Geoscience Reference
In-Depth Information
11.1.5 Erosion of cohesive sediments
Erosion modes
Erosion of cohesive sediments is affected by flow conditions, sediment properties,
and bed configurations. The first erosion mode is surface or floc erosion, in which
sediment is eroded from the bed in particles, due to the breaking of inter-particle
electrochemical bonds under the action of the flow that exceeds a critical shear stress.
The second mode is mass erosion, in which sediment is eroded in layers, due to bed
failures along planes below the bed surface when the applied shear stress exceeds the
bed bulk strength. The third mode is sediment entrainment due to bed fluidization
followed by the destabilization of the water-sediment interface (Mehta, 1986).
Critical conditions for incipient motion and erosion
The critical velocity or shear stress for erosion (or incipient motion) of cohesive sedi-
ments is related to the properties of bed materials, such as plasticity index, void ratio,
water content, and yield stress, but general accepted relationships are not available,
especially for consolidated muds. Determination of the critical flow conditions must
be based on laboratory or in-situ field tests using the natural muds of study.
Dou (1960) and Zhang (1961) studied the incipient motion of newly deposited
cohesive sediments and proposed several empirical formulas for the critical depth-
average velocity. The Zhang formula is
h
d
0.14
1.8
γ
d
0.72
1
/
2
−
γ
γ
0.000000605
10
+
h
s
U
c
=
gd
+
(11.16)
where
U
c
is the critical depth-averaged velocity for the incipient motion of sediment
(m
s
−
1
), and
d
is the sediment size in meters. When the sediment size is larger, the last
term on the right-hand side is negligible and Eq. (11.16) reduces to Eq. (3.28), which
is for the incipient motion of non-cohesive sediments.
The newly deposited mud is a kind of Bingham fluid. The critical shear stress for
erosion is related to the yield stress by Migniot (1968) as follows:
·
⎧
⎨
⎩
1
/
4
0.95
τ
τ
<
15
B
B
U
∗
c
=
(11.17)
1
/
2
0.50
τ
τ
≥
15
B
B
s
−
1
), and
where
U
∗
c
is the critical shear velocity for incipient motion (cm
·
τ
B
is the
cm
−
2
).
According to many experiments, Otsubo and Muraoko (1988) related the critical
shear stresses for surface erosion (
Bingham yield stress (dynes
·
τ
ce
1
) and mass erosion (
τ
ce
2
) to the yield stress
τ
B
as
0.56
B
0.94
B
τ
=
0.27
τ
,
τ
=
0.79
τ
(11.18)
ce
1
ce
2
