Geology Reference
In-Depth Information
It should be noted that the patterns of dominance
referred to above represent generalities that average
out a great deal of local variability, both temporally
and spatially. For instance, it is widely observed that
the channel thalweg tends to be ebb dominant, whereas
the flanking tidal flats are flood dominant (Li and
O'Donnell
1997
; Moore et al.
2009
). In addition, the
morphological irregularities that exist because of the
presence of channel meanders and elongate tidal bars,
which are slightly oblique to the flow, create localized
areas of ebb- and flood-directed residual movement
of sediment. This is commonly expressed as a series of
mutually evasive channels. Typically, the two sides of
an elongate tidal bar, or the upstream and downstream
flanks of a tidal point bar, experience opposing direc-
tions of net sediment transport (Dalrymple et al.
1990
;
Choi
2010
), because they are alternately exposed and
sheltered from the reversing current. In addition, tem-
poral variability in the strength of the tidal and river
currents can cause temporary reversals in the direction
of net sediment transport. As a result of these com-
plexities, spot measurements of currents and sediment
transport have the potential to be misleading. The geo-
morphic setting and temporal context of a measure-
ment station must be documented with care before the
significance of a data set can be assessed.
wedge
, and a residual seaward flow in the lighter over-
riding fresher water. The currents associated with this
circulation are extremely weak and have little or no
influence on the transport of bed material, but they do
control the longer-term movement of the suspended
sediment (Dalrymple and Choi
2003
).
Flocculation of the river-born suspended sediment
as it moves into the area with measureable salinity,
coupled with the density-driven residual circulation
(termed
baroclinic flow
; Dyer
1997
), tends to trap
suspended sediment within the estuary, generating a
turbidity maximum
(Fig.
5.3c
), within which sus-
pended-sediment concentrations (SSC) can be elevated
to very high levels (Dyer
1995
). The peak of this tur-
bidity maximum typically lies near the tip of the salt
wedge (Allen et al.
1980
), although the broader zone
of elevated turbidity can stretch from the fresh-water
tidal zone near the tidal limit, out beyond the mouth of
the estuary (e.g. Guan et al.
1998
; Uncles et al.
2006
).
Suspended-sediment concentrations in the water col-
umn generally decrease upward from the bed, and vary
in phase with, but commonly with some lag relative to,
the speed of the tidal currents (Fig.
5.7
) because of ero-
sion and resuspension of material from the bed (Allen
et al.
1980
; Castaing and Allen
1981
; Wolanski et al.
1995
; Ganju et al.
2004
). During slack-water periods,
however, the suspended particles settle to the bed and
can generate a thin near-bed layer of very high concen-
trations. If these concentrations exceed 10
g/l
, then this
dense suspension is termed a
fluid mud
(Faas
1991
;
Mehta
1991
). They are being found in a growing num-
ber of strongly tide-influenced or tide-dominated estu-
aries (Thames Estuary: Inglis and Allen
1957
; Gironde
estuary: Allen
1973
; Castaing and Allen
1981
; Bristol
Channel—Severn River: Kirby and Parker
1983
; James
River: Nichols and Biggs
1985
; Jiaojiang River: Guan
et al.
1998
) and deltas (Fly River delta: Wolanski et al.
1995
; Dalrymple et al.
2003
; the Amazon delta: Kuehl
et al.
1996
; Seine River: Lesourd et al.
2003
; Weser
River: Schrottke et al.
2006
), apparently because the
strong tidal currents resuspend large amounts of mud;
it is possible that such high-concentration suspensions
are present in most tide-dominated estuaries.
The intensity of the turbidity maximum is highly
sensitive to the strength of the tidal currents, with the
highest turbidity generally associated with spring tides
(Allen et al.
1980
; Kirby and Parker
1983
; Wolanski
et al.
1995
), because of their ability to resuspended
more sediment. Its location is strongly influenced by
5.2.2
Salinity, Residual Circulation
and Suspended-Sediment Behavior
The interaction of marine and fresh water generates
longitudinal and vertical salinity gradients within an
estuary (Haas
1977
; Uncles and Stephens
2010
). The
location of the longitudinal gradient is highly sensitive
to both the phase of the tide, moving up and down the
estuary with the flood and ebb tides, respectively, and
also to variations in river discharge, potentially mov-
ing down river a considerable distance when the river
is in flood (Uncles et al.
2006
). Turbulence associated
with the strong tidal currents minimizes the tendency
for density stratification, producing partially mixed or
well-mixed conditions (Dyer
1997
). Stratification is
least pronounced during times of weak river flow and at
spring tides, but can become better developed when the
fresh-water input is greater (Allen et al.
1980
; Castaing
and Allen
1981
). Such density stratification generates
so-called estuarine circulation, which has a net land-
ward-directed residual flow in the bottom-hugging
salt
