Geology Reference
In-Depth Information
critical shear stress for deposition. Adopting and expand-
ing the concept of critical shear stress for deposition,
Mehta and Partheniades (
1975
) conducted an extensive
laboratory investigation on the deposition of cohesive
sediment, with an initial sediment concentration ranging
from 1 to 10 kg/m
3
, which were much greater than the
concentrations used in the Krone (
1962
) experiments.
After a short period of rapid deposition, an equilibrium
concentration (
c
eq
) was typically observed for a specific
set of conditions (Fig.
2.6
). Furthermore, the ratio
between the equilibrium and initial concentrations
remains largely constant and is independent of the initial
concentration (
c
o
). This leads to an insightful conclusion
that a given flow can maintain a constant fraction of
sediment in suspension regardless of the absolute value
of the concentration. It was found that the
c
eq
/
c
o
ratio
depends solely on the bed shear stress, leading to the
development of several empirical formulas relating the
c
eq
/
c
o
to the bed shear stress (Mehta and Partheniades
1975
). Three deposition regimes were distinguished
based on the bed shear stress, or the ratio of the equilib-
rium concentration and the initial concentration. They
are full deposition, hindered or partial deposition, and
no deposition. It is worth noting that wave motion,
which generates additional shear stress, may prevent
full deposition during slack tides.
The complicated flocculation processes and the
settling/depositional behavior of flocs, as discussed
briefly above, can be applied to understand the com-
monly observed turbidity maximum in tide-dominated
estuaries. Turbidity maximum is one of the most dis-
tinctive regional scale sediment transport phenomena
in meso- and macro-tidal estuaries with abundant fine-
grain sediment (Nichols and Biggs
1985
; Dyer
1986
).
It is a zone with suspended sediment concentrations
that is higher than those in the input river as well as in
further seaward in the estuary. The turbidity maximum
typically occurs near the head of the salt water intru-
sion with its formation controlled by erosion due to the
tidal flow, interaction between fluvial and tidal flows,
salinity (typically 1-5‰), and mixing patterns of
freshwater and seawater (partially or fully mixed). The
influences of these factors on fluctuation and the set-
tling of flocs lead to the formation and maintenance of
turbidity maximum. The location and suspended sedi-
ment concentration of turbidity maximum vary with
variations of fluvial discharge and tidal fluctuations.
Because of fluctuation, settling of fine-grain particles
towards the bed can be quite rapid around slack tide.
Fig. 2.6
Variation of suspended sediment concentration with
time. After an initial rapid decrease, an equilibrium sediment
concentration is reached. For a given flow condition, the ratio
between the equilibrium concentration and the initial concentra-
tion remains constant and is dependent of the initial concentra-
tion (Modified from Partheniades
1986
)
If the sediment flux is high, the water trapped between
the flocs may not escape, resulting in the formation of
a high concentration layer, i.e., fluid mud, above the
bed. Fluid mud can also be formed at locations with
overall low energy. Fluid mud can be easily eroded by
the tidal currents during the subsequent tide.
Extensively developed fluid mud can significantly dis-
sipate the incident wave energy (Wells and Kemp
1986
). Fluid mud may behave like Bingham plastics
(Sills and Elder
1986
). During the flow-accelerating
phase of the next tide, the increasing shear stress at the
top of the viscous fluid mud layer may re-suspend
some of the sediment. Alternatively, the shear stress
may also induce failure at the bottom of the layer, lead-
ing to the “flow” of the entire fluid mud layer.
Laboratory studies on erosion of cohesive sedi-
ments, or the initiation of motion for non-cohesive
sediment, are conducted from a similar approach as
