Geology Reference
In-Depth Information
50
2.5
Saltation-Abrasion Erosion
40
2.0
30
1.5
20
1.0
Fig. 8.2
Erosion by saltating bedload and
suspension.
A. Erosion by saltating bedload alone is predicted
to vary as a function of the rate at which clasts
impact the bed, the energy per impact, and the
fraction of the underlying bedrock exposed.
Because both high impact rates and abundant
bedrock exposure favor erosion, the most rapid
erosion occurs where these competing factors are
roughly balanced. Note that the cited erosion
rate represents the annual erosion that would
occur if flood conditions persisted all year. The
actual erosionrate, therefore, depends on the
frequency of such floods. Modified after Sklar and
Dietrich (2004). B. Effects on maximum saltation-
abrasion erosion rate due to changes in a single
variable, when others are held constant. Rates
increase with greater clast size, sediment load, and
flood frequency, because each tends to increase
the total energy delivered to the bed. Changes in
discharge or roughness, while other variables are
constant, shift the slope at which maximum
erosion occurs, but not its magnitude. Modified
after Goode and Burbank (2009). C. Predicted
erosion due to impacts by both bedload and
suspended load. The solid line represents erosion
when the channel slope is fixed (0.0053), such
that depth increases with higher transport stages.
Alternatively, if depth is fixed (0.95 m), variations
in slope can drive accelerated erosion at high
transport stages (thick dashed line). Erosion by
bedload only (thin dashed line) is shown for
reference. In all models, 60-mm-diameter clasts
are assumed, but sand is also included in the
saltation-suspension model. Note that transport
stages of
≥
100 times the critical shear stress are
considered exceptional flows. Modified after Lamb
et al
. (2008).
10
0.5
0.0
0
0
10
20
30
40
50
A
Sediment Supply (kg/s)
Increase erosional capabiltiy via:
Bedload-
Saltation
Erosion
•Increase flood frequency
•Increase clast size
•Increase sediment load
Decrease slope for
peak erosion via:
•Increase
discharge
•Increase
roughness
Decrease capabiltiy via:
•Increase rock strengh
•Increase channel width
•Extreme sediment load
B
Increasing Channel Slope
10
2
Saltation plus
Suspension
10
1
10
0
Total load (Slope = 0.0053)
Total load (Depth= 0.95 m)
Saltation-Abrasion (bedload)
10
-1
10
0
10
1
10
2
10
3
10
4
C
Transport Stage (
τ
*
/
τ
*
c
)
Intuitively, the incision of the bed of a river
is more related to specific stream power than
to total stream power, both because specific
stream power relates to energy expenditure
per area of the bed and because shear stress
exerts the force along the bed that moves
sediments and removes bedrock. Nonetheless,
neither total nor specific stream power actually
provides much insight on the processes that
cause a river to erode its bed. Many numerical
