Geology Reference
In-Depth Information
waves were often measured at a fixed location on a
tidal flat during higher tides due to less friction related
wave dissipation (Lee et al.
2004
; Talke and Stacey
2008
). Sediment transport by wave forcing can be
significant locally, as well as during storm conditions.
Bottom shear stress, and therefore initiation of sedi-
ment motion and transport, is also strongly influenced
by water depth, which varies substantially in tidal
environments. When the tidal water-level fluctuations
are confined by channels, e.g., tidal inlets and creeks,
strong tidal-driven flows can be generated. As com-
pared to other types of channelized flow, tidal flow
reverses direction periodically with a slack water period
in between, which may create unique bi-directional
sedimentary structures. In the case of tidal inlets
between barrier islands, large flood and ebb tidal deltas
can be deposited through the interaction of tide and
wave forcing. The cyclical rising, slacking, and falling
tide and the associated flow variation leave signature
sedimentary records through geological history, pro-
viding valuable information for understanding earth
history (e.g. Kvale et al.
1989
).
Sediment grain size in tidal environment typically
ranges from non-cohesive medium sand to cohesive
clay. Compositionally, tidal sediments can be silici-
clastic, carbonate, and organic materials. A variety of
sedimentary structures, ranging from millimeter-scale
sand-mud laminations on tidal flats to subaqueous
dunes of tens of meters in tidal channels, exists in
tidal environments, indicating a wide range of sedi-
ment transport and deposition processes. Transport
and deposition of mixed cohesive and non-cohesive
sediments are poorly understood and provide cutting
edge research topics (Van Rijn
2007a, b, c
)
Given the wide range of both cohesive and non-
cohesive sediments, and the energetic and highly vari-
able hydrodynamic processes driven by both tides and
waves, sediment transport processes in tidal environ-
ments are extremely complicated. This chapter aims
at providing a basic review of the principals of sedi-
ment transport applicable in tidal environments.
Various transport formulas and their general applica-
tions in tidal environments are discussed. It is worth
emphasizing that methods of computing the rates of
sediment transport are largely empirical, based
heavily on field and laboratory experiments.
Calibration and verification based on site-specific
data are essential to accurate applications of the formulas.
The transport principles and formulae can also be
applied qualitatively to interpret the sedimentary
processes observed in the field, and to design field
experiments. More detailed and further in-depth
mechanics of sediment transport can be found in
several dedicated texts, e.g., Mehta (
1986b
), Fredsoe
and Deigaard (
1992
), Nielsen (
1992
),Van Rijn (
1993
),
Pye (
1994
), Allen (
1997
), and Soulsby (
1997
).
2.2
Principles of Sediment Transport
Transport of sediment in coastal environments results
from the interaction between moving fluid (seawater in
this case) and sediment. Present knowledge on sedi-
ment transport processes is largely empirical, based on
numerous field and laboratory experiments. Insightful
parameterization is crucial in describing the compli-
cated fluid-sediment interaction. In the following
section, key parameters describing fluid motion, sedi-
ment, and fluid-sediment interaction are discussed,
followed by the presentation of the commonly-used
methods for the calculation of non-cohesive and
cohesive sediment transport, respectively.
2.2.1
Fundamental Parameters
Fluid motion over a sediment bed exerts a horizontal
drag force and a vertical lift force. Generally, when
these forces overcome the gravity of a sediment grain,
transport is initiated. A theoretical analysis of the ini-
tiation of motion of an individual grain typically starts
with a force balancing between the drag-lift forces and
the gravitational force on the grain. The sediment grain
is put in motion if the moments of the fluid drag (
F
D
) and
lift (
F
L
) forces exceed the moments of the submerged
gravitational force (
F
G
) on the grain (Fig.
2.1
). However,
due to our limited understanding of the very compli-
cated fluid-sediment interaction, sediment transport in
the natural environments cannot be quantified from the
force analysis of each grain. Instead, it is quantified
empirically through insightful parameterization of sediment-
fluid interaction, as discussed in the following.
When viscous fluid, e.g., seawater, flows over a sur-
face, a shear stress is generated by the fluid flow. This
shear stress is responsible for entraining and transport-
ing sediment. On the other hand, the friction at the
fluid-sediment interface exerts a drag on the fluid flow,
