Sediment and streams

1. STREAM SEDIMENTS
A. Introduction
1. Types of energy: potential, kinetic, thermal
thermal energy: expended on internal friction
& friction with channel perimeter
potential & kinetic energy: mechanical work -
erosion, transportation & deposition
2. Entrainment: set of processes that initiate
particle motion
competence: size of the largest particle a
stream can entrain under a given set of
hydraulic conditions

2. Sediment Entrainment
• Entrainment channel erosion and sediment
transport begins with entrainment of
sediment from the channel bed
• depends on several factors;
– Sediment
• grain size
• shape of particles
• packing of sediment
– imbrication
– flow conditions
• velocity or shear stress of flow
– competence of flow

3. 3. Thresholds: under what conditions will
a river do the mechanical work necessary
to modify or maintain channel
morphology?

5. B. Driving Forces Affecting Entrainment
Critical bed velocity
a. sixth power law:
r3 = kv6
1. Velocity
• average velocity
and bed velocity
• harder to initiate
motion than to
maintain motion
Hjulstrom diagram

6. 2. Shear stress
force: a push or a pull in a particular direction
stress: force/area
shear stress: force per unit area parallel to the
bed
Critical bed shear stress (threshold for
entrainment)
bed shear stress
tb = R g r s
where: R = density of water
g = gravity
r = hydraulic radius
s = slope

7. c. Shields diagram

8. 3. Velocity gradient
 a. Rate of change in velocity with distance
from bed or banks
 b. Velocity gradient creates a vertical
pressure gradient, which results in an
upwards force
 c. Velocity gradient steeper for turbulent
flow than for laminar flow

9. For a given discharge and area:
 wide, shallow channels - steeper
gradient along bed than along
banks - promotes bed erosion
 narrow, deep channels - steeper
gradient along banks than along
bed - promotes bank erosion

10. 4. Stream power: w
w = r g Q s / w
Where,
r = density of water
Q = discharge
s = slope
w = channel width

11. C. Resisting Forces Affecting
Entrainment
1. Size and density
2. Sorting of bed material
• equal mobility hypothesis
• downstream fining
3. Layering or packing
4. Particle shape and orientation

13. D. Bank Erosion
1. Fluvial entrainment: corrasion
(Mechanical erosion or abrasion)

14. • 2. Weakening and weathering
processes
a. reduce strength of bank material
b. soil moisture condition is most
important control
positive pore pressure
lubrication

15. E. Transportation
1. Suspended load
• concentration decreases with distance from
the bed
• estimated transport rate = depth-averaged
sediment concentration x mean flow velocity
x depth
Qs = Cs x v x d
• main control over suspended sediment
concentration: spatial & temporal variability
in supply of sediment to stream
2. Bed load
3. Dissolved load

16. F. Deposition
1.Ripples
• form primarily in fine & medium sands
(< 0.7 mm diameter)
• height < ~3 cm; length <~40 cm
• height & spacing increase as grain size
increases
• relatively smooth water surface

17. 2. Dunes
• form in medium & coarse sands; may
form with gravel if flow conditions high
enough
• height up to several m
• height & spacing increase as flow
depth increases
• small surface waves out of phase with
dunes

18. 3. Antidunes
•surface waves in phase with
dunes
•not very common