Sediment Transport

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

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

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

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

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

= 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)

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

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