Sediment Transport
Flowing water transports sediment as:
1. Bedload – particles roll, slide, or bounce along bottom
2. Suspended load – particles carried above bottom by fluid turbulence and
grain collisions (“dispersive pressure”)
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
Lift
Fluid Drag
+
Gravity
Friction and
Electrostatic
-
Forces promoting movement
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
u∗
= Cs
=
FM = Z2 D2 (rfluid) u2
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