance of grain size?
What do sedimentary structures tell us about flow conditions?
Controls on grain movement during bedload transport
Relationship between bedform type and flow conditions

4. What forces act on a sediment grain in moving fluid?
Forces hindering movement Forces promoting movement
Lift
Friction and Electrostatic
Fluid Drag + -
Gravity

5. Forces hindering movement Forces promoting movement
Fluid Drag
Gravity
Resisting force due to inertia:
FR = m g
Simply mass × gravity, but grain mass is awkward
FR = 4/3 p r3 (rgrain – rfluid) g
Replace mass by volume and density
FR = Z1 D3 (rgrain – rfluid) g
Combine constants into single term Z

6. Forces hindering movement Forces promoting movement Fluid Drag
Gravity
Fluid velocity necessary to create moving force:
Egrain = ½ V (rfluid) u2
Mass is volume × density
FM = Z2 A (rfluid) u2
Energy is force × distance
FM = Z2 D2 (rfluid) u2

7. = Forces hindering movement Forces promoting movement
Fluid Drag
Gravity
At initiation of grain movement, inertia = fluid drag
FR = Z1 D3 (rgrain – rfluid) g =
FM = Z2 D2 (rfluid) u2
u ∗ =Cs D (rgrain –rfluid) g (rfluid)

8. For typical river conditions:
u ∗ =0.06 (rgrain – rfluid) g D
Shields’ Criterion
Describes the maximum particle size (D) that can be moved by a current of velocity u – called the competence of the flow – shown by Hjulström diagram
Relationship doesn’t apply at fine grain sizes because Shields’ criterion doesn’t account for friction or electrostatic forces

9. Fluctuating current velocity in natural settings results in alternating erosion and transport with deposition (changing competence and capacity)
This is the main reason why sedimentary rocks are layered